Engine Rebuild - Every Size Bearing Is Too Large!

Detailed writeup by my father to follow. Summary:

Last summer I laid down my Bonnie and the shift pedal punched a piece of the casing off inside. We bought a used set of casing of eBay. Dad is rebuilding it in another province. The guts have been transferred over, and they're buttoning it up - crank won't turn. Never fear, the realize that different cranks and casings have different tolerances, hence the different bearing sizes. Get parts professionally measured. Order proper bearings. Reds (2nd smallest) don't fit. Order whites (smallest) bearings, also don't fit. WTF? Seems that the casings are slightly out of spec for being too tight of a gap for the crank. Did we buy a lemon crankcase? NB: the white bearings they took out of the used casing (that we purchased) showed signs of heat damage on one side, as if they had undergone significant friction.

Writeup from dad begins:


----------------------------------------------------------------------------------------------------------------
We started out by opening up both cases and moving all of the contents of the original motor over to the replacement casing. We moved the main crankshaft journal bearings over, taking care to move them to the same spot on the new one as the old one.

We put oil or assembly lube on everything and sealant on the horizontal split mating surfaces between the upper and lower case, and after some issues related to keeping it in neutral and positioning the transmission gears accordingly, we got the two halves together and bolted them up per the manual. At first, everything finger tight, then torqued to 10Nm, then to final torque, or in the case of the main crankshaft casing bolts, turned through 75 degrees to tension them. We reattached the oil pumps, clutch, starter, etc. And then found that the crankshaft wouldn’t turn. At all.

It would turn if we untensioned the main crankshaft bolts, so we realized after some thinking that we missed a step: the new casing had smaller holes obviously for the crank bearings and the old inserts were too thick and squeezing the crankshaft. Eventually we took the motor to an engine building shop as,they had the measurement tools and experience to accurately measure the crank journals and crankcase boring holes. They also repeated the assembly torquing with very sophisticated Snap-on torque and angle wrenches and confirmed that they had the same result.

They measured the shaft and outer holes (very hard to access the inner pair, and in any case, the 4 holes would have been line bored together in a single operation when made) and we looked up in both the shop manual and the Haynes manual on an identical chart to see what the combination of male and female part sizes called for. We found that the the crank journals were slightly under the spec limit (though still within the service limit) and so were the crankcase holes! (There is no stated “service limit” for the holes because they are not considered a possible wear item). So we went to the upper right cell of the selection chart, for the smallest crank dimension range was listed, along with the smallest hole size range and selected the Red replacement bearing part number. I ordered 8 (for some reason they are sold singly even though you would never use just one).

We installed them as before and same result. Crank was seized. The shop measured the crank, the holes, and the bearing inserts and confirmed they were too thick by measurement to provide the needed clearance (0.0007” min to 0.0017 max.). Nowhere in the Triumph manual or parts list do they tell you what the nominal thickness of the four different sizes of inserts are, just that they are
White, Red, Blue or Green. But we did some detective work and also still had one pair of the bearing shells from the replacement casing, which had a faint white mark on them, and we could measure their thickness. White were the thinnest. So I ordered 8 of the, which they had in stock. If the replacement casing had been in a working engine, and it apparently had, and had tight holes, and since the crank in your bike was already undersized, and the Whites were the thinnest bearings you can buy, it didn’t seem possible that it wouldn’t work.

It didn’t work.

When I got to the shop yesterday, and keep in mind that these people have been in business for decades and engine tolerances and even building racing engines are their business, they measured the bearing inserts and said that they were thicker than the samples we had from the replacement casing, a few tenths, and so the resulting clearance would be only 0.0004 or 0.0005” tops. It was possible that the crankshaft would turn but under load and at high speed it might seize with insufficient oil access. We did not bother to take it apart and reassemble it. I would not go ahead when an expert has told me that your engine could seize when you were going down the highway, not for anything.

At this point, we discussed that the replacement casing bearings showed signs of heat and contact, slight rubbing. The bearing surface was slightly blue/straw coloured at one edge but the heat had not carried through to the back side. This could indicate that the replacement engine had had problems with too little cleanse from the beginning also and that the crank and bearings had had to wear in together. This is not optimum. Or it could just be that the original owner let the oil get very low or old and there was oil starvation at that location.

So we discussed options:

1. Linebore (hone) the four holes all at once and open them up by 0.0005 to 0.001”. The engine shop didn’t have a 1.6 inch mandrel to use for this, their smallest was about 1.8” and they are very expensive tools to buy. But in any case this would be making the holes bigger and if they go a bit too far, it is irreversible and now you have a scrapped casing.

2. Grind the crankshaft journals smaller. They discussed among themselves and said that if the crank set up perfectly they could do it. It didn’t sound like a slam dunk. Same problems, you go a bit too far and it is all scrap plus you are taking an undersized component and making it more undersized, more out of spec.

3. Stop messing around with an out of spec replacement casing and replace the original broken casing lug with weld buildup machined to suit. We have the replacement casing as a template and the shop has a CNC mill to remake the shape. Also it holds a decent arm which is not a highly toleranced fit.

The shop owner and I both agreed the third option was best. It is not irreversible, the heat input is not in an area that would distort key features, and we go back to a casing/crankshaft combination that we know worked fine and where the old bearings showed no sign of previous distress.

The costs of both fixes are comparable and there is a good chance that we can use one of the two bearing sets we already bought. If we don’t use the whites I can return them within 30 days for a 75% return of the money.

Engine Technology will do us a favour and do the work next week. He has a 99% plus confidence this will work.
------------------------------------------------------------------------------------------------------------------------------------

Writeup from Dad ends.

We are now heading down the road of repairing the original damage to the original casing, which we know works with my crank. But I'd like to solve this mystery! Anyone ever hear of this? Know of a solution that doesn't involve modding parts? Is it just a lemon?

Anyone have thoughts?