Crank Shaft Balance V8 Rover/TVR

[markjtaylor]

Advice would be nice!!

I have a Rover/TVR 3.9 bottom end, all OK. Due to acquiring a big valve head and more interesting camshaft, valves now brush the piston tops.
Pistons are now out having pockets ground in the top to miss the valves.

As the same amount will be removed from each piston, will it be necessary to re-balance the crank, or safe to leave alone???

Any info gratefully awaited

[Tim Falce]

Rebalance the pistons and rods.

[oneofapair]

Originally Posted by markjtaylor

Advice would be nice!!

I have a Rover/TVR 3.9 bottom end, all OK. Due to acquiring a big valve head and more interesting camshaft, valves now brush the piston tops.
Pistons are now out having pockets ground in the top to miss the valves.

As the same amount will be removed from each piston, will it be necessary to re-balance the crank, or safe to leave alone???

Any info gratefully awaited

If your crank is a typical 90 deg "rover" type V8 the balance of the crank WILL change if you are altering the weight of the piston. If it now has a flat plane crank shaft it will not. The 90 deg type crank has to be balanced with a weight representing the total weight of the " big ends" of the con-rods (rotating mass) plus a percentage of the "small end"(reciprocating mass) assembly of the con rods , clamped to each crankpin. Due to the 90 degree configuration any weight change will alter balance of crank . The weight removal from the piston will have less effect than weight removal from the "big end " area due to the fact that it is a reciprocating mass and its direct effect is less than rotating mass.
So balance will change to an extent if weight loss is minimal(?) out of balance will reflect this (?) . Its your choice (?)

[graham bahr]

in theory changing the weight of the pistons will cause the crank to need to be rebalenced as v cranks need balencing with piston weight etc taken into account, BUT i doubt the few grams that the pistons will be lighter by wil make any noticeable difference

[Al Weyman]

It will cause it to be out of balance actually on a V as the total weight of the piston, rings, conrod and bearings is weighed and a bob weight of the same weight bolted to the journal when balancing bit different than doing a four or straight 6 or at least in theory it could be ever so slightly of balance but at our level for a few grams of alloy off the pistons I very much doubt you would notice.

Sorry Graham we cross posted!

[oneofapair]

Originally Posted by Al Weyman

It will cause it to be out of balance actually on a V as the total weight of the piston, rings, conrod and bearings is weighed and a bob weight of the same weight bolted to the journal when balancing bit different than doing a four or straight 6 or at least in theory it could be ever so slightly of balance but at our level for a few grams of alloy off the pistons I very much doubt you would notice.

Sorry Graham we cross posted!

That s not quite right. The bobweight that's bolted to the crankpin journal consists of the big end weight complete with the shell bearing plus a PERCENTAGE of the small end & piston weight complete with rings,circlips etc. The big end and small end weight is determined from the centre weight point of the con rod.

[Al Weyman]

You are of course correct which means a few grames off the piston would make even less difference.

[markjtaylor]

I now have the pistons returned, minus 'eyebrows', I am told that the same amount of weight has been removed from each, which would appear to be minimal.
I was hesitant to get the crank balanced as I was told by this machine shop, not to be named, that balancing the crank would cost between £150 to £300, which I felt was excessive.
Hope this does not bite my ass, clearly an engine removal and re-build will be much more costly and painfull.
Keep you posted