Author Topic: Con Rod Bolt Torque settings  (Read 3196 times)

Offline shabashow

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Con Rod Bolt Torque settings
« on: 15.04. 2009 13:12 »
Hi folks,

Does anyone know why there are two different torque values for the conrod bolts, 8.5 for earlier models and 22 ft lbs for later ones? The bolts would appear to be the same, for the small journal, at least, through the production run, apart from the earliest A7s.
Would there be any problem if a bolt in an earler engine was torqued up to 22 instead of 8.5?
The reason I'm asking is I've fitten new -40 shells, and when fitted, the ends were a thou or two proud of the con rod and cap, so there is a gap between the cap and the con rod, when loose fitted. I was told that these will squash down when the bolts are properly torqued up. The gap wasn't taken up at a torque of 8.5, and didn't close up until a torque of about 12 ft lbs was reached.
Common sense would tell me that if the same bolt can take the 22 lb ft torque, there should be no problems with a lesser setting. There's no up and down play, and the rods rotate easily around the crank.
Any thoughts or comments before I close up the crankcases?

John

Online RichardL

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Re: Con Rod Bolt Torque settings
« Reply #1 on: 15.04. 2009 14:09 »
I think it might be normal for the bearing to sit proud of the cap landings when laid in by hand, then, take better to the inside diameter of the big end when slightly tightened. If it stays proud of the end, that sounds either like the landings have been dressed without change in big-end diameter or faulty bearings. I don't think we are supposed to depend on bearing ends shmushing together to fit properly. Having said this, I hope others (probably, most) with more experience will chime in and confirm my thinking or correct me if I have mistated.

One thing, however, proceed with extreme caution if you are thinking of honing out the inside diameter of the big ends (maybe, just "don't"). I have a miserable story of one shop's attempt to do this, which ended in them having to provide me with new rods for free (upside: they were billet beauties, but after a lot of grief).

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Offline shabashow

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Re: Con Rod Bolt Torque settings
« Reply #2 on: 15.04. 2009 15:13 »
Richard,
the ends of the rods haven't been honed. The old shells were a nice snug fit, no protrusion past the landing. Unfortunatly, they were gubbed after one of my little ends broke into a million pieces.
I took my rods and new shells to my local engineering firm (Engine Resource in Dundee) and was advised under no circumstances hone or file down the proud edges.
The caps nipped together fine in the end, but the torque is a wee bit higher than the recommended 8.5 ft lbs. Like I said in my previous post, later engines with exactly the same crank/rods/bolts torque up to 22 ft lbs.
What I need is confirmation that my engine won't throw a rod if the bolt is torqued up at 12 ft lbs, rather than 8.5 or 22 ft lbs.
John

Online RichardL

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Re: Con Rod Bolt Torque settings
« Reply #3 on: 15.04. 2009 15:20 »
Like I said, "...people with more experience...," very possibly, yourself. In case you haven't already done so, you might try doing a search from our home page with the words, "con rod torque" (without the quotes). This will show you numerous forum threads on this topic.

I agree with not honing down the proud edges because, I think, the bearings don't truly take to the inside diameter until forced to do so by tightening of bolts.

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Online BSA_54A10

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Re: Con Rod Bolt Torque settings
« Reply #4 on: 15.04. 2009 15:38 »
Did you et the rods "shut & cut" before you replaced the slippers ?
The rods can & do go oval so naturally will take more torque to get proper crush on the slippers
Bike Beesa
Trevor

Offline shabashow

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Re: Con Rod Bolt Torque settings
« Reply #5 on: 15.04. 2009 16:07 »
No Trevor, I just refitted them as is, with new shells. As said previously, the previous shells, also -0.040 fitted in flush. Could be just a different manufacturer.
John

Offline trevinoz

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Re: Con Rod Bolt Torque settings
« Reply #6 on: 15.04. 2009 22:47 »
John, the torque setting was changed when the thread form was changed. BSF to Cycle or vise versa, I would have to look at the bolts to confirm.
  Trev.

Offline shabashow

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Re: Con Rod Bolt Torque settings
« Reply #7 on: 16.04. 2009 14:50 »
They're new, modern bolts, so can it be assumed that the appropriate torque will be 22? My engineer at Engine Resource is of the opinion that 8.5 ft lbs is very low, and has suggested going to the 22 ft lb level. He's rebuilt more engines that I've had hot dinners, and would value his experience and judgement highly. He suggested to try 22 lbs and plasigague it, which I will tonight. 'll aslo measure up for ovality. If the bearings are tight, or the rod doesn't rotate and binds, there will be some engineering work needed on either my rods, the shells, or both.
Thanks for the pointer to the threads.
John

Online RichardL

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Re: Con Rod Bolt Torque settings
« Reply #8 on: 16.04. 2009 16:08 »
It's somewhat of s sticky area, with respect to ovality. One plastigage 90 degrees from the rod split will tell nothing about ovality. I haven't tried placIng one near the split, but I think the near-tangential contact with the journal would tend to smear the plastigage, either in tightening or in removal.  You could clamp the bearing into the loose rods, using the appropriate torque (I think 22 ft. lbs. is correct, and I've used it, though I'm worried about sticking my neck out with such recommendations)  and use an internal micrometer to measure in line with the rod and, say, 80 degrees off from that.

Then, the whole other question comes from those who say to replace all used rod bolts.  Should we consider our rod bolts used once they've been torqued, regardless of actual use? I think it's a fine line and those, or someone, promoting such replacement may chime in.  I have been told that such "every time" replacement may apply to race engines, but easy-runners are far less likely to fail. Of course, one can use the used bolts for trial fittings, saving the new ones for the fnal fit (probably a good idea.)

Richard L.
Plan on signing up for the world-wide 2020 DISTINGUISHED GENTLEMAN'S RIDEon September 27, 2020 (if it's not cancelled and we are free to move about by then). Watch website at https://www.gentlemansride.com for details.

Offline shabashow

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Re: Con Rod Bolt Torque settings
« Reply #9 on: 20.04. 2009 19:33 »
Worked on it Saturday and finally concluded that I needed more expert help after mashing up one of the castleated nuts. Took the crank and rod into Engine Resources this morning - got it back this afternoon, all torqued up with the self locking variety of nut. The prognosis - the rod was about 2 thou oval, so every time I went to fully torque it, the shells were squeezed too tightly onto the crank and wouldn't spin freely.
Thanks for all your comments and advice.
Off to fit it into the cases now,
John