Author Topic: Main bearing (bush)  (Read 938 times)

Online Rex

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Main bearing (bush)
« on: 03.10. 2017 21:03 »
1951 A7 Star Twin in many bits.....the bush and crank end appear visually fine, but is there a quick-and-dirty test to ascertain if that's actually true? Thanks!

beezermacc

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Re: Main bearing (bush)
« Reply #1 on: 03.10. 2017 22:28 »
If the journal is worn there is sometimes a visible lip on the outer edge of the journal which you might be able to feel with your finger nail. To be sure you need to measure for ovality. Decent calipers might do but I'd only be happy with a micrometer.

Online Rex

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Re: Main bearing (bush)
« Reply #2 on: 04.10. 2017 09:08 »
I have mikes and Vernier calipers and that's clearly going to be the next step, but I was just pondering if there was any sort of perceived wisdom, like, any movement between crank and bush means the bush is shot....sort of advice?

Online Greybeard

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Re: Main bearing (bush)
« Reply #3 on: 04.10. 2017 10:29 »
I have to admit that when rebuilding my A10 as I don't have a 2" Micrometer I just looked for any up/down movement of the shaft in the bush. I've recently repeated that check, (using a tyre lever on the crank end) and don't see any movement. I did put a new bearing in the other side. I fitted a new supposedly high output oil pump, (not from SRM), run on straight SAE40 oil, have an oil filter, change the oil frequently and do not thrash the engine so have convinced myself that the bottom end will be OK, (for a while). I've been reassured by chaps saying that these bikes should not need as much mollycoddling as we sometimes think.

Postscript: My bike, (that has it's original engine) was off the road for over 40 of it's 62 years so really has a much lower mileage than most other A's around today. The pistons are 30 thou oversize and the big end shells are 20 thou under size. That's how it came to me. UPDATE: I replaced the piston rings and big end shells.


LATER: I guess the main bearing bush may have been changed at the same time as the other work.
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Online Black Sheep

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Re: Main bearing (bush)
« Reply #4 on: 04.10. 2017 14:32 »
If it looks fine it probably is fine. For some reason people seem to be panicked into timing side needle roller conversions. For the life of me I can't see why. A plain bush has a much higher load capacity than a retro-fitted needle roller. BSA actually knew what they were doing, that's why there are still so many around for us to enjoy today.
2 twins, 2 singles, lots of sheep

Online JulianS

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Re: Main bearing (bush)
« Reply #5 on: 04.10. 2017 14:42 »
I went needle roller over 30 years ago because I was fed up with poor quality pattern bushes which lasted 12 - 18 months, and fed up with setting end float on the crank.

Has proved perfectly reliable and would not want to go back to the bush setup.

Online KiwiGF

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Re: Main bearing (bush)
« Reply #6 on: 04.10. 2017 19:53 »
1951 A7 Star Twin in many bits.....the bush and crank end appear visually fine, but is there a quick-and-dirty test to ascertain if that's actually true? Thanks!

Maybe try getting a feeler gauge in the gap? I'm not sure of the wear limit but if you could get a 002 feeler gauge in the gap, that would indicate a need for further investigation.

Whilst there, it would be a good idea to check the end float. That really needs to be done with a dial gauge tho.
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Online Rex

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Re: Main bearing (bush)
« Reply #7 on: 04.10. 2017 21:15 »
Well, it's miles away from being assembled enough to check the endfloat, and there's not a hope in Hell of any feeler gauge going into that main bearing gap! I just wanted some reassurance, really.
For many years and resto's I would've replaced it as a matter of course, but, as someone further up said, modern pattern parts are so often shite that it's possibly better to stick with what you have (if possible) than replace with sub-standard parts which won't last.
It'll be getting a new drive side race and new B/E shells, and hopefully those plus a hone and re-ring plus valve grind will make a sweet-running engine. I'm impressed...it's a nicely designed unit.

Offline coater87

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Re: Main bearing (bush)
« Reply #8 on: 05.10. 2017 04:01 »
Rex,

 Mine was already in bits when I started.

 I measured my crank.

 Then I used a snap gage to take 10 or 15 measures on the Bush. Mine was easy enough because things were not round, plus I was 4-5 thou. over size.

 But the thing was the snap gauges let me take good inside measures. Even a dirt cheap set will work, you always do the same measurement a few times with those anyways.

Lee
Central Wisconsin in the U.S.

Offline duTch

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Re: Main bearing (bush)
« Reply #9 on: 05.10. 2017 04:18 »

 Coater, excuse my ignorance,  buyout what's a 'snap' gauge (gage) ?

 When I did mine, I think my digital calipers are just big enough to  measure  the crank,  and to do the inside I used some 3/8" threaded rod with big washers at each end (each held captive between nuts), as a crank substitute and measured the outside of the washers- All a bit bodgy, but couldn't think of anything better (Other than some fancy $measuring$ weapon *conf*)
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Online orabanda

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Re: Main bearing (bush)
« Reply #10 on: 05.10. 2017 06:23 »
Lee,
The out of round you have measured is common. I have found the outer bore will distort between 0.010"- 0.015" subject to the pounding (high rpm,etc) the engine has taken.
To prevent the loss of the crankshaft's life blood (oil) between the casing and outside of the bush, it is a good idea to have the bore machined true, then make a bush with interference (0.002") to suit the new boring. Then you can be confident of delivering all of the pump's oil flow to the crankshaft, and maintaining oil pressure.
Sometimes I have been able to detect air coming out from between the outside of the bush and an out of round bore, after removing the outer timing cover and the oil pump, and blowing air into the port in the crankcase for oil supply to the bush (& crankshaft).

Richard



Online RichardL

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Re: Main bearing (bush)
« Reply #11 on: 05.10. 2017 14:31 »

 Coater, excuse my ignorance,  buyout what's a 'snap' gauge (gage) ?


Dutch,

I take Coater to be referring to telescoping, spring-loaded, inside diameter transfer gauges, where you measure the locked-in telescoped length with your outside micrometer. I believe the term "snap gauges" has been co-opted (not specifically by Coater) for this type because, it appears, actual "snap gauges" are outside diameter go/no-go calipers.

As for the telescoping gauges, I got so frustrated trying to get repeatable trustworthy measurements with them that I bought an inexpensive dial bore gauge  on eBay. Much easier. Of course, I'm not ruling out that Coater may have mastered the technique where I have not.

Here's a picture: https://goo.gl/images/X36ARL

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Offline coater87

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Re: Main bearing (bush)
« Reply #12 on: 05.10. 2017 15:34 »
 Yes,

 Those are them. Snap gauges, they make a "snap" sound when you use them correctly.

 I have no idea why you cannot get accurate readings with these Richard, they are a very simple tool to use. Unless, you are trying to use both hands to do it, which will mess it up every time.

 Just make sure to use he same Micrometer on the crankshaft and the snap gauge.

 Lee
Central Wisconsin in the U.S.

Offline jachenbach

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Re: Main bearing (bush)
« Reply #13 on: 05.10. 2017 15:53 »
Richard, if you have a method that works, go for it. But the method I was taught and use for the internal telescoping gauges is to lay it and the micrometer on your flat palm and work the micrometer dial with the other. If you try to hold both with fingers, it's about impossible to get a good measurement.

Offline Sluggo

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Re: Main bearing (bush)
« Reply #14 on: 05.10. 2017 19:45 »
The out of round you have measured is common. I have found the outer bore will distort between 0.010"- 0.015" subject to the pounding (high rpm,etc) the engine has taken.

Richard
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Thats a great point to show there, thank you for doing that. (Well illustrated).  ALL of these vintage bike alloy case engines distort to some degree over time. I find it amazing that few people are willing to acknowledge that after 60-70 years that these things distort.

I have not extensively surveyed early BSAs but I can attest that multiple Unit twins certainly do distort and a tool & die maker friend used to blue print cases for people and found a lot of BSA preunit singles were badly out of wack or even manufactured out of tolerance.  I have a file somewhere where I documented much of his procedure and need to re-locate it but the last one he did before he died was the Main bushing bore WAS distorted like you show but ALSO off center to the other bearing bore, and the cam bushes were also out of line but in a different direction.  When you bolted up the cases it locked everything up.  Took quite a bit of work to correct.

Not all doom and gloom however, its fixable but every engine should be checked.

As to using a Mic, calipers, snap gauges or anything I am sure there is probably tutorials online these days, perhaps some Youtube videos?  But the best thing is find a skilled and seasoned machinist and bribe them to give you a tutorial.  Then have them measure stuff and you do the same until you get repeatable and accurate measurements.   People who have not done this in a while should also do a refresher as there is a goldilocks touch to it.
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