The V8 Owners Forum -

Hi all I'm wanting my pistons pocketing to fit the crower 50232 cam but have trouble finding anyone these days thats got machine to do it so my question is dose anyone know anywhere Yorkshire midlands that can pocket pistons???....will I need crank rebalancing too???

Thanks

Chessman engineering at Coventry first class work they have a website, Cheers Mark.

Thanks mark ill have a look at the website and give them a bell

"" Will I need crank rebalancing too ""

The crank balance will not be affected by having the pistons altered. What you need to do is make sure that after pocketing the pistons that they all get weight matched.

That's not correct.

Will the crank balance be affected - yes it will, when manufactured the crank was balanced taking a nominal piston mass into account.

Does it NEED to be re-balanced. Not necessarily, a few grams of valve relief probably won't be noticeable but if you were building a race engine you would match individual piston, rod and rod end to end masses and then balance the crank to those.

I phoned chesman this morning and they said it would need balancing to do job right

russell_ram wrote:That's not correct.

Will the crank balance be affected - yes it will, when manufactured the crank was balanced taking a nominal piston mass into account.

Does it NEED to be re-balanced. Not necessarily, a few grams of valve relief probably won't be noticeable but if you were building a race engine you would match individual piston, rod and rod end to end masses and then balance the crank to those.

So if all the old pistons and rods weighed X.
And you lightened them by the same amount to arrive at Y.
Why would the crank need to be rebalanced if all the rod and piston assemblies weighed the same as before but a little lighter?

I'm just a simple plumber so I may be spouting a load of crap. But I'm always ready to learn about engine stuff

When I had my engine balanced recently I had to give accurate weights of all the piston assemblies - As these had all been matched as separate items and the rods also matched end to end it was not a problem to do. These weights are then used to make up weights to attach to the crank .

http://www.enginebuildermag.com/2014/03 ... cing-work/

To do the job "right" you need to match all pistons weights to the lightest - match all the rods to the lightest and then balance the rods end to end then you have the crank balanced for those masses - you have the clutch cover and the flywheel matched and balanced and you have the front pulley balanced.

As I am going 7500RPM assembly its worth the extra effort. Thankfully on mine there was sufficient weight in the new crank casting to have the weight removed - it gets tricky and more expensive when you have to add heavy metal to the crank to get balance.

Also bear in mind that the RV8 is externally balanced by a bit of old thread bar hammered inside the lip of the front pulley - change a pulley and that balance is gone.

Andrew

Because the crank has a component of the piston mass considered to rotate with it ie acting on the crank pin, as well as the 'bottom half' of the conrod. The calculation is 100% of rotating mass(big rod ends and rod bearings)+ 50% of reciprocating mass(pistons,pins,locks and rod small end) per cylinder. You can include mass of oil in the galleries too if you're particularly picky. A V8 will have a bob weight of the equivalent of twice that added at each pin when it is spun up and balanced.

So mass matching components to each other and conrods end to end is important but so is the total component mass per calc above.

Will a few grams of piston cutout be noticeable, probably not but bear in mind the crank was only balanced to 'nominal' numbers at the factory and is at best a compromise. The further you get away from those the more vibration your motor will produce, the higher the stresses in the crank etc. The higher the engine speed you plan to use, the worse those get (speed squared effect). Build a race motor and you do it 'correctly' every time.

Some time ago I had an engine balanced and it vibrated more than it did before balancing. I then realised that the balancer had used the weight of Clevelite big end shells when he assembled the bob weights. The shells I actually used were 9gms lighter.
I ended up drilling the counterweights opposite each crank pin to loose the equivalent weight and compensated for the error.

The engine was then smooth throughout the full rev range.

Changing the weight of a piston will of course only affect things by 50% in 90 degree crank v8's as pistons are considered as reciprocating weight.

russell_ram wrote:Because the crank has a component of the piston mass considered to rotate with it ie acting on the crank pin, as well as the 'bottom half' of the conrod. The calculation is 100% of rotating mass(big rod ends and rod bearings)+ 50% of reciprocating mass(pistons,pins,locks and rod small end) per cylinder. You can include mass of oil in the galleries too if you're particularly picky. A V8 will have a bob weight of the equivalent of twice that added at each pin when it is spun up and balanced.

So mass matching components to each other and conrods end to end is important but so is the total component mass per calc above.

Will a few grams of piston cutout be noticeable, probably not but bear in mind the crank was only balanced to 'nominal' numbers at the factory and is at best a compromise. The further you get away from those the more vibration your motor will produce, the higher the stresses in the crank etc. The higher the engine speed you plan to use, the worse those get (speed squared effect). Build a race motor and you do it 'correctly' every time.

OK! I see now
So it could be possible to accurately weight match all the pistons and rods and then end up with vibration when fitted to the untouched crank?
That makes sense
Regards
Pel