Hi all,

I wanted to build a new bigend today of an M21,
In order to convert an M20 to 600cc.

I disassembled an old M21 flywheel, cleaned everything and then started to assembled it all:

A NOS original army stock bigend bearing pin with a NOS cage, and a NOS conrod.

I put in the 24 rollers of 1/4x7/16 which I bought as “standard” and there was just no way I could fit the conrod on..

Thinking I might have an oversize rollers, I took another bag of rollers I had and inserted it into the cage,
But again- it just did not go.

I tried to use another new “Alfa” bearing but it still wouldn’t go into the conrod race..
Then I tried the same with the old bearing cage, thinking may be the new one has too much spread,
But again no luck.

Is there any way that the bigend pin is too fat or (because it did not go with 2 pins)
The conrod race is too small?...

The only other option is that I had to sets of oversized rollers, which I can try to replace again,
But it doesn’t seem logical..

Thanks for any thoughts,
Noam.

email (option): noam10@gmail.com

The outer rod race distorts slightly when it is fitted and for this reason they are supplied slightly undersize, so it will probably need honing out, most of the car engine rebuilders have a honing machine, so it would probably be best to take it to them, along with the pin and bearings and get them to hone it to what is called a "plug fit" (they should know what it means). Failing that you can machine up a lap that is a very snug fit on the inside of the rod and use grinding paste on it, moving it along the lap and turning the rod on it from time to time

email (option): davmax@ntlworld.com

Yes, I replaced mine NOS also and the outer race had to be machined to fit, trip to the machine shop and voila! Make sure you know your bearing tolerances though.

The clearance should be 0.0006" (6 tenths of one thousandths of an inch) to 0.0012" (one thansandths and 2 tenths of one thousandths of an inch)

email (option): davmax@ntlworld.com

If you can get it done the original method used at manufacture is preferable (more accurate) for sizing the big end eye..They were internally ground to finish size. This method ensures there is no ovality in the eye, produces a parallel bore and tolerance can be easily controlled, though careful initial setting up is required..All the new big ends I supply are finished this way..I only mention this to indicate theoretical 'best practice', the other methods mentioned will produce an entirely serviceable big end assembly if carried out with care....Ian

email (option): ian@wright52.plus.com

Thanks for the replys.

When I did another bigend before and also used all NOS parts like here,
all went well and smooth and no action was needed, so I tought it should always go like this..

OK, I will go and hone the bigend..

Thanks,
Noam.

email (option): noam10@gmail.com