SOHC.co.uk Forums > CB750
Con Rod bolts
K2-K6:
I'll add a few things that I believe about the suject but please feel free to make any points if you have more relevant experience. Also if I use the wrong terms, again please add to it.
First is that if a rod lets go then there is usually one hell of a mess of most of the components near it, with that in mind you have to make your own choice as to what you do but I'd guess it's quite a sickening crunchy noise when it happens.
There are many examples on the bike of bolts being torqed and undone again with no material detriment, wheel spindles are a good one and you don't change them. This proves that it's possible given the sizing and specs being of the correct range and you simply don't routinely replace them. In fact to me all of the bike design seems to be of this caliber.
There is a basic difference between "stretch bolts" and bolts that are stretched a little when when torqued to a lower level. A "stretch bolt/stud" as I understand it is specified to be pulled past it's elasticity limit during assembly and remain there during use to give correct assembly and when dismantled would remain at that elongated length i.e. if you were to use them again you would be pulling them even longer the next time and they would probably fail in use.
A normally torqued bolt/stud (non "stretched" application) is specced to be pulled to a point under it's elasticity limit and therefore when removed should return to it's original length, you can use a micrometer to check this theory. I believe the 750 rod bolts to be of this intention.
To this you'd have to consider their history if you know it, possible engine over-revs, previous assembly exceeding the designers torque setting etc.
Another influence is the lubricant/loctite used during torqueing as if you don't conform to spec for example if you put something really slippery onto the threads during assembly then the tourque measured at the spanner can be within range but the load on the bolt material can be at a level above design intent.
Even saying all this it seems that these rods and bolts have a pretty good capacity/reliability (tough) for normal non-tuned engines and there seems to be no real reports of failures in these circumstances i.e. re-using them.
Hope that makes sense.
Voxonda:
Hello K2-K6,
+1 on you. But.......The rodbolts can not be compared to wathever bolt in the bike. The loads on the rodbolts are of another dimension then on a axle for that matter. Like AshD stated if you're not using the bike you can consider leaving them on the bike. But IMHO it still is a bad saving. All the SOHC engine builders I know, myself included, opt to replace the bolts.
Cheers, Rob
K2-K6:
I agree with your making up your own mind and would never want to hear a motor being wrecked like that.
I feel I need to clarfy my reasoning of stressing in that I was only intending to compare an example of tightening a bolt to below it's eleasticity limit and not the direction of the loadpath (the axle being essentially in shear as opposed to the rod bolts in pure tension for their duty cycle) so hopefully that makes sense.
I think people that use them as you do can bring a very qualified experience to this discussion and a very nice bike as well that you have Rob.
Thanks.
UKROBK7:
Good discussion ! I have been in touch with Mark Paris and he says:
" you can reuse them so long as they are still straight enough to not have to be driven out or into the rod holes, and have not been overtorqued. For example, in engines that are run at high RPM quite a bit (lots of 6500+ RPM use on a continuous basis, like track racing), the bolts take a 'set' and show a slight bend to them. This often requires they be tapped out of their holes to disassemble the rod cap: if they tap out (or slip out) without much force, they are not "bent" and can be reused.
Almost all of them have a tiny bit of bend from normal use, which you will notice as this: they go into the rod hole in one orientation easier than the other. When you go to install them in the rod, test it by inserting it one way, then turned 180 degrees and inserted again. You may well find that one orientation is easier to insert: use that orientation. This prevents the bolt from being stressed back in the other direction, which causes them to stretch a bit. If you're not racing, they will be fine. The occasional trip to redline we all enjoy won't hurt them, either."
Started the rebuild - long way to go yet so I have time to make my mind up!!
Rob
UKROBK7:
Update! Just spoken with a chap I knew from a Honda dealership. He says "Always replace the bolts - they are stretch bolts. Use the old bolts for plastigauging etc but then through them away".
Prices for the K7 bolts from Honda up to engine number 2733086 (mine) =
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