Right-Side Main Bush Inside Diameter and Clearance

[RichardL]

After work by my "machinist," my main bush is 0.003" bigger than the main journal, rather than the 0.001-0.002" (max) described in the books. "Machinist" says the correct value is 0.0015-0.003". Does anyone care to offer an opinion as to which or whom is correct? "Machinist" is sending a new bush for installation, by others, if necessary. But if 0.003" is OK, I would love to move on, just not at the risk of loss of oil pressure.

Richard

[terryk]

I spoke to a fella here some time ago about this. He does a lot of work on these, line boring etc and he said you really need the closest clearance. He said 1 and a 1/2 thou is correct so twice that is probably a bit much I would guess. Especially if you are reconditioning  the whole engine you should do it so its got room to wear not be on the limit. He said it may seem tight but thats the oil supply to the bigends and if they are not a good fed then it doesnt get splashed up to the cylinders, cams etc and blows out the bush back to the sump. Anyway its worth replacing I think, but maybe someone else has experienced it as being ok. Terry

[a10 gf]

Agree with terryk, go for the min. possible clearance. Else it will have to be redone way too soon. & get the new bush reamed absolutely paralell to the crank, using only the correct tool.

e

[groily]

Nothing really to add Richard, having just seen this query. We never want to start at the outer limit, so 3 thou is too much for comfort (quite a lot of wiggle, that), and, ?, is the bush even parallel after the ministrations of your 'machinist'? Has to be worth getting it just right while you're doing everything else. There's a thread here somewhere on which bushes are better - solid bronze or lined steel - might be worth having a look to recap (I don't know the relative merits). Reaming has to be done 'in line', I assume therefore with the crankcase halves assembled and the drive side main bearing in place, so its inner race can be used to support a mandrel and keep the reamer absolutely square. No magic in this, just need the right man with the right weapon, and it'll be perfick. Trying to get the finished dimension in the bush and parallel without having the 'other half' to hold the tool is pretty much impossible I'd have thought given the size of the hole and its relative lack of depth. I thought it but didn't act on  it when I was young and penniless and messed about with bushes as if they were cheese - and always lived to regret it when trouble ensued. Having got so far . . . . be a shame to waste all the effort! Groily

[RichardL]

One thing troubling me is the sequence of machining to get the correct tolerance. I was of the understanding that the way it's done is ream/hone the bearing until round, then grind the crank to the desired outside diameter. Now, my crank is already ground, unless it is reground, it is the bush diameter that must be made to match, +0.001" (or so). Seems to me the crank grinding folks are better prepared to hold tolerance than one gets from the line reaming/honing operation.

Richard

[groily]

Not sure Richard. But it should be poss to get a bush reamed to fit a given dia journal almost perfectly and certainly to the thou or so required for optimal fit. You don't want to be grinding away at the crank too much - there are limits, I think, to the undersize journals you can get . . . and making one from scratch to fit an underground crank will be a labour of more than love! Took me ages to make a new sleeve gear for a turned-undersize gearbox mainshaft, and that is complete child's play by comparison. If your journal is round, personally I'd get the bush made to fit it, not the other way round. Subject, natch, to other better-informed opinion . . .

[G/F DAVE]

Hi, Richard my engineer grinds the crank timing side journal first then line reams bush to fit, you never grind crank to fit bush as bush can become oval once fitted to cases. If you can find a lead bronze bush as per original BSA LB1 material these are the best as they wear the bush not the crank as some of the phosphur bronze solid type bushes do (pb1 material). Aphla bearings in the uk used to make a LB1 bush these are steel cage LB1 lined if you can find one of these in the size you need. I have one fitted to my goldflash smooth motor no vibes and as my crankshaft is on its first grind -010 I,d like to keep it that way .hope this is of help to you . all the best G/F DAVE

[RichardL]

Yes, my new machinist (so far, no quotes around that word) tells me he grinds first and reams after (no joke intended). I will look for some kind of markings on the bushes I now have to see what type they are. If there are no markings, how could I tell the type?

Richard

[G/F DAVE]

Hi, Richard here is a photo of a alpha bush .These are a steel cased bush as you can see also have a small brass retaining screw for inner part. There is a guy in the uk who is making LB bushes for bsa A50/65 models,  but not A7/a10 at the moment unfortuanately. I have seen a few of these old stock bushes on the ebay site it,s where I bought my last one.They should be scribed with size of bush on outer case ie; steel part. There are cheap types of these bushes  that were made but you can see the quality of these so hard to make a mistake, besides I think the cheaper type have a steel retaining screw which does not show thru to inner bush. Regards Dave..

[RichardL]

Dave,

Thanks for the photo. It is very informative, especially showing the color and appearance of the bearing material, which looks just like the 0.003"-over bearing now fitted. It is going to be interesting to see what arrives from "he who cannot be named."

Richard