Author Topic: Conrod bolt torque  (Read 3941 times)

Offline unclemeat

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Conrod bolt torque
« on: 19.06. 2007 23:22 »
According to the service sheets, early 22 TPI bolts should be tightened to 10 lb.ft. Later 26 TPI bolts to 8.5 lb.ft and from 1956 onwards 22 lb.ft.

I have a '53 A10 Golden Flash with small journal crank. Its using 26 TPI bolts but 8.5 lb.ft torque just seems a little on the small side.

What do you guys (and girls !) think...and what torque have you gone to ?
BSA A10 GOLDEN FLASH 1954
CHANG JIANG COMBO 1968
TRIUMPH THRUXTON 2007

Online fido

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Re: Conrod bolt torque
« Reply #1 on: 20.06. 2007 07:32 »
I've never seen my conrods as the motor has not been apart but I think if I were doing a rebuild I would degrease the nuts and bolts and do them to the specified torque but with Loctite to stop them working loose.

Offline dpaddock

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Re: Conrod bolt torque
« Reply #2 on: 20.06. 2007 23:05 »
I used 22-23 lb ft on my DA-10 engine with 1.68-inch journals.
I doubt that journal size determines the tightness. When in doubt, go with the factory spec.
What is the engine number?
David
'57 Spitfire


Offline unclemeat

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Re: Conrod bolt torque
« Reply #3 on: 21.06. 2007 20:09 »
Engine number BA10 02976.

Had another look at the specs and still cant work out why we have these different figures. According to standard bolt torques, a 5/16" bolt should be around 9.5 lb.ft. There is variation depending on thread and, of course, these are not just standard bolts, however, if anyone can explain why there are these different figures quoted in the BSA service sheets, i'd be grateful.

Any thoughts ?

What have other people torqued to for their small journal rods ?

BSA A10 GOLDEN FLASH 1954
CHANG JIANG COMBO 1968
TRIUMPH THRUXTON 2007

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Re: Conrod bolt torque
« Reply #4 on: 22.06. 2007 08:37 »
It is logical to use a higher torque for a coarser thread as there is less friction surface area and they have a greater tendency to work loose. The higher figure on later engines probably corresponds to a change in steel grade to a higher tensile strenth type. According to Service Sheet 215 the early bolts use a castle nut and split pin. Correct fitting of this sort is time consuming as you may have to file the nut down until the holes line up at the correct torque. The higher tensile bolts could be used without split pins, thus saving on assembly time.

Offline dpaddock

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Re: Conrod bolt torque
« Reply #5 on: 23.06. 2007 01:31 »
BA10 02976 is a very strange engine number; please recheck it. What is the nominal diameter of crank journals ?
David
'57 Spitfire


Offline unclemeat

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Re: Conrod bolt torque
« Reply #6 on: 23.06. 2007 11:26 »
Currently at 1,4400" (-020) and typo....should be AB10 02976.(now this number seems starange but it is what i have).
BSA A10 GOLDEN FLASH 1954
CHANG JIANG COMBO 1968
TRIUMPH THRUXTON 2007

Offline dpaddock

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Re: Conrod bolt torque
« Reply #7 on: 23.06. 2007 22:23 »
I agree, uncle; that number is stranger still. As noted in other forums, this incorrect number is probably the result of a PO who fitted an NOS replacement crankcase (which was blank) and forgot what the original number was (or decided to make up his own).

Anyway, except for 1950, A-10 engines were manufactured with 1.68" crankpins. Only A-7 engines (and the 1950 A-10 engine) have 1.44" pins. Perhaps someone else can comment on this.

Given the weird engine stamping, I wonder if the PO blew the crank and substituted an A-7 (or 1950 A-10) crank. I've heard of stranger things . . . Check the stroke. The A-7 is 72.6 mm; the A-10 is 84 mm.

What is the history of your bike?
David
'57 Spitfire


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Re: Conrod bolt torque
« Reply #8 on: 28.06. 2007 23:30 »
The big end journal diameter on the A10 Golden Flash was 1.460 until 1957. From 1958 the Flash used the same crankshaft as the Rocket. According to the parts book, the Rocket used the small journal crank in 1954 but at some time it was changed to the one piece heavy large journal crank, completely different to the later large journal two piece crank. There is little information available about the one piece crank, but they are much heavier than the later type.
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Offline dpaddock

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Re: Conrod bolt torque
« Reply #9 on: 29.06. 2007 00:45 »
Trev:
My data was taken from BSA Service Sheet No. 207. You apparently have a later (and better) information source. Thanks.
D/
David
'57 Spitfire


beezabob1956

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Re: Conrod bolt torque
« Reply #10 on: 02.09. 2007 20:21 »
i think the lower torque figures you quote are definately correct as per bsa service sheets,please note however that these original conrod bolts were manufactured with a hole near the end and were meant to be used in conjuntion with castellated nuts, with very thin washers underneath,and split pins.the later bolts used a locking nut of the staytight or aerotight variety.

Offline a101960

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Re: Conrod bolt torque
« Reply #11 on: 03.09. 2007 20:49 »
Maybe this teaching you to suck eggs, but I will say it anyway. Never ever under any circumstances re-use the original bolts. Sooner or later they will fail with disastrous results, and never ever use machine cut threaded bolts. The bolts must have rolled threads, machine cut threads introduce potential stress failure areas. This is absolutely crucial, especially if you are fond of winding the motor on a bit! Under no circumstances torque the bolts beyond the recommended figure. There are two reasons for this. The first is that the bolts are pre-stressed and over tightening will alter the stress factor, and the second reason is that you will crush the end cap. The cap will "give" in response to over tightening. If you are aware of all of this I do apologise, but I thought that it was worth mentioning.