The BSA A7-A10 Forum
Technical (Descriptive Topic Titles - Stay on Topic) => A7 & A10 Engine => Topic started by: Topdad on 26.01. 2016 10:54
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This is where I show how limited my engineering knowledge is.
To be fair I can spanner with the best and am meticulous in prep and rebuilding and can say that nothing I've ever put together has failed be it jag engines bikes etc.
However certain terms and requirements have been stored away in my memory under the " ask that at a later date when I've more time " well I'm at that date !
Firstly I know that our A10 cranks when ground for new shells need a "radius inc " on the journal and I 've heard all sorts of problems happening when engineers not familiar with the type have left this out of the job. So is that the case ie if not incorporated do you prepare for a wrecked engine ? next why would that be the case and finally just what does the engineer do to finish the journal properly ?
I feel lots of other questions coming but for now over to you guys to explain, thanks in expectation *respect*
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Not quite sure what you are saying.
However here we go.
if the corner is ground square then you have a stress raising point which can cause the journal to shear off.
However now days there is a wonderful heat treatment process called nitriding.
Costs about $ 20 and is just about mandaory now days.
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Of course, what Trevor said about the radius is accurate, but I'm going to try to embellish it, a bit. If you were to assume that the corner between the journal and the web was absolutely square, then there is an infinitely (theoretically speaking) small area over which bending forces can be distributed. With a radius, small though it may be, you add an infinite number of points (again, theoretically speaking) over which those same forces can be distributed. You've, of course, seen or used the technique of drilling a small hole at the end of a crack in plexiglass to stop the crack from growing. This is similar.
Real mechanical engineers here will correct me if I'm wrong, I'm sure.
Richard L.
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Does the nitriding process actually overcome the problem of lost radii then Trevor?
The only crank I have ever broken (not on a BSA) failed due to the crankpin radii being removed/reduced to near-zilch in the (careless) regrinding process. Do you think nitriding would make a crank that is good in all other respects serviceable?
Not something to suck and see without being pretty darn confident, owing to the collateral damage that comes with failure.
I can't recall being told the cranks I've had reground in recent years had been nitrided - but maybe it's standard practice and taken for granted? (But in that case I wouldn't have expected to have had some items classified as 'doorstops' & not fit for reuse - due to lack of the radii!)
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Ok so at least I hadn't dreamed the practice up. How does an engineer achieve this when doing the job ,is it just a small area that is left to either side of the journal ?
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In my limited experience, a machine shop won't want to grind a journal whose radius has already 'gone'. There are pix (in the cheapo Haynes books even!) for Norton twins; data for AMC twins' radii is also out there and widely regarded as ESSENTIAL to follow.
The grinding wheel used has to be narrower than the full width of the journal - but maybe they shape the wheel's edges to create the correct blended profile 'all in one'. Dunno.
If I were in your shoes Topdad, I'd be careful, is all I can say. If the radii are still there, shouldn't be a problem: they want preserving and can and should be preserved. It's when they have been worn away or ground off by previous regrinders that I'd worry because what's gone is gone and can't be reclaimed.
But I don't know, hence my Q, whether nitriding can get round the problem by restoring strength/integrity in different ways.
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G'day Topdad.
All of the above. The radii is to be 0.090" and the grinding wheel is shaped to this before grinding. I've had two A7 cranks snap due to incorrect radii and finish (oh, and 8000rpm at 14:1). Finish (smoothness) of the radii is just as important.
Cheers
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Right , thanks everyone that's cleared a little space in the brain and I feel I understand ...reasonably ! Be prepared for next question when I think of it !!
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Are we all worrying needlessly about this?
This ol' donk is a fairly understressed, agricultural unit even in RGS tune, so whilst I'm sure the radius needs to be in the ballpark, unless we're doing a racing engine, WGAF?
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Hi jools ,not been stressing about this just that if I don't understand the reason something is required for a job it niggles in my brain .I'm touching wood but I don't need a crank grinding although I do have a large journal crank which I'll get reground when I can afford to and it is better to be able to at least comment on the spec they'll use. As to weather it is absolutely needed, whilst taking on board your thoughts, I think I'll stick with the majority who are all more experienced in this than I am, cheers bob
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And when it is out get the nitriding done.
Can be done before or after machining as it wont affect the surface .
Makes the crank a lot tougher
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I'm sure you're dead right to go about it with great care Topdad. Lucky for me I haven't had to do my journals yet, but if I had to I'd probably tread very carefully like you.
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I had a camshaft ground and nitrided by Tighe cams once.
It came back bent.
when I took it back, the bloke reckoned the heat in the process did that and smacked it with a hammer.
I wasn't too impressed.
another lesson learned.
cheers
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when I took it back, the bloke reckoned the heat in the process did that and smacked it with a hammer.
I wasn't too impressed.
another lesson learned.
cheers
So they straightened it the same way you build a Harley crank. But those are 5 pieces, and the only way to coax them into alignment is with a big cold chisel, a large "C" clamp, and the use of a 15 pound lead wacker.
I dont think its a good idea for a 1 piece BSA crank. *sad2*
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I had a camshaft ground and nitrided by Tighe cams once.
It came back bent.
*eek* they did mine I think when they were on Montague Rd Sth Briz... I might have to check it ; a bit hard when it's in use