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MG MGB Technical - Wrist Pin wear

Hello, if you haven't read my over size bearing thread on the General board

PLEASE DON'T

Now then. Is it possible to tell if the wrist pins are worn by pulling and pushing on the piston when it is out of the engine. I don't really want the hassle of replacing them as it all seems very hard to do. I live in a small NSW country town and the likelihood of anyone here having the tools to do the job is remote. Is a visual inspection likely to show any problems. The engine is an 18GBH with the circlip wrist pins.

Tony
Tony Oliver

Rather than pulling and pushing would waggling the crank end of the con-rod from side to side, along the line of the bore of the main bearings, be a better test? Since they only go back and fore a little bit and not round and round like all the other bearings in the engine (excluding rockers I suppose) is wear ever an issue? Can't say I recall any mention of someone having to replace them on this or any other forum.
Paul Hunt

Tony. As a follow on to Paul's comments, I would note that, here in the US, wrist pins are not available as new parts. This would seem to be an indication that they are a low wear item.

However, the proper method to check the wear of any circular shaped part is to measure the part, then, take a second measurement at 90 degrees to the first and compare the two readings. Ideally, the two readings are exactly the same. If they are not, the part is worn and, when the amount of wear exceeds certain limits, the parts need to be replaced. This process works well if you have some form of defined specifications as to what are the acceptable limits of wear. Is a wear limit of .001" acceptable? Is a wear limit of .0001" acceptable? All depends on the application and such limits to wear must be developed by hypothesis and validated by experiment.

We have no source of wear limits as they are not contained in the factory workshop manual, indicating that the factory never considered this to be a significant factor at the dealer/dealer's mechanics level.

It would be possible to make a guess as to what might be acceptable wear limits by researching to see if there are published wear limits for other wrist pins. But, because of materials differences, heat treatment differences, and design differences, they would only be a guess and of little value.

Unless the wrist pins show obvious signs of damage--such as rusting or roughness, I would not be concerned about them.

Les
Les Bengtson

Thanks Gents. I was just a little concerned as before I stripped the motor down I had a slight tapping noise at about 3000rpm under very light acceleration. I thought this may be the pins.

I'll see if I can get a caliper up in the gap and measure them.

Cheers

Tony
Tony Oliver

I had an 18V with bad wrist pins. At idle and lower revs and load I got a sound a little bit like a diesel rattle. On load and at higher revs I couldn't hear them. It was not a typical rod or main bearing sound.

When I tore down the engine and tested the pins by rocking against the piston I couldn't feel anything. Then I cleaned them up in solvent and the rocking was evident. The heavier engine oil disguised things.

My machinist said usually wrist pins never wear out except from dirty oil. The tolerances are in the 0.0001" range. It was evident after further teardown that the engine had suffered from dirty oil. As these were the press fit pin design new pistons were needed. You of course get new pins with these pistons.

I think the ultimate failure with loose pins is a broken piston.
Robert McCoy

Tony, a tapping during light acceleration/deceleration can be a sign of a worn rod bearing at the crank. If you don't here it when the engine is cold but only when the engine gets hot and the oil thins out it would also be an indicator of a loose bearing.
John H

Thanks gents. I can't feel any play on the pins but will try again after giving them a good clean up. From the comment by John it could have been rod bearings as the ones I have removed are fairly worn.

Tony
Tony Oliver

If after cleaning you don't feel play when rocking the piston against the pins then you're probably okay there.

If the pins pass caliper the rod journals to see how they measure up. Then use the old bearings and plastigage to check your clearances. Hopefully you won't have to machine the crank.

Aftaer this you should have your answer. Excessive pin clearance is usually from contaminated dirty oil which often shows up as wear everywhere including cam bearings. Most likely rod bearings are your issue.

In any event you'll want new bearings now that things are opened.
Robert McCoy

This thread was discussed between 08/10/2008 and 16/10/2008

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