Author Topic: Small end problem  (Read 454 times)

Offline bikerbob

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Small end problem
« on: 05.07. 2017 16:38 »
I have a1956 swinging arm A7 for a few weeks now I have had noise coming from the engine at certain revs about 25 mph in second gear 35 in third gear and 45 in fourth gear. So decided to take head and barrels off, on first inspection could find nothing wrong but when I removed the pistons one of the small end bushes has turned in the con rod blocking off most of the lubrication hole. This has resulted in the internal diameter of the bush being 5-6 thou bigger than the gudgeon pin. I have ordered a new bush but suspect that it will not be a tight enough fit in the con rod the original worn bush still had to be removed using a home made extracter but it came out too easily. How do I make it a bit tighter would bearing loctite be the answer or I could do what we used to do years ago when I worked in enginering and we had this problem we would tin the outside of the bearing then press it in this was the solution before loctite. Or does anyone have another remedy. Thanks.

Online Triton Thrasher

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Re: Small end problem
« Reply #1 on: 05.07. 2017 17:14 »
If the small end of an alloy rod has been softened by heat, allowing the eye to become oval, the rod is scrap.

Offline bikerboy

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Re: Small end problem
« Reply #2 on: 06.07. 2017 00:16 »
They do make oversize small end bushes I know +2 and +4 are available

Offline bikerbob

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Re: Small end problem
« Reply #3 on: 06.07. 2017 15:31 »
Thanks for that info I have checked the con rod and it is still round and no bad scoring just a few minor scratches no sign of any heat problems, have ordered a new standard bush did not realise you could get oversize ones and will see if it is a tight enough fit if not I have a small lathe and will turn a bush to suit, I am by no means an expert on lathe turning but will have a go and if it does not work out I will order an oversize bush. Looking at the bush that turned it would appear that sometime in the past a previous owner has fitted a bush that was maybe a little to small and has tried to compensate by putting a series of pop marks all over  the bush to make it a tighter fit, I have had the bike for about 3 years now so it did work for a while.

Online muskrat

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Re: Small end problem
« Reply #4 on: 06.07. 2017 21:11 »
G'day bikerbob.
Pop marks are the 1st bodge used, followed by knurling.  *bash*
Good job you found it before she went bang. Buy two +4 oversize bush's and take them and the rods to an engineer to fit. They will need to line ream them to size and fit squarely. It can be done in the bike (but they might not like to).
If I found that I'd be pulling it down to see if the PO had done any more dodges. I've found shims under big end shells etc.
I think the BSA Factory service life of the rods is 80K miles. If she's getting close to that a full rebuild with new rods might be warranted. Something to think about.
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Online Black Sheep

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Re: Small end problem
« Reply #5 on: 06.07. 2017 21:31 »
If the bush has been bodged to make it fit it has come loose before. You might be OK with an oversized bush, but I would be wary of a small end eye beginning to elongate. Given the choice, I would fit a replacement rod. 
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Online KiwiGF

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Re: Small end problem
« Reply #6 on: 06.07. 2017 22:17 »
Thanks for that info I have checked the con rod and it is still round and no bad scoring just a few minor scratches no sign of any heat problems, have ordered a new standard bush did not realise you could get oversize ones and will see if it is a tight enough fit if not I have a small lathe and will turn a bush to suit, I am by no means an expert on lathe turning but will have a go and if it does not work out I will order an oversize bush. Looking at the bush that turned it would appear that sometime in the past a previous owner has fitted a bush that was maybe a little to small and has tried to compensate by putting a series of pop marks all over  the bush to make it a tighter fit, I have had the bike for about 3 years now so it did work for a while.

Making a new bush from scratch if you are not an expert will be tough! A possible alternative would be to turn a mandrel that is (say) 001" bigger than the bush you've bought, and use it to expand the standard bush a tiny amount. This assumes you will still be able to ream the bush to size afterwards, to fit the gudgeon pin. I've done similar jobs by making a mandrel where just a short portion of the mandrel is made slightly bigger diameter (with smooth edges/ramp) and the rest of the length acts as a guide. Just a thought  *smile*
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Offline bikerboy

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Re: Small end problem
« Reply #7 on: 07.07. 2017 01:00 »
I should have added that, if you fit a new bush it will need to be reamed out afterwards. There you go better late than never :)