Nonsense Found Building Bikes

mdt-son said:

Re. bearing problem: Clearance while heated is insignificant. If the bearing bore has become slightly elliptical (at ambient temperature) for unknown reasons, the bearing will be pinched diametrically. This may be sufficient to deform the bearing in the elastic domain, causing a locking effect at the inner race. The outer race - although steel - isn't that solid!

- Knut

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I know it doesn't matter when hot - that was just a data point. I always keep the inner with the outer until ready to install on the crank so I don't mix them up - should not matter if I did, just anal about such things.

I will know later today. Of course, I'm measuring before I put my last in-stock, very expensive bearing in.

Most would probably just run it - it does turn smoothly once in, but I don't do things like that on customer engines or bikes I'm building for sale!

marshg246 said:

Way too many good crankshafts available to spend the time to find someone to do that here! I've already got one good 750 crankshaft out for a minor Gorilla-caused repair that has been over three years waiting its turn - no more of that and the other one I sent with it just recently came back costing more that any used crankshaft I ever bought! Better to buy 2-3 used and find one gem! Most I've bought are fully in-spec. In the US, the majority of Commandos are very low milage and there are a couple of "breakers" that can be trusted and a couple more that usually are OK.

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Greg

I am sure there is a workshop in the US with sensible turn-around times. 3-6 months isn't uncommon. You'd have to queue up at Seager Engineering as well - I have waited 10 months for my timing cover to be repaired.
The good ones are overwhelmed with tasks - no wonder.

Do you magnaflux every used crankshaft bought? Many used cranks - although of low mileage - will fail this test (typically 2 out of 3 samples tested). If I had a tested and trusted crank on hand with surface wear at the shafts, I would be inclined to spend some money on getting it fixed.
Tested and trusted cranks are valuable!

I have a Mk3 crankshaft which is still on STD grind - haven't had it magnafluxed yet. Shaft wear doesn't concern me, as the engine obviously hadn't been apart since leaving the factory.

- Knut

marshg246 said:

Engine has 78k miles, apart when brand new for a piston problem, never apart since. Was running fine but smoking a little on the drive side, had a top-end noise, and gearbox had a problem so the owner wanted it checked over.

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Some smoking is to be expected from an air-cooled engine at 78K miles !
Another consideration, why does so many engines develop a smoking habit at the left cylinder & head? Could it be the whipping crankshaft causing accelerated cylinder and piston wear at the left cylinder ?
At first glance one thinks the whipping shape is symmetrical about the C/L - this isn't necessarily so.

- Knut

mdt-son said:

Greg

I am sure there is a workshop in the US with sensible turn-around times. 3-6 months isn't uncommon. You'd have to queue up at Seager Engineering as well - I have waited 10 months for my timing cover to be repaired.
The good ones are overwhelmed with tasks - no wonder.

Do you magnaflux every used crankshaft bought? Many used cranks - although of low mileage - will fail this test (typically 2 out of 3 samples tested). If I had a tested and trusted crank on hand with surface wear at the shafts, I would be inclined to spend some money on getting it fixed.
Tested and trusted cranks are valuable!

I have a Mk3 crankshaft which is still on STD grind - I haven't had it magnafluxed yet. I am not concerned about shaft wear, as the engine obviously hadn't been apart since leaving the factory.

- Knut

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When you're running a business 3-6 months is not acceptable if there is an alternative. I use Jim Comstock but these days only with up-front begging for a decent turn around and then hounding when he takes longer. 100% of the work he's done is perfect. I no longer send him anything saying there is no hurry - that means never going to happen. I'm trying out a new place recommended by guys here for boring. As of yesterday, he's past his promised turn around (he set it, I accepted it) and I've heard nothing. If this bottom end ends up together this weekend and the cylinders are not back Monday - I'll be truly annoyed!

No, I magnaflux every used crankshaft I choose to use. Don't waste my time on those that are out of spec or have other issues I don't like - for instance, clearly been apart but not marked so might be back together wrong. There's one on eBay right now that looks good but has that issue so I won't buy it.

I wasn't concerned about this crank having shaft wear until I found the shaft wear! The journals on this crank look brand new and are completely in spec.

OK, call me crazy - I can take it.

Checking the first two bearings I had in (of course, after they were out):
1st bearing I had in: 2.8345", 2.8346", 2.8347", 2.8344" and the inner race is snug now but is easy to put in and take out. This was very tight when installed.
2nd bearing I had in: 2.8344", 2.8344", 2.8345", 2.8345" the inner race is fine now but was tight when installed.

I got a new one out of stock, same batch number as the other two. Measured 2.8347 but after installing it I spilled alcohol on the paper and can't read the rest. The difference between smallest and largest diameter was .0003" - I do remember that.

I studied/measured the hole in the crankcase and found one spot that I'll call a bump. I looked at/measured the bearing that came out of there and it had a depression in one spot. I'm guessing that the bearing was dropped in the factory and installed anyway. Over time, the case conformed to the depression and made the bump. Hard to measure but about .0015".

So I carefully reduced the bump, reheated the case and dropped the new bearing in. Once cool, the inner race was fine so I'm calling that fixed.

The Loctite 620 took care of the inner race on the other side. I used a rubber strap and tried very hard to make it move - won't budge. I'll put the crank in without rods and make sure everything is as smooth as it should be and if so, finish putting it together.