Hello Everyone,

I have been wondering about clearence between the crank and crank bearings in the at/ct bases. The notion first came to mind when I saw five different crank shim thicknesses offered for the primary gear side of the crank. On the three bottom ends I have done, I have put the shims that were in the engine back in without any thought. I am in the process of building another engine. During disassembly I noticed that there was no shim on the primary side. I made the discovery of there being various sizes offered when I looked for a part number. I then looked for a spec in the manuals that I have. I did not find one. I watched a Dale Sweger video where he said (paraphrased) "you will not find a specification". He then went through the procedure of measuring the depth of each case half from the parting line to the newly installed bearing, adding those two numbers and subtracting the width of the crank. I don't remember the figures he quoted, but it was somewhere around 1.00mm. The flywheel side shim is .3mm, leaving .7mm. I think he chose a .4mm primary side shim leaving .3mm clearence. I have asked a couple other guys that I know who have experience with Yamaha enduros, one of whom sold me the engine that was lacking the shim, and neither had a reasonable answer other than  "..it'll be fine". I can except that it will be fine, but I thought I'd ask the brain trust.

Anybody have any hard data, or thoughts regarding this?

Thanks for reading.
Jamie Galvin

The only thing that really matters is that the web width of the crankshaft is 56mm +.05mm -.10mm, if this spec is met it will fit in the cases. You don't need a shim on the mag side for when you tighten the crank gear nut on the clutch side it will pull the crankshaft all the way to the clutch side. I measure the cases, but you will not find a spec for that clearance. I use the case measurement and the crank web width to insure I don't have too much shim in there so as to keep the cases from coming together or to cause the crankshaft to bind. The manual does not tell you to measure the cases. That is just something I do to satisfy my mind. So, if you had one on both sides leave the one out on the mag side and add one thin one to the clutch side. If you had one only on any side add it to the clutch side. The shims offered in the parts manual are selective, you don't have to have any, some engines have no shims, some have them on both sides, some only on one side. I like to add a thin one to the clutch side to ensure the crank web does not contact the cases.

Thanks Dakota, good explanation.

Shims on early models are used to center crank in cases.  DEET wrote a post years ago on how to measure and select.

Best practice is generally to use the shims that were there.  If a shim is lost/missing, often crank will rub or be really close on right side case when clutch nut is tightened...  so at the very least I install the crank in right side case and tighten nut before engine assembly. 

I'm not sure I've ever come across an early AT/CT that didn't need a right side shim.

Thanks Mark, I’ll try to find Deet’s post. Meanwhile, the engine is together, I followed what Dale Sweger did, I’m sure it’ll be fine.

I found Deet’s post. Copied and pasted here:


Replied by  DEET  on topic  DT250 Rebuild problem
Apparently you didn't check crank clearance before installing.
That would have alerted you to a potential issue.
With the empty cases together, measure the distance between bearing faces.
Measure the width of the crank at the bearing face contact area.
Subtract crank width from bearing opening. That tells you the clearance.
Divide the clearance by 2 and that gives you the shim dimension.
Shim goes on the primary side.
The Yamaha service manual tells you the crankshaft maximum width. It can't be any wider than that.
Pressing bearings into cases is not the recommended method of installation.
A crankshaft installation tool should be used for crank installation. Install primary side first.

Take some measurements... you should soon see the problem.

As usual, excellent information provided quickly by this forum. I’ve said it before, but this forum is the best I have used. Thanks everyone.

Jamie Galvin

Thanks for the repost of DEET's instructions Jamie!

If ya doing mains just be sure the mag side hasn't been turning on the crank. That happens more with motors up to 100cc using a smaller bearing but nothing worse than bolting the cases up then--oh its still loose on the left. If it's been slipping spinning it'll be obvious. Don't panic, but if that's the case it'll be tight to fit until the last bit where bearing sits. Then go loose.