Where does the crank go?

O

olly

Senior member
So we're rebuilding the engine on my '73 SF1 750 and found the crank slinger was rubbing on the bottom case. By pushing the crank tight up against the ball bearing end and massaging the slinger a little it not longer touches.
But now i'm thinking. How do you decide where's best to put the crank as a shim behind the ball bearing would allow it to run more centrally but means we've got to decide how thick and then send it off again.

Heres a picture of the other end thats a 1.27mm gap. How critical is primary and cam chain alignment?
IMG-20211018-WA0005.jpg
 
is primary chain alignment governed by shims?
There's no need to modify the case - the crank can be positioned so it does not rub. The question is what's the right place (beyond the obvious answer of so it doesn't rub) - or maybe that is the answer and it's not at all critical.
 
The ball bearing on the right locates the crank transversally, via the groove in the bearing and half ring in the lower case. The rest of the crank floats transversally in 3 cylindrical roller bearings. The middle 2 bearing outer races are located by the stepped insert in the upper case. The left bearing outer race should sit hard against the half ring. If things aren't right at that, something is amiss. Crank pressed together incorrectly perhaps? No shims in setting up a lav bottom end, they are supremely simple.
 
The ball bearing 1/3rd of a ring is a bit loose in the case and quite a bit more loose in the bearing - all in all the bearing outer race can move 0.43mm. That's way less than the 1.27mm gap at the half ring roller bearing end. Actually it's more than 1.27 now I think after we've pushed it all tight to one side. Is it even possible to make the crank too short? Maybe the replacement bearings are different widths to the originals and that's the explanation
 
sound to me as if you NEED a crank rebuild, or if you had had one (a partial job perhaps) then it is wrong, my very good friend and learned Laverda technician, Steve, beat me by 17 minutes to say exactly what I wanted to. Basically there is no separate thrust bearing, the RHS ball type does two jobs, end float and a main bearing, the others are rollers to allow for side slip, and the heavy loads, the C rings are a vital component regarding this.

You don't happen to have two centre ball bearings do you? that was a factory cock up on a grand scale and was the subject of factory rectification, which Slater was still doing fifteen years later, (FOC if you could prove he sold the bike in the first place) I last did one about seven years ago. They do still crop up. Whats the engine number?


CLEM
 
The crank has just been rebuilt by Keith Nairn of Laverda Scozia. I'm not sure what you mean by partial job, it's my understanding the whole lot came apart as it needed oversize crankpins and new big end bearings. The centre bearings were not renewed. I'll ask about the centre bearings and get the engine number. I'm not with the bike at the moment.
 
CLEMTOG said:
the C rings are a vital component regarding this.
Click to expand...
so the 1/3rd of c rings on the ball bearing side is not tight in the grove of the case and less tight in the groove of the bearing, overall there is 0.43mm play. Relying on the c ring for anything other than insurance and location (albeit not super accurate) seems a bit sketchy. Once the whole thing's bolted down surely those bearings don't move - we were even going to use Loctite.

The full half ring on the roller bearing side is only going to prevent movement in a single direction. Again is it for insurance? location? both?

Please can you explain the purpose of the half and 1/3 C rings?
 
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Yes, we're in touch with Keith on this - I'm tapping into the hive mind too.
With comments such as 'something is amiss. Crank pressed together incorrectly perhaps?' and 'sound to me as if you NEED a crank rebuild, or if you had had one (a partial job perhaps) then it is wrong' I think it's important that I try to become as knowledgeable as I can and listen to as many opinions as possible really.
 
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Rob said:
This is ridiculous. You should be talking to Keith and be guided by him, not wasting your time casting around for opinions here. I've seen Keith at work on cranks. He's rebuilt a couple for me. What he doesn't know ain't worth knowing
Click to expand...
No it's not ridiculous at all - I'm trying to figure out what's what and what the purpose of the 'C' rings are and how best to reassemble. Whether you mean to or not you're hinting people on here haven't got useful knowledge to share and I don't think that's fair.
 
Can't imagine Keith buggering up here either, he certainly knows what he's doing.

No, not really possible to have the crank too short, unless the rod big-ends have been narrowed. Check big-end axial clearance, should be between 0.15-0.30mm, 0.35 is max. If that works out, crank length will be within spec.

Centre the crank so it doesn't rub, spread the inner roller races so they fit over the stepped insert of the upper case easily, place the LH outer race towards the locating half ring, button it up. The RH ball bearing will find its sweet spot on the other locating ring all by itself. Do not use adhesives unless absolutely necessary (visible wear in the bearing bosses). The outer races will always spin a slight amount, no worries. Certainly ain't no rocket surgery...

As Steve has already stated, the crank is located axially by the RH ball bearing, the LH ring prevents the outer roller race from falling out. The factory could have machined a step here, but line-boring and adding a groove is far quicker/easier/cheaper.

Primary chain alignment is managed through shims behind the primary sprocket and/or shims on the mainshaft locating the clutch basket. The basket should run between 2 x 1mm shims, placed on either side of the needle bearing, this can be varied to a slight degree, but not advisable. Shim primary sprocket as required. Chain alignment is critical! Cam chain will take care of itself if the cam is centred in the head.

With all due respect, if you don't know what the half-rings are for, I get the impression you are exceeding your capabilities. Re-fitting the crank to the cases is the simpler part of re-assembly.

piet
 
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