Author Topic: Con Rod Nuts  (Read 451 times)

Offline Zander

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Con Rod Nuts
« on: 24.06. 2017 08:12 »
Got a text from my son last night saying that he'd met an ex-BSA owner who told him that I should replace the existing con rod
nuts n bolts, as he'd had one come adrift and wreck his engine.  During the course of my rebuild, I have removed and refitted the existing fixings, which, upon inspection looked perfectly useable.  The bolts are clean and tidy - no thread wear or damage, and the nuts are still a very snug fit on the bolts.  I thought they were ok, and prior to refitting the con rods, (using a Torque Wrench )I looked in my manual  to see if there was any warning about re-using said nuts n bolts.  As there was no mention of this, I've not replaced them. Currently, the crankshaft is still on the bench so  I could fit new ones easily, so would welcome the perceived wisdom of the group!   I try to work to the premise that if it ain't broke, don't fix it, but now I think I'm in danger of becoming paranoid about this engine!!!!!!!
'59 GF

Online bsa-bill

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Re: Con Rod Nuts
« Reply #1 on: 24.06. 2017 09:29 »
These bolts are stretched when torqued up, I have no idea how many times you can stretch and then release them but ..............
Do you have any idea how many times they have been torqued before ??
All the best - Bill
1961 Flash - stock, reliable, steady, fantastic for shopping
1959 Rocket Gold Flash - blinged and tarted up  would have seizure if taken to  Tesco

Offline Sluggo

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Re: Con Rod Nuts
« Reply #2 on: 24.06. 2017 11:15 »
The only proper way to torque them is rod bolt stretch and measuring with a Micrometer which is almost a 2 person job to do it right.  The bolts are elastic and proper torque is verified by the amount they stretch.  If they dont meet spec, they should be replaced.  Whether 6 months old or 50 years old.

However,, judging by age alone and unknown history it would seem prudent to replace regardless and have a known entity.  The issue is being certain of the replacement bolts/nuts quality.  ARP is of course the best name out there but I dont know if they make them for your application.

Some vintage motors use a cotter key arrangement on the rods as well. This is always terrifying to me, as a cotter key can fret, and stress fracture if allowed to vibrate or shake.   On any nut that is a pinch lock or locking nut, they are one use only.  Throw away after each use.

Not to instill paranoia.. (Knock-itis)  But alloy connecting rods have a finite service life.  They dont live long, nor do they prosper.  If in doubt, replace as well.  I find it amusing and troubling those who advocate high rpms on 50 year old alloy connecting rods.   Triumphs factory race program stated in memos that they considered the useful service life for a racing connecting rod to be ONE 500 mile race.  Makes you think doesnt it???  I build my vintage motors as low RPM motors and discourage any use near or above 5000 rpm and ideally below 4500.  If you want to rev the snot out of them then new billet cranks and rods.

Jim Schmidt makes some nice stuff for Nortons,. Pearson and BSAF make a wonderful crank and rod set for Goldstars.  But for twins not a lot of choices.

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Offline Zander

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Re: Con Rod Nuts
« Reply #3 on: 24.06. 2017 16:43 »
I've previously mentioned that this bike had been restored, and judging by the condition
 of the pistons, i.e. Crowns bright and perfectly clean, it's not done any work.  The con rods and bolts look new, but, I've had them off twice, so, as a matter of interest, I'd like to know, if possible please, where I can find the stretch tolerance dimension. I dare say I'll end up with a new set anyway!
'59 GF

Offline Sluggo

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Re: Con Rod Nuts
« Reply #4 on: 24.06. 2017 19:02 »
If I was totally organized I would have here in my office a chart or 3 ring binder with specs and could rattle that off for you, but I am not.  In general you can find on the internet several charts for known graded bolts (IE: Grade 8 etc) and metal grades (materials) and there should be charts showing a correlation between stretch and torque.  But its totally variable based on a number of factors you have to consider.
ARP has a proprietary mix of metals they use and you should follow their specific ratings, A unknown bolt from a vendor can be any number of materials.   But the basics are,, a rod bolt is made to stretch under load. After a service life is exceeded, it wont stretch to spec.  A well made bolt will stretch multiple times but eventually if it does not, then time to toss it.  So that is WHY that is the proper method of checking.

That being said, on a tangent, some manufacturers specifically manufacture one time use only stretch bolts.  I doubt BSA ever did this or many other period Brit bike manufacturers but its something to be aware of.  Many modern auto manufacturers use these now and have for some time.  Ford seems to love using them and can be expensive to purchase replacement head bolts.  There is discussion in a number of forums about "WAISTED" Bolts and ARP makes some, and for some applications its a great asset to have.  Some of the Norton people seemingly just discovered this application and are using it for their troubled and leak prone top ends which I find amusing as its been around a long time.

While many deride HD,, they did an amazing job with their all alloy engines, and I own multiple EVO sporties and Buells and their engineering staff performed miracles in getting the old iron head design into an all alloy engine that is so durable, leak and nearly maint free with few issues.  If you study the evolution of the BSA Goldstar engine and the challenges they had, what HD did with their engines is nothing short of amazing. (Crdit where due)  But to get an all alloy engine to seal, and stay sealed with all the expansion/contraction that an all alloy engine experiences is a feat in itself.
Remember that any advice received on a free internet forum is generally worth about 1/2 of what you paid for it.
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Offline chaterlea25

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Re: Con Rod Nuts
« Reply #5 on: 24.06. 2017 19:24 »
Hi All,
Zander,
As the original length of your bolts is unknown theres no way of knowing if they have been over stretched?
If you post photos of the nut ends they may be identifiable as original or not,
Maybe they are ARP already?
ARP make bolts for the A10's and you need to follow their instructions carefully,
measure length,  torque, measure , release and measure 3 times using the supplied lube
From memory the stretch is 0.005
When ARP bolts are fitted you need to check clearance to the crankcases
SRM for one supply the ARP bolts also rods
Other rod suppliers are Thunder engineering and MAP in USA

John
1961 Super Rocket
1963 RGS (ongoing)

Offline mugwump

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Re: Con Rod Nuts
« Reply #6 on: 24.06. 2017 19:36 »
Hi Zander, I think you are being a little pessimistic in worrying about the bolts. Just how much of the engine are you intending to replace.The manual for my AMC twin just says to renew the nuts as they lose their self locking ability once removed. Use the old ones for dry assembling parts and then fit new when happy.
John

60'Matchy G12
58 AJS 18s
58 Ariel Huntmaster]

Offline Zander

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Re: Con Rod Nuts
« Reply #7 on: 24.06. 2017 20:49 »
Hi John,  I wasn't worrying about them at all until the ex BSA owner told my son about his experience.  I happily put the bottom end together as it is until I hit the bearing touching the flywheel web problem!  I certainly wasn't  expecting con rod bolt issues
and not finding anything in the manuals about not reusing them, was content to proceed with the build.  I only set out to correct the end float, not to strip and renew everything in sight,  and have spent an inordinate amount of time in the process. 


'59 GF

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Re: Con Rod Nuts
« Reply #8 on: 25.06. 2017 07:12 »
Replace the nuts certainly. One problem that has cropped up over the years is bolts of uncertain specification. Better an original high-spec bolt or a new unknown one? I was taken aback by an advert that showed how you could tell new con rod bolts for Nortons from supposedly inferior originals because the new ones had cut threads rather than rolled ones. I nay not be much of an engineer but this rang alarm bells.
As for alloy rods. Alloy quietly fatigues. The fatigue curves are a bit sobering. A reason to keep the revs down a bit on my twins. Steel rods are fine pretty much indefinitely unless you exceed Young's modulus which is why you have to check for stretch if racing. Having said that, there are Rudges being thrashed mercilessly on the race track with 80+ year old rods. 
2 twins, 2 singles, lots of sheep