15 replies to this topic

[Bradley Gaunt]

I will need to replace one of my conrods after one came loose in my engine. Is it important to match its weight to the other 3? The set I have are standard but have been ground smooth.

[bmcecosse]

Yes - I would say so - unless you never intend to rev about 6000.

[Notay]

being an graduate auto engineer, i would say yes it is important in terms of overall engine balance. I would imagine the engine to vibrate slightly more at the higher rpms rather than the low rpms - where you would probably do most of your driving for the road. however, if the original conrods are cast, which i think they are (someone please correct if I am wrong), then you expect there to be some wide degree of tolerance - of which i do not know.
why not check the weight of all four first and report back - you might find that the 3 original ones are not equally balanced.

[carbon]

Like bmcecosse says they need to be balanced, both for overall weight and for end-to-end weight especially if you're making up a set from mixed rods. The out-of-balance vibrations get worse at high rpm.

Someone on the forum should be able to give suggested weight tolerances, but as a starting point a set should probably be matched to within 5 grams both overall and end-to-end.

PS. what type of rod bolts were you using?

[bmcecosse]

I was going to ask -how on earth did one 'come loose' ????

[Bradley Gaunt]

I was going to ask -how on earth did one 'come loose' ????

It's a new build. The half engine was put together by Martin at old MRA, it is hard to tell how it came loose but would appear that one bolt sheared first and then the second failed. I had been experiencing a strange slapping sound and was trying to diagnose it so had been careful not to take it over tick over speed. Luckily the engine was only turning at less than 1000rpm and it doesn't appear to be any other damage.

[Cooperman]

Normally con-rods don't need balancing unless you intend to regularly rev to over around 6500 rpm so long as they are all part of the same original set.
However, yours have been polished by grinding so any replacement will be a significantly different effective weight when revving, so balancing is essential.
That is a very unusual failure mode. Sounds like either incorrect torque setting when assembled or a sub-standard bolt or nut.

[mini-luke]

That is a very unusual failure mode. Sounds like either incorrect torque setting when assembled or a sub-standard bolt or nut.

As such, I'd be tempted to replace all the con rod bolts to be sure.

[Bradley Gaunt]

Normally con-rods don't need balancing unless you intend to regularly rev to over around 6500 rpm so long as they are all part of the same original set.
However, yours have been polished by grinding so any replacement will be a significantly different effective weight when revving, so balancing is essential.
That is a very unusual failure mode. Sounds like either incorrect torque setting when assembled or a sub-standard bolt or nut.

I agree but I wouldn't know how to prove it either way... bolt torqued wrong and the liability lies with the engine builder, sub standard bolt (not sure what type of bolt is used) and maybe it isn't the builders fault... The engine is fairly highly strung and will rev to 8.5k. Next question... where can I get a decent set of conrods which don't break the bank? Can decent machine shops exchange the pistons onto new rods as they are Cosworth forged and kinda want to keep them...

[Cooperman]

If you can get a set of Innocenti 1300 Cooper con-rods they are very good. Just polish and balance them and they will take the revs you want. Having them crack tested first might be advisable.
It's impossible to say whether the pistons will come off the old rods without damaging them, but it's worth a try. It sounds like a defective bolt. Use APT bolts & nuts next time.

[Tupers]

Specialist Components do a very nice set of H beam rods for a very very good price. Clicky

[mini-luke]

It's impossible to say whether the pistons will come off the old rods without damaging them, but it's worth a try. It sounds like a defective bolt. Use APT bolts & nuts next time.

I think Peter meant to type ARP - http://www.minispare...|Back to search

[Bradley Gaunt]

Gotta be honest the ones I have look just like the ARP ones but I guess they cant be as they wouldn't have failed... Im a bit concerned now that the pistons wont come off the old rods...

[mini-luke]

Specialist Components do a very nice set of H beam rods for a very very good price. Clicky

That is a very good price indeed bearing in mind it's £90 for the ARP bolts alone. Better than the inno ones, readily available and ready to go no crack testing etc required.

[Cooperman]

Yes, I did mean to type ARP. Thanks - just having a 'senior moment (like "Do I come here often?!).
I think for over 8000 rpm I would prefer a Cooper 'S' block, EN40B crank & polished balanced pistons. Not cheap these days though with a full race 'S' engine costing a small fortune. I reckon my 115 bhp 'S' rally engine with all straight-cut transmission would be well over £8000 to buy fully built these days.