250SL Con-rods

[Bianchito]

I've heard of the weakness of the 250's con-rods in comparison with other models, and Im wondering if they can be susbtituted by those of a 280, whhich are supposedly stronger.

Can anybody confirm this and if there is any well know source for them??

Thanks!

[Paddy_Crow]

From what I know, I would say no. The 250 and 280 have different stroke and same crank centerline to deck height, so it is likely the 280 rod is shorter (depends on piston pin to crown height being the same). I'm sure Dan or one of the other guys that actually works on the motors can confirm.

[ja17]

Hello Bianchito,

Actually, I did some experimenting with this switch years ago. However the passages in the crankshaft, bearings and connecting rods are different.  The cranshafts are different in other ways also.  You would have to replace the crankshaft and the connecting rods as a unit using the correct bearings.

My experience has never shown me that there was a real weakness in the 250 enine. I do believe that when Mercedes designs a new engine, that they try to "improve" and incorporate latest design features in the newer engine. So as far as being "defective or weak", I probably do not agree. As far as latter engines being improved, possibly!

I know lots of high mileage 250SLs. The secret to longevity is good maintenance, and good storage. The earlier engines 230, 250 were a lot more foregiving as far as oveheating...more space between cylinders. However any of the engines can be ruined from severely overheating.

Joe Alexander
Blacklick, Ohio

[Paddy_Crow]

I just checked on the bore and stroke of the three SL engines, I got it wrong. The stoke change was from the 230 to 250, not the 250 to 280.

[Benz Dr.]

There are a lot of variations between the 3 different engines. Early 230SL pistons used 4 rings and 24mm pins, intermediate used 3 rings and 24mm pins and 3rd version used 3 rings and 25mm pins. I think the early version used a drilled conecting rod similar to the 220SE of which the engine was designed from.
 The 230SL used 3 different pistons while the 250SL used a similar piston as the 3rd version 230SL but it didn't have the long skirt like a 230SL. Like most us, I prefer a short skirt...

The 230SL and 250SL have the same bore but the 250SL has a longer stroke. The piston is also a 3 ring type and has a 25mm pin. The heads and head gaskets are interchangable on the 230 and 250.

The 280SL uses a 3 ring piston with a 25mm pin. There were two engines made which are quite different in design and the head gaskets are totally different. You can't mix them up. The conecting rods in a 280SL are quite short and have a small hole drilled into the upper side of where the bearing fits. Every time the journal in the crankshaft passes this small hole a shot of oil sprays out which hits the bottom of the piston to help keep it cool and to oil the pin bushing.
 
The 280SL is the cutting edge of what the factory could do with this engine. Everything is stresed higher and further than ever before and even all the torque values are much higher. Angle of rotation is the norm rather than straight torque values and the use of stretch bolts is used. Conecting rod bolts are started at a set torque and then 90 degrees after that. You would think they would snap off but they never do. Same for the flywheel bolts.
Overheating is  a problem with some 280's and seems to be a bit worse with the late US model. This could have something to do with the distributor timing used but a lot of this problem relates to the size of the cylinders relative to the block dimensions. AC in these cars can really overheat the engine due to the confined engine bay space. This didn't seem to be as large of a problem in the sedans of the same era due to the extra room under the hood on those cars.

As for the 250 being a weak engine prone to self destruct, I think that's nonsense. I've seen good and bad examples in all 3 engines. Oddly, only the 280 seems to have suffered from heavy camshaft wear in the examples I've seen. I've seen more engines with '' death rattle '' pistons in the 230 and 250 line but I think they were from older and higher mileage engines. They are older so it stands to reason from that perspective.

The 280SL had several designs that made it somewhat  better. External oil cooler, larger oil galleys, cooled pistons, visco fan clutch, better ignition system, two oil pressure relief valves and yet they still shared a number of parts from the 250 engine.
 The heaviest wear areas I see are cyinder walls and valve guides in all engines. If the cylinder walls are really worn then the crank bearings will be too. Heavy blow by into the oil pan usually dilutes the oil with gas and other contaminates and the bearings suffer from this. Proper maintenance is the only way to prevent or slow this from happening. Given the low miles these cars are now driven every year that should never be an issue any more.  


Dan Caron's
 SL Barn
benzbarn@ebtech.net
 slbarn.mbz.org
  1 877 661 6061

[Bianchito]

Thanks guys!!

[enochbell]

Dan,

I usually find your posts to be interesting and sometimes entertaining, but that description of engine differences was amazing, thanks for your contributions.

g

'64 230sl, fully sorted out...ooops, spoke too soon