The BSA A7-A10 Forum

Technical => A7 & A10 Engine => Topic started by: olev on 15.02. 2009 01:57

Title: conrod bolts
Post by: olev on 15.02. 2009 01:57
The star twin's small journal crank and rods should be back from the grinders some time soon (I hope)
I have all the information and parts required to rebuild the bottom end except for conrod bolts.
The general concensus is to buy new bolts and nuts. The internet forum searches I've done has only put the wind up me. I haven't found a recommended bolt and nut set anywhere. Even SRM gets bagged and nobody gets a tick. which is a worry considering the cost of the things.
They talk about rolled and machined threads and different steels. I know..Iknow I read too much and should spend more time in the pub.
The motor will not be travelling huge kilometers, and there is a temptation to just buy anybodys or use the original bolts with new nuts. however I'd prefer to do things right and do them once.
Has anyone had trouble with conrod bolts stretching??
Is there a quality set available out there??
Is the whole saga a load of manure??
Will Elizabeth eventually find true love??
Title: Re: conrod bolts
Post by: Richard on 15.02. 2009 19:49
 Definately fit new bolts and try to find bolts that have had a rolled thread as these are less prone to fracture problems
At least that is what the experts have told me.
I secured mine from Kidderminster Motorcyles (UK) who are knowledgable and extremely helpful
Title: Re: conrod bolts
Post by: beezalex on 16.02. 2009 16:39

I disagree about the new nuts and bolts.  Under normal running conditions, there is no reason the originals should wear out.  I know they're old, but a)they never see enough heat to change material properties and b)they are under CONSTANT tension unless they've seen a catastrophic bearing failure.  I DEFINITELY would not put a generic bolt in there, even if it's supposed to be high-tensile (grade 8).  As long as the threads are OK on the stock items, I re-use them on street bikes (and cars, for that matter) and I have not had a problem.
Title: Re: conrod bolts
Post by: Richard on 16.02. 2009 18:58
I can not agree with your view on this as it is not a case of them wearing out
1) the nuts are the locknut type and once undone one could argue that they may not lock securley

2) the bolts are torqued up so therefore it is reasonable to say they are already stretched so when reused they are stretched some more as they are retorqued up.

Of course if reused they may never fail but it only takes one to fail and ruin the motor.
Most  dealers that know anything about engineering should only stock high quality bolts
Title: Re: conrod bolts
Post by: A10Boy on 16.02. 2009 19:42
When I re-build [comparatively] high revving engines I always use new big end bolts, I would never use "generic" bolts. I would have thought that items supplied by SRM would be top quality and am surprised to hear an opinion to the contrary.

Where did you hear that ?

Title: Re: conrod bolts
Post by: beezalex on 16.02. 2009 20:53
Richard, with all due respect, I realize your two points represent the conventional wisdom, but that conventional wisdom conforms neither to engineering principles or what I have heard from others.  I have not personally heard from one person who re-used good stock conrod bolts in a street application and had them fail.  I've personally heard from two people who have had failures with aftermarket replacements.  I realize it's not statistically significant, but it is the data I have.

1)We're still talking about the bolts, right?  I said nothing of nuts, but in this case, the original nuts are not locking, so it doesn't matter and there is no problem re-using them.
2)The bolts are preloaded and torqued to be elastically deformed ONLY.  That is what they are designed to do and to stay under constant tension regardless of external load.  Therefore, unless there is a catastrophic failure of some sort, there is not reason the bolt should plastically (irreversibly) deform.  I suppose that there's a remote possibility that the bolts may have been overtorqued by some ham-fisted mechanic without damaging the threads, and I guess that is a concious risk I'd be willing to take over an un-tested bolt of dubious origin.

BTW, EVERY mass-produced bolt has its threads rolled, but I would not put a grade 5 bolt from the hardware store on a con rod.
Title: Re: conrod bolts
Post by: Richard on 16.02. 2009 22:16

 Firstly my second post was meant for beezalex not Olev

I can see your point of view regarding using old bolts, BUT one may not know how many times these bolts have been used for instance if you obtained a bike and stripped it down and the shells were 20 thou oversize it may mean that the crank could have already have had two regrinds (unless you knew the complete history of the engine) and the bolts could also, if the engine was being rebuilt by some one with your view, have been refitted twice more since new also they could well have been over tightened.

I have the view that having bought new bolts and nuts with rolled threads from a decent supplier with a reputation to uphold I would be a lot happier than if I was trusting old fatigued bolts and nuts.

We both have different opinions and may not have helped Olev with his original question, and for that I must apologise but hey this is life and it would be a boring world if we all agreed 100% of the time!!
Sorry Olev I hope someone else wades in and it makes it a bit easier to decide
Title: Re: conrod bolts
Post by: olev on 10.03. 2009 12:14
I've received a set of conrod bolts and nuts for the A7's small journal crank.
The suppliers (MCA) use part number 67-1535 which doesn't match anything in my books.
Maybe its for a small journal crank after 1952 where my parts book stops. I don't really care but would like to use the correct torque setting for the particular bolt. They look good quality which is comforting as they cost an arm and a leg. The new ones have a finer thread and locknut rather than the castellated nuts and pin.
The service sheet reckons 10 ft/lbs for the early ones, 8.5 ft/lbs for the later type and 22 ft/lbs from 1956 on. MCA are a bit slack answering emails so my guess is 22 ft/lbs.
Anyone know where the 67-1535's come from and/or their torque settings??
Title: Re: conrod bolts
Post by: bsa-bill on 10.03. 2009 13:08
 Draganfly list this as

67-1535 - Big-end bolt, small journal - A7/SS -1960-62
Suprceded to - 10419-54Anut

The year seems a bit funny I thought small journals were an early thing - perhaps not or maybe just a typo

22 lbs is correctI think

All the best - Bill
Title: Removing con rod bolts
Post by: BSA500 on 26.05. 2016 21:34
A hopefully easy one. How do you remove the con rod bolts from the con rod without any damage occuring. They appear to be a tad tight-heat and a light tap ????
Title: Re: conrod bolts
Post by: chaterlea25 on 26.05. 2016 23:30
No heat,
They should move when tapped with a  mallet
Make sure to align the eccentric  bolt head when fitting the new bolts
Search the forum for ARP rod bolt tightening, You should get some of their thread lube with the bolts

Title: Re: conrod bolts
Post by: BSA500 on 27.05. 2016 08:31
Thanks I just like to be careful when waving a mallet/hammer near a con rod  *smile*
Title: Re: conrod bolts
Post by: bikerbob on 27.05. 2016 14:11
I would never knowingly buy anything from MCA I have had push rods that were too short valve adjusters that were very brittle and magneto brushes that were too soft this was admittedly about 20 years ago but once bitten or three times bitten in my case maybe they have improved their quality but I will avoid if possible. I am not surprised that you have communication problems with them I had the same problem 20 years ago before emails. In the case of the push rods they denied there was anything wrong even though I provided them with a BSA original rod they just told me that if not satisfied with their products to go elsewhere.
Title: Re: conrod bolts
Post by: edboy on 27.05. 2016 19:36
 i have found that the conrod bolt holes burr easily and the conrod bolt becomes too tight. as already posted the bolts should be easy to move when tapped so i ream out the conrod. 22ft.1b with no friction thanks. i reuse the conrod bolts if they look in good condition and not bent.i also clean up the oval part where the conrod bolt sits- no burrs or swarf wanted there either. nuts are cheap and best to renew.
Title: Re: conrod bolts
Post by: bsa-bill on 27.05. 2016 19:57
I dare to question wiser heads but I understood the critical thing with bolts was how much they had stretched during  torquing.
If the bolts stretch and stay stretched when removed are they still allright to torque and stretch further or do they once removed revert to their original length (before torqued )
Title: Re: conrod bolts
Post by: chaterlea25 on 27.05. 2016 21:55
Hi Bill,
The bolts should stretch when tightened and return to original length when undone
Theres a sequence to tightening the ARP bolts, measure, tighten measure, release measure and so on
THe ARP bolts have a different torque setting to the BSA bolts, different thread pitch is one reason
From memory 28ftlbs when used in BSA rods?????
The stretch achieved is more important than the actual torque figure


Title: Re: conrod bolts
Post by: edboy on 28.05. 2016 21:33
the point i was trying to make was that conrod bolts should be free to move in the conrod and not bind. if not carefull the rod will settle and you could end up with a loose bolt after milage. whatever torque you choose the rod must be free and in my opinion care over the rods is the most important part of the rebuild
Title: Re: conrod bolts
Post by: bsa-bill on 28.05. 2016 21:44
The bolts should stretch when tightened and return to original length when undone

Cheers John - that's the bit I wondered about, I used bolts from a certain BSA expert (who may be here) they were made to his spec and torque was a bit higher than 22 and taken up to torque/stretch  three times, I wondered if after constant use they perhaps settled at full stretch
Title: Re: conrod bolts
Post by: Triton Thrasher on 28.05. 2016 23:27
Renewing the bolts was good practice when you could get original bolts.

Replacing originals with inferior bolts is bad practice.
Title: Re: conrod bolts
Post by: Tomcat on 29.05. 2016 07:28
Lots of relevant points to consider Olev. Here's my 20c worth, I'm an ex mechanic and have done a lot of 'Rings and Bearings' on cars and trucks. On all of these I have re-used the con rod bolts and nuts, with only one conrod ventilating the block, the cause of this was extreme revs not the re used bolts! None of the engines I rebuilt were performance engines so there was little need to go overboard like say, a drag race engine.
 As previously mentioned your bolts may have been re used many times over the last 50 years, so in this case new bolts and nuts are a good idea. When I rebuilt my YA7 engine 10 years ago I bought NOS bolts & nuts as I don't wish to see the inside of the engine again. Cheers Tomcat