View Full Version : 121-940 Engine Rebuild

11-25-2008, 05:17 PM
Following my drive to the Laguna Beach Convention and back, I discovered that I had burned an exhaust valve. So, with the option of rebuilding my current head or preparing my 121-940 engine that has sat in the corner of my garage for many years, my enthusiasm was for the engine swap.

After looking at several of the previous threads, I found a lot of information and experience. I don't intend to repeat the information in the previous threads but just to hit the highlights.

I attached several pictures that show the differences. The 940 flywheel was for an automatic and was 11 inches in diameter, the manual transmission flywheel was 10 inches in diameter. Likewise, the front damper pulleys was different. I took them all to my machine shop and will let them sort out how to balance the assembly. From previous, it looks to be a fairly easy balance task.

The oil pump is a different issue that I have not heard how it has been solved in the past. The 121-940 has a higher capacity oil pump, the pump gears are 7mm taller than the 121-921 oil pump. A silly 7mm does not sound like much, but it puts the oil pickup tight against the oil pan. The 121-921 oil pickup fits on the 121-940 oil pump. I used cardboard placed on the pickup to measure the clearance; the 940 pump has about 3/8 inch clearance where the 921 pickup on the 940 pump has less than 1/8 inch. So, I need to raise the pickup about 0.30 inches.

Has anyone else solved this issue before? How?

My 121-940 cam is rusted. I also have a 121-924 cam but would like to use the 940 cam because it has a longer duration. I am hoping that it can be ground and polished. Anyone have a good spare 928 or 940 cam?

11-25-2008, 05:51 PM
The easiest thing to do for the cam is to use the 190SL, unless you have another grind that you think will make you happy.

Below is an ancient picture of my 940 short block and the oil pump mod I did. It is a little tough to see, but the ponton oil pick up is mated via heliarcing to the stock 200 oil pump.

11-26-2008, 04:24 AM
I had the same issue with my oil pump. I decided to use the one from the 921 engine as I was uncomfortable with such low clearance on the one from the 940 ..... too near all the sludge that would eventually settle.

The oil pressure reading is currently excellent but all of my bearing play figures are within recommended limits, which makes a difference.

If ever it gets to the stage where the engine is so worn that the pressure does eventually drop below acceptable limits, I'll just put the rebuilt 921 block back in! Hopefully not for many years yet though!

11-26-2008, 07:51 AM
Alan ... I have also thought of that. If the oil pressure exceeds the regulator valve pressure, additional volume is just released back int the sump.

What is your normal oil pressure?

11-26-2008, 12:13 PM

For the novices, what is the advantage of the 940 engine? Is it just a matter of of the crankshaft and bearing configuration?

Martin Chin

11-26-2008, 03:59 PM
regarding the oil pump, I think there were two options - one early and one late. The early pump was identical to the 921 pump and in this configuration there was no pressure relief valve (this was what I found in my 940). In the later configuration there was a larger pump an a relief valve (the one you describe). Does anyone have the tech manual to check this??

11-26-2008, 04:42 PM
Does anyone have the tech manual to check this??
"Some" information "shouild" be in the M121 engine catalog. I will dig it out later. My engine is about serial number 700. Seems Jim's is around 7000. I think, but am not sure, I used the pump the engine came with. I know I used the correct, if not latest, pump. It has been 25 years ago so I am not sure any more.

RE: Pressure. My pressure shows just like any other 190SL. Pegs when cold and runs about 80 - 85 psi driving hot. At idle it shows more than half scale in Summer weather, engine hot. Oil is 15 - 40.

11-26-2008, 04:46 PM

For the novices, what is the advantage of the 940 engine? Is it just a matter of of the crankshaft and bearing configuration?

Martin Chin
5 main bearing engine with the latest cylinder head. That setup should last your lifetime. It is a bolt in. It is so similiar to the 190SL engine that I am told it was fitted to some pontons too, but that is a rumor at this point.

Something like a 4 cylinder 220 would require more work to fit in. But, I think has been done and, as I recall, it mainly revolves around relocating the oil filter setup, but I am sure there is more.

02-24-2009, 06:02 PM
Well it has been a couple of months and I finally received my engine back from the machine shop. Everything took longer; my pistons were not able to be reused and I need to have a set made and then the rods needed to be bushed to fit the new wrist pins.

The engine was balanced with the smaller 921 fly wheel and the single sheave damper. The head was also rebuilt.

I painted the block and just began the assembly when the first glitch arose; the crank was ground to the first overhaul specifications .... by the book. But the first oversize main bearing set was for the smaller crank journals AND an wider thrust bearing specification. So the crank is back to the machine shop to have the thrust journal ground to match the bearing.

Nothing serious, just another detail and another delay.

02-26-2009, 09:20 AM
Well .... I cannot report progress but I keep learning. Looking up the part numbers for the oil pump and none appeared to be available. After inspecting several old pumps, a new one seemed to be the prudent choice. I was able to finally locate a part number that was available; 121 180 24 01 (60+ available in Germany). If you need an oil pump, remember this number so that you don't need to expend the efforts finding it that I did.

02-26-2009, 11:49 AM
Well .... I cannot report progress but I keep learning. Looking up the part numbers for the oil pump and none appeared to be available. After inspecting several old pumps, a new one seemed to be the prudent choice. I was able to finally locate a part number that was available; 121 180 24 01 (60+ available in Germany). If you need an oil pump, remember this number so that you don't need to expend the efforts finding it that I did.

Oil pumps are VERY fixable. Did you open yours up? Why do you think you need a new one?

The usual case is you need to surface the little cover. Clean the rest real well. Clean the screen by tapping it on wood around the edges as all sorts of stuff will fall out. You can even "port" them inside some for better flow.

Remember the 200 pump is a high volume pump to cope with the 2 extra main bearings. That means there are bigger surfaces inside. Look at them closely for scarring. If the bores where the gears go are not bad, the rest can be worked.

Done this so, so many times. The only reason usually to put a new pump in is laziness. They don't go bad to the point of no repair very often. Look at yours, post some pictures, and let us know.


02-26-2009, 01:47 PM
Walter .... The problem that I noted was a looseness of the lower bushing. I tried assembling it several times and could not get the pump to rotate smoothly. The inside of the pump looked OK, without significant grooving or wear. If I could have gotten it to feel right, I would have used it. With it not feeling right, I just thought that the risk of an oil pump failure on a new engine to be too high.

02-26-2009, 03:41 PM

Yes, there is some feeling that must be applied when assembling the pump. Did you surface the cover? Did you deburr the bushings in the cover? You need to progressively tighten the cover, but not too tight. It is not a simple matter of bolting it together. A little plastic hammer helps here too. Parts ain't cheap. I prefer to recycle when ever I can do it safely, as you have tried to do too.


02-26-2009, 04:27 PM
Walter .... Yes, I de-burred the gears and tried taping the cover in several directions to get it centered. I got it where it would stick in only a portion of the revolution. It just didn't feel right.

The main problem in my mind was that the lower bushing was not tight and I could feel a gap. I believe that this bushing should be firm so that the gear would not rub against the inside of the pump housing. If I though it would last 100K miles, I would have used it.

03-01-2009, 09:44 AM
Now some real progress. The thrust surfaces of the crank have been reground and assemble began. I fitted the crank and checked the bearing clearances with plastigage, perfect ...0.051 mm. I installed the pistons and checked the rod bearing clearance and they were also correct.

Then the problem; the piston crowns extended above the block by about 0.025 inches. Because of the tightness of the intake valves to the pistons, I will need to pull the pistons and the crowns shaved by 0.025. The head looks good and I had the shop insure that the intake valves were recessed 0.035 inches.

This reinforces the importance of checking everything during assembly.

03-07-2009, 10:55 AM
Glad to hear of your progress.

Do you think the problem with the pistons was due to the way they were made or because the block was skimmed?

I assume that you had them made because it was cheaper than the price of those supplied by Mercedes? I was tempted to do the same until Per told me of the problems he had with his pistons, specially made by Venolia.... it convinced me to buy from MB.

03-07-2009, 02:01 PM
Alan ... I had them made because MB pistons were not available. I believe that there was just a mis-measurement when the sample was measured. 0.025 inches (0.60mm) is not a whole lot. I have had problems once with a piston touching an intake valve so I am extra careful.

My latest delay is that I broke a ring when installing the pistons after they came back from the machine shop.

03-07-2009, 02:36 PM
My latest delay is that I broke a ring when installing the pistons after they came back from the machine shop.


Hope the rest are OK!

03-08-2009, 10:15 PM

0.025 inches is exactly one revolution if a micrometer or many other measurement devices. I have seen this mistake many times. I myself made it a couple of times while on the bench.

03-13-2009, 09:18 AM
One more little part needed for the 940 block; the bracket for the carburetor support strut. The bracket is available from MB; 121 094 04 41, $25. It bolts to the 940 engine mount location.

03-13-2009, 12:00 PM
It's available from SLS in Germany (Section 09 - item 16a) at Euros 10.80. I will be visiting their stand at Techno Classica in Essen on April 2 and can ask them to bring one with them, from Hamburg. Happy to mail it to you .... the least I can do after all your helpful advice!

If you can wait that long, drop me an email with your address and I'll mail it to you on April 3.

03-13-2009, 04:09 PM
Alan .... Thanks for the offer but I already have one. I was just posting the details of all of the special little stuff in converting a 121-940 engine for use in a 190SL.

I also should mention that the studs for the intake manifold, water fitting and the chain tensioner need to be replaced; 10x30, 10x35, 8x18 and 8x20. You can just extend the existing threads and then cut then to the required length.

03-22-2009, 11:23 AM
Another step forward and more issues.

I bought a new oil pump and installed it. Dave Heiple sent me an email about a friend of his that installed a new MB oil pump and it made noises so that they had to pull the engine and remove the pump. So I put a couple of quarts of oil and an oil filter on the partially assembled engine, inserted a rod into the distributor gear area and turned the pump with my drill. Mine also made a funny pulsing noise that just didn't sound right. It was very metalic. After pulling the pump I ran it again in a pan of oil to confirm the noise. Then I took the pump apart, there were scrape marks on the pump flats and rough places on the pump gears. After touching up the pump gears with a small file, I assembled the pump and adjusted the housing. The pump now turned and pumped quietly. I did find some shavings in my oil pail after testing. These would have been picked up by the oil filter but it did not instill confidence in my new pump.

My other issue was that I received a used head which had been ported and polished with heaver valve springs that I decided to use. I first sent it to my machinist to weld up the water hole between #2 and #3 and to lap the valves to insure a good seal. The problem surfaced when there was not enough movement in the adjuster to set the exhaust valve clearance. I checked the Technical Data Manual for the specification (940 engine - min 16mm, max 18mm from the bottom of the head to the exhaust valve crown). So it was back to the machine shop for a new set of exhaust valve seats.

Once more, it pays to check and double check everything.

04-01-2009, 10:59 AM
Still another issue for anyone working with a 940 engine. The fly wheel alignment pin in the crank shaft on the 940 engine is longer than the one on the 921 engine. My 940 engine had a spacer between the crank shaft and the fly wheel so it had a longer pin.

Of course, I did not notice this until after the crank/flywheel/damper had been balanced and I was doing final assembly. As it turns out, there is just enough space for this pin to clear the block and clutch disk with about 1/8 inch to spare. It does require relocating the pin closer to the block.

04-24-2009, 08:13 AM
Did you use the head from the 940 too? If so, I'd be interested to know what you did about valve seals, if you fitted new ones?

I never did resolve the issue that I had with the new MB supplied ones slipping off the guides, so I had to re use the old seals!

05-08-2009, 09:03 AM
Seems that I never answered your question on what my normal oil pressure is on the 940 engine!

It pegs at 90 when started from cold. At normal running temperature it is 75 at low idle speed (about 700 - 800 rpm), rising to 80-85 at high idle speed. At normal runing it pegs again at 90.

05-08-2009, 10:17 AM
Alan ... I also missed your question. I have rebuilt two 940 heads; I installed new valve seals on the first (they did not slip off of the guide) and reused the seals on the second (they were in good condition.

My engine is now running and my oil pressure is about the same as yours at idle (75 psi) and pegged with the engine off of idle. This is a fresh rebuild so I would not expect any oil pressure issues.

05-08-2009, 12:24 PM
My 57 always pegged at 90 & ran there until the oil got warm. But then I had all of the gauges rebuilt. It then pegged at 90 for about the first minute or less. Then it fell to 75. and woul drop off to 30 at idle. So a lot could be said for an accurate gauge.

05-11-2009, 11:12 AM
The engine is running and I now have about 50 miles on it. After spending time tuning the Webers, I discovered that the welded up manifolds are more touchy to tune as they do not have the channels between adjacent cylinders. I have also discovered that the carburetors need leaner jets; 110 main and 45F2 idle jets.

The engine is thus far running well but I still has some questions. I took a compression check this morning and it was #1 125, #2 140, #3 145 and #4 140 (my 121-921 engine had 160 in each cylinder before it burned a valve). I set #1 at TDC and pressurized it with 120PSI and could not hear any leaking air out the exhaust, intakes or the dip stick hole. I interpret this as the cylinder is sealed and that the reading is from the head or cam configuration.

This engine has the cam out of Gary Cox's old race engine so the intakes may not be closing soon enough for the compression test.

On the other hand, the engine wants a LOT of ignition advance. It ran poorly at 40 degrees at 3000RPM, ran very well at 45 and runs even better at 50 degrees BTDC. At 50 degrees, the engine still idles smoothly at 25 degrees advance but does not have much torque in the 1500-2000 RPM range. I have set the timing back to 45 degrees and I'll put some more miles on it.

05-23-2009, 09:48 PM
Another update. Well the lack on leaking with pressure in the cylinders was caused because I had not removed the schrader valve from the compression testing hose before pressurizing it. So, I just pressurized the hose .... DUH.

Anyway, I put about 100 miles on the engine and took another compression test; #1 145, #2 155, #3 155 and #4 155. So the problem was that the engine just needed time for the rings to seat. I am hopeful that with more miles that #1 will seal in the same manner as the other cylinders.

So, I will be off to put another couple of hundred miles on the engine before I spend much much time tuning and worrying.

05-24-2009, 10:33 AM
With aircraft engines, they are run at max rpm under load until the rings seat. The oil consumption goes down accordingly. Does this hold with auto engines as well?

05-24-2009, 11:44 AM
Cecil ..... I don't know how or when piston rings seal. I am "learning" but I am not sure that my experience is repeatable. My rings are not M-B rings. They are JE Rings (http://www.jeproseal.com/):
Top Ring: Plasma moly-coated ductile iron (barrel shape)
Second Ring: Phosphate-coated cast iron (tapered face)
Oil Ring: Chrome-plated carbon steel (low or standard tension)
Complete ring sets

05-24-2009, 04:31 PM
I guess it's a black art!

06-19-2009, 07:39 PM
I now have about 700 miles on the engine and it was time to put it on the dyno to see what does. When Robby and I put our cars on the dyno the last time in 2001, his car produced 73HP with 82 ft-lb torque (http://www.190slgroup.com/tech/images/rob_dyno.jpg) and mine produced 83 HP with 91 ft-lb torque. (http://www.190slgroup.com/tech/images/jim_dyno.jpg) My 940 engine produced 87HP with 98 ft-lb torque.

I was hoping for more but the numbers are accurate. Just as important, the 940 engine has significant more torque in the lower RPMs with most of the torque coming under 4000RPM. This low end torque greatly enhances drivability. The dip in torque above 4000RPM was caused by the engine running a little rich.

The second line on the dyno chart is my 2001 run (they still had the results on their computer).

06-19-2009, 10:52 PM
All that torque begs for different gear ratios. I find myself in top gear a lot. A final drive of 3.25 might be a better fit than even the 3.7.

I can dream, can't I?

06-20-2009, 09:58 AM
How does the five main affect vibration?


06-20-2009, 04:08 PM
How does the five main affect vibration?

If anything, it affects it in a good way. I can not from the seat of my pants determine it is better, but it sure is not worse. Like most engines, a lot has to do with how well it is balanced to begin with. I do not get the high speed droning a lot of cars get. So maybe it is better damped in that respect.

06-20-2009, 05:11 PM
Walts (East & West) .... I believe that the 940 is significantly smoother that my old engine. As "West" noted, the low end torque is the real difference.

I will add another comment. I am running 28mm chokes in my Webers to increase drivability. I believe that that is the reason that the torque curve decreases at higher RPMs. The dyno guy normally works on vintage race cars and I assume that his comments are based on high reving engines. He says that my engine would produce much more power with 32mm chokes. He also prefers setting the air/fuel ratio richer that what I prefer. In fact he shut down early on my first run because the air/fuel was about 14 (the way I had set it up). He said that it was way too lean. His target was 12:1. In the time on the dyno, I tried several main jets to get the A/F under 12. The chart that I posted had an A/F between 11 and 12. The flat spot between 3500-4500 was when the A/F went from 12:1 to 11:1.

My thoughts are that I could get additional high end power with 30mm chokes but I might loose the nice smooth torque in the 2500-3500 range.

"West" ... I think that the 3.7:1 rear end is about right for both around town and highway driving. I had my big tires on the trip west last year and they worked good on the highway but it made in town driving a 2nd/3rd gear experience.

robby ackerman
06-20-2009, 11:54 PM
I might as well insert this here. Jim put his O2 sensor up my tail pipe today and the car misbehaved as if on cue -- just when I needed it to act up. The temp was mid-nineties and it bogged down seriously under acceleration. The O2 meter displayed an A/F ratio of 10:1. We drove back to Jim's garage and he swapped my 1.35mm main jets for 3 @ 1.25 and 1 @ 1.30 (the one at 1.30, well that's another whole story) and that brought it from 10:1 to 11:1. Then we swapped in 1.20 mains and the A/F ratio fell into the sweet spot between12:1 and 13:1 and the car pulled smartly.

Idle jets are .45, Air correction 2.0 and acceleration jets .52. Displacement is 1943cc.

06-22-2009, 04:07 AM
Jim, when you put your car on the dyno, did you end up changing your ignition timing or is it still set at 45 btdc at 3000 rpm?

My 940 was set at around 37 btdc at 3000 by the guy who was maximising the bhp at 4000 rpm. However, I still have the kangaroo effect under load when driving up an inclined slope at low speed. No problem when accelerating with more pressure on the gas pedal or when driving on a flat road. I subsequently advanced the timing at 800 rpm from 12 to 15 which seemed to improve it a little but as this was the max stated in the manual, I thought I had better not exceed this ... maybe I should? Would be interested to know what your advance is at 800 / idle.

I have also tried changing idle jets (F9 55 to 50 & 45) but still the problem persists. Normal driving over 1700 rpm is excellent. I also tried fitting the 1844 Pertronix electronic ignition but then it sounded as though it was running on only 1 cylinder .... I think the unit is faulty.

I am using the 190sl distributor as the original one from the 940 needs a vacuum port, which I don't have with the Webers.

robby ackerman
06-22-2009, 09:23 AM
Jim got rid of my kangaroo effect -- surging at low steady speed -- Saturday when I dropped by his home.

In my situation we made two changes -- 1) Jim opened the the idle mixture adjustment screws on cylinders 2 & 3. I had 1 & 4 opened 1.5 turns and 2 & 3 about 3/4 to 1 turn. I had closed 2 & 3 closed down some to make the engine idle smoother. However, when I pulled one plug one off at a time and measured the rpm drop, there was almost no change in idle speed compared to pulling the wire off 1 & 4. So 2 & 3 obviously weren't getting enough fuel.

And 2) as I mentioned in my post of Saturday, the other change he made was closing the main jets down from 1.35mm to 1.20mm. The 1.35s, which I've run for 25 years, let way too much fuel in, which was verified with an O2 sensor in the tail pipe and a digital display.

Everything is interrelated. My timing remains set at 38 degrees.

06-22-2009, 10:15 AM
Alan .... I tried several "pulls" on the dyno with different ignition timing. I was surprised that 38 degrees (at 3000) provided the best power at all RPMs. So I learned something and have now set my ignition at 38 degrees.

Your "kangaroo" effect is probably caused by the linkage out of sync. At idle, set all four idle mixture screws the same (1 1/2 turns usually work for me) and then set your throttle idle screws so that you have a good idle and moving either screw will change the engine speed (this means both carburetors are providing the same air). Check the linkage by lifting the rod at the back of the linkage (not a linkage that goes to the carb). Both throttles should lift off of the idle screws at exactly the same time.

The other method is to remove the screw cover over the progression holes and look through the hole to see the butterfly. As you lift the throttle, you should see the butterflys cross the same holes at the same time.

06-22-2009, 12:30 PM
Many thanks Robbie and Jim for your comments. I already synchronized the carbs by viewing the butterflies through the progression holes. There is slight play in the linkage but the butterflies all start to move at the same time. The idle mixture screws are all unwound roughly the same amount ... around 1.75 turns.

Because the problem occurs under 2000 rpm, I had assumed that the main jets (135s) were not the problem ... maybe I am wrong here?

I also thought that maybe it is the advance curve of the non-vacuum distributor but I assume that you have the same mechanical advance that I do Jim.

I am just about out of ideas on this one!

robby ackerman
06-22-2009, 01:03 PM
Those 135s are what my Weber book says are needed for the DCOEs on a 190SL, though instrumentation says otherwise. My observation is that some fuel is coming through the main jets even at idle. Since I changed to 120's I've had to give the car more choke for cold starts. Also, vacuum in the nozzle is what pulls fuel through the main jets, and whenever the engine is running air is being sucked through the auxiliary venturis and chokes so some fuel, albeit a small amount, is being delivered.

Back to the subject of 135s and 120s -- some engines may will require 115's or smaller. My intake manifolds and cylinder head have been ported and polished and have increased airflow, hence a larger fuel requirement, over the stock manifold & head.

And yes, I'm using a distributor with a mechanical advance, not a vacuum advance.

06-22-2009, 01:32 PM
You all using number 15 cams?

06-22-2009, 03:08 PM
Alan .... At low speed cruising and low throttle positions, your idle jets provide most of the fuel through the progressive holes. My jets are 0.45 F2 while Robby has 0.45 F9.

A vacuum leak can also cause the "kangaroo" at low speeds (one or two cylinders working more than the rest).

06-25-2009, 04:34 PM
Jim, what can I say! Yet again you were spot on ..... it was the manifold!

I bought a can of carb cleaner and sprayed it around both sides of the inlet manifold. When I sprayed the engine to manifold joint of the rear carb, the engine revs dropped. I took the carb off, tightened up the two manifold nuts and the stuttering was fixed! Not only that but with the vacuum leak fixed I was able to use smaller idle jets. I am now running with F9 45. I tried F2 45 but the F9s were much better for my DCOE 40s

Many thanks Jim, you have really made my day!