Commando crank shimming

Not open for further replies.
Jan 6, 2005
Newbie here looking for advice on procedure for shimming the crank. Have never done this before so any help or tips much appreciated.
Permissable end float is usually listed as 0.010-0.024". This is usually adjusted with shims behind the outer bearing race in the crankcases, although some engine builders will use shims between the crank and the bearing. The only factory shims available are 0.003" (P/N 067569) and fit between the crankcase and the bearing. These typically are sufficient unless you are mixing and matching 750 and 850 parts. A friend putting a 750 race crank in a set of 850 MkIII cases found he had .060" end float! If you find this is the case, air-cooled VW crank shims will work.

If you are using superblends, install the races in the crankcases by heating the cases in an oven and chilling the races in the freezer, then quickly drop the races in with only a light tap to seat. Install the bearings on the crank and assemble the case halves with crank and torque the case bolts. Set up a dial indicator and measure the end float. Disassemble the cases and heat them in an oven and the races will drop out with a sharp rap of the case half on a wooden surface. Install the appropriate shims and re-install the bearing race after heating the cases again. Re-check the end float with your dial indicator.

Norton didn't pay too much attention to end float and it has been said that they often didn't shim them at all. However, I have never opened up a motor that I thought was untouched that didn't have at least one shim.
Thanks very much for the info, it'll be a great help. Am building an 850 Nourish/Triumph but using a Commando crank hence the question. I'm also going to fit a Commando clutch cos I want to run the primary dry.
Thanks again
Good tip there Ron L.

Having put a 750 crank into 1973 850 cases for the first time, I found I had .042" end float. The Air Cooled VW shims fit ok you say.

I will check it out.
That tip about the VW shims was good timing Ron L, I puchased some of the VW shims yesterday.

They would seem to protect the crankcase better from any "bearing spin", if it were to happen also. I like the way they fit.

I do have a question about "ideal" endfloat though. As it happens........ 5 different people will tell you the "perfect" end float is anywhere from 0 - .024".

After playing around with different amounts of shims yesterday, I found that with about .012" with the shimmed case still "warm", I only had about 5 thou once it cooled down. The crank now seems to spin "just nice" bolted up.

It seems to me that once the engine is running & heats up, I will have the larger endfloat again.

So, I am thinking that the way it is at the moment is "right" for this set of cases.

Would anyone like to clue me in ?
Thanks for the heads up.
Your figure of .007 was about what I had in mind at room temperature, so maybe I read that figure in another post of yours.
I will play around again today & get it between .007" & .012" then.

The reason I thought that the "case" shim may be "better" was really because it kept the outer bearing "fully" on the inner race. Whereas the "crank" shims appear to push the inner race & crank away from being fully into the main bearing. I am sure you will know what I mean by that ...?
At the risk of "obfuscating" I offer the following comments regarding crank shimming.

The Norton Mark 3 shop manual states the "Permissible end Float" for the crankshaft as: 0.010"/ 0.024".

Ideal end float should therefore be .017" (mid tolerance).

Shimming between the crank and race does NOT change crankshaft position compared to shimming between the bearing and case.

In other words, pushing the race into the bearing is the same as pushing the bearing into the race as far as crankshaft position goes.

If you want to keep the crank in the middle of the engine cases, then you would need to shim each side of the crank shaft equal amounts.


No "Obfuscation" allowed on this forum Jason :!:

Thanks for your input.

It probably should not surprise you that I also have workshop manuals as well as aftermarket manuals to find info.

The reason I sometimes ask questions on this forum sometimes, is to verify information I may have been given by other parties. This is the case here.

Experienced engine rebuilders that frequent this forum have given me information that confirms information I have received already.

Therefore, I will go by their recommendation.
If you pull an 1973 850 set of cases apart one day, you may find that the drive side is the one that is shimmed. The timiming side has no case to use a crank shim against. So shimming both sides with norton crank shims does not appear possible to me. I will be corrected I am sure if I'm mistaken there.

Having said that, using the VW "bearing shims" you could shim both sides as they are larger and it could be done. But, as no one else has suggested shimming both sides, nor have I read it anywhere else but in your post, I will just shim the drive side.

I will let the forum know if this doesn't work.

I replaced my 750 main bearings a while back now & it has done many miles since without a bottom end problem. It did not require shimming FYI.
Not open for further replies.