Small end bush interference

[Danny.Lee]

Hi guys

there was some discussion in December 2013 on this board on the subject of the 'correct' interference for a new small end bush. Now call me dense, but I dont think I got to the bottom of that one, hence this post.

My VB is getting a new everything and this week I have to fit the repaired crank back into the cases and also a new small end bush.

Drags supplied my bush which I measured and found to be about 10thou more than the little end eye. The bush I pressed out was smaller than this, but still a tight fit in the eye.

I have the facility to alter the bush size if required, so my questions are:

do I need to?

what is the maximum recommended cold interference before I press it in?

Thanks again for any help, its always appreciated.

Dan

[nevhunter]

Depending on how rough the rod eye is, you may find it shaves a fair bit off as you install it. 10 thou is more than you need. The bush is fairly thin walled and will crush in a fair bit, but I reckon .006 is enough. Some of today's bronzes are not very hard especially the extruded ones. If they are too soft they will lose their interference and crush in on the pin and be too tight, after a bit of service. (Heat cycles). Nev

[John.Cheetham]

Hello Danny,

With regard to my previous post in December 2013, I settled on an interference fit of 2-1/2 to 3 thou and felt I achieved a good result. The bushes for the long stroke BSA A7 (incidentally also from Draganfly) appeared to be of a reasonable grade of phosphor bronze.

It is important to mount the bushes in such a way in the lathe that the full length van be accessed in one pass as turning them round to finish the other end will result in the O/D not being a true cylinder. This will allow only partial surface contact within the con rod eye with more chance of the bush moving.

I pressed my bushes in using a bench vice fitted with aluminium jaws and the fit felt OK to me.

Hope this helps.

Regards: John

[Danny.Lee]

Thanks Nev, John, much appreciated

I still have the original bush I pressed out, (on reflection I might have been better leaving it in). This seemed to suggest an interference of around 3 thou and was a tight fit.

I wondered though if there was an element of 'spring' in phos bronze, especially as it is of such thin wall thickness
and if you cut down the new bush to a nominal 3 thou, is there a risk it would end up less than this because the metal is crushed or 'sprung' by the act of pressing it in?

So did my original bush start out at plus 3 thou or was it bigger to start with?

On the lathe, my intention was to turn up a pin to a close internal fit, full length, then 'glue' the bush on, size it, then put the lot in the oven to
release the glue.

Any suggestions on suitable 'glue' ? bearing fit Loctite??

Dan

[John.Cheetham]

I don't think the issue of "springing" is really a problem and your measurement of the old bush at 3 thou' oversize agrees with what I said in my previous post regarding the size I reduced mine to.

You could make a longer mounting spigot / mandrel to mount in the lathe with a threaded end using the old bush as a spacer then a nut gently tightened to hold everything in place for machining. This would save the bother of gluing the bush in place and the subsequent need to break the bond.

If you go down the Loctite route I think the bond will break down around 220 degrees Celsius.

Regards: John

[Bob.Murphy]

On the lathe, my intention was to turn up a pin to a close internal fit, full length, then 'glue' the bush on, size it, then put the lot in the oven to release the glue.

I haven't checked, but I suspect that phosphor bronze has a higher coefficient of expansion than mild steel.

If you turn the bar a thou' oversize you could warm the bush and shrink it onto the bar for machining, then warm it to release it. It shouldn't require a huge amount of grip during turning as its not like turning '316 stainless' .

Or would that process result in the bush O/D being undersize when it was released from the bar, even with a very light interference fit ??

Bob.

[Danny.Lee]

Hi Bob
I think I will try it your way. It seems from what Nev and John say that a fit in the range 3 to 6 thou should work so if I go for around 4 thou it wont matter if the machining interference does affect the final size.
But I will report back
Regards
Danny

[Bob.Murphy]

I'll be interested in the result.

I spend hours on my lathe, and I'm always learning .

Bob.

[Danny.Lee]

Hi All, update

after much re-measuring I came to the conclusion that the little end eye was oval and the new bush was also oval but not as much. All the permutations came out as
maximum interference 7 thou min less than 3 thou so I decided to press it in as it was, as I didnt want to take anything off the minimum value.

Nev was right, the conrod actually shaved some of the bush on pressing in. I immediately went paranoid thinking that it had shaved to dead size so no interference at all, but then I tried the new
gudgeon pin which had been a fairly loose fit in the new bush. It would go nowhere near it. I tried a 13/16 reamer but I think that was worn because the pin still wont go in.
So far as I can tell, even after an undersize ream the hole is still about 3 thou smaller than it was before pressing in.

So I figure it is probably tight enough. Didnt move when I drilled the oil slot and the reamer took some serious effort to turn and it didnt move so I am going to settle for that

The new bush was about 2.5mm wall thickness

Thanks for all your interest, your comments have been very helpfull.

I hope this info helps someone else

Regards

Danny

[John.Cheetham]

Sounds as though you need to get hold of an adjustable reamer to finally finish the bush. Tracy Tools may be able to supply one unless you are able to borrow one local to yourself. The bore is a bit small and hence fiddly to try and hand scrape it to size.

Regards: John