Contra piston tight

I have a racing diesel with a problem! The contra piston will move when cold but when she runs it locks up almost solid straight away. I guess it needs a 'microshave' off. Anyone done it before and what's the best way?
Adrian

I had this same problem with a PAW tbr 15 and I sorted it with fine emery paper (600 I think)on a flat file. I used a micrometer to avoid ovalising the contra piston and just gently papered it down a few microns. It is very much a trial and error process and I have no idea how much I took off but I remember that I spent several hours on it. I laso had to take some high spots off of the piston as well. I was using this engine for Britiah Goodyear racing and the engine was reasonable quick.

Regards

Mike

It would help to know which engine you have and what kind of head it has.

If the piston will adjust cold but not hot, it is very close to the right fit so, very little needs to come off of it. Sometimes just polishing a ruff surface finish will do the trick.

Some counter pistons are simple iron slugs tightly fitted in the top of the sleeve. If the piston has a threaded hole on the top side you can mount it on a mandrel. If not you can hold it between 2 smaller pieces in a vise.

Use some 1200 grit aluminum oxide paper on the O.D. then polish it with something like Duponts #7 or some other metal polish. Only go over the surface once with the paper and polish then try the fit.

Any bit of grit that gets into the assembly will cause major damage due to the tightness of the fit. Degrease then clean both parts with hot soapy water then grease before assembly. I cannot stress the cleaning enough.

I find that once I have the compression set for the fuel I'm using, little or no adjustments are nessesary. You might want to just put up with the way it is and make small adjustments when the engine's cold to find the flight setting.

Hope that helps,
Giffy

Thank you both. It's a racing MVVS2.5 Rear exhaust, rear induction. The contra piston is just a slug with nothing to hold it. I think I'm just going to have to try with 1200 paper and polish. Thanks for the cleaning tip Giffy. The compression will be left alone once it's set but finding that setting when it binds up fairly quickly is a real pain. I will also look for ovality or any high spots. What a nuisance!
Adrian

I've got the ABC glow version. Where do you get parts for that engine? Do they make a 2.5 ABC diesel?
Does yours have a plastic back plate?

Give Dave Wallick a call. He had a batch of heads for the MVVS diesel made up by Yakov Mazniak. If Dave has any left, I think they went for $55. It is a modern push-pull contra piston design and improves the engine's handling.

For those with an ABC MVVS, the Mazniak head needs a spacer made to properly fit the ABC liner because the ABC liner is not as tall as the diesel liner.

Adrian - having established that it's not one of mine.....
The best methods are a little light lapping or honing. If these are not available, the cures already mentioned are all very viable. You could take out the contra, get some 500 to 800 wet-or-dry sanding paper, lay it on a piece of plate glass, wet the paper, hold the contra between finger and thumb, then with a rotating motion make 5 light strokes across the paper with the contra. Move the cotra round in your fingers about 5 degrees, and do another 5, making a total of 10. Continue doing this in fives up to 70. Then do another 4, then 3, then 2, then 1. Then clean it eal good and try it. Remember light strokes and you'll get there in the end.

Hello there

I had an MVVS 2cc diesel with a tight contra piston. 0.0002 inches was nearly too much.

John Quigley

Here's what you do:
Push the contra-piston down through the cylinder and out through the bottom.
Clean everything spotless, place the cylinder top down on the bench, lightly oil the contra and push it back into the cylinder with a firm push of the thumb. You will obviously need a piece of wood or something under your thumb. Carefully note the distance from the top of the cylinder to the top of the contra-piston. Push the contra back out again through the bottom of the cylinder.
Now remove a very small amount of metal from the contra with your choice of the methods recommended by others, clean the contra, oil it and push it back in the cylinder again with your thumb. Try to use the same amount of force as before.
Aim to move the contra no more than say 3/32" (2.4mm) further up the bore. When you have reached this point, reassemble the engine and see how it feels. It should now be OK.
If you can push the contra right up near the top of the cylinder with your thumb, don't bother to reassemble the engine. Order a new piston/ cylinder assembly!
Dave