Piston decision.....

[IJ.]

Ross heaviest>JE Lightest, everything else was in between, you'd have to check manufacturers sites for exact weights I didn't save the list I compiled.

Never used an Arias in a 7M before but have used them with great success in SBC's over the years and can see no reason why they can't make a good 7M piston.

 

[jdmfreak]

Pros...cons ......heavier lighter??

 

[IJ.]

Heavier are more detonation resistant, lighter are happier to rev hard.

 

[jdmfreak]

Hmmmm lol

 

[supraguy@aol]

Thanks, IJ.
Im vassilating between Ross and Arias.

 

[jdmfreak]

So ian.... in your opinion which would be best for a 7m?

 

[IJ.]

If you have a solid base tune and a good fuel system I'd go for the lighter pistons, the Ross are cut from a generic blank so the crowns are very thick/heavy.

I used JE's in my last 7M build, no probs at all but my fuel system was 2x what I actually required.

 

[Dan_Gyoba]

I bought the Arias pistons myself.

Haven't installed them, but the block is finished at the shop, anyway, I have to go pick it up, but waiting for bearings anyway.

 

[supraguy@aol]

On a different subject, IJ, is it sensible to have my crank rebalanced?
Obviously, the factory did some balancing, but was it adequate for higher powerlevels?
Also, how does using forged rods/ pistons affect the need for balancing? My understanding is that dynamic crank balancing is performed 'bare'. Should the counterweights be lightened to account for a lighter piston/ rod combo?

 

[IJ.]

An I-6 has very good inherent balance, but anytime you change anything weight wise in the reciprocating assembly it should be rebalanced, A good shop will balance the Pistons with the chosen rings pins and clips then balance each rod end to end then build a bob weight for the crank and spin it up to balance it, the Damper and Flywheel/Driveplate should be 0 balanced on their own.

 

[CyFi6]

I thought bob weights were only used on V configured engines? Not that I know anything about engine balancing, but I always wondered how they compensated for a lighter piston/rod assembly when all they seem to do is match each piston to the others, and all rods to the others on each end, then the crank is balanced on its own, without any compensating for lighter piston/rod.

 

[IJ.]

A Bob weight is just a dummy/stand in for the Piston/Rod assembly so you can clamp them on the crank and spin it up.

 

[supraguy@aol]

Ah-ok, that makes perfect sense now about the bobweights representing the new piston/rod combos.
I assume that any rod/ piston combo I end up with, is going to weigh less than the oem? I assume that forged pistons will be clearly heavier than stock, but will the rods weigh less?
I have Cunningham forged rods to use in my build.

 

[CyFi6]

Everything I can find online says that bob weights aren't needed for inline engines, but not really sure why. I remember Rich saying that bob weights are not typically used for inline cranks as well, and when my crank was balanced there were no bob weights installed.
http://books.google.com/books?id=VX...#v=onepage&q=bob weight inline engine&f=false

 

[supraguy@aol]

I searched youtube for videos of 'crankshaft balancing', and every single video I found except one, was of inline cranks with bobweights, so...

http://www.youtube.com/watch?v=LvgVI2PTKzI

http://www.youtube.com/watch?v=2UNLrXUL49w&feature=related

Supra crankshaft:- 6M
http://www.youtube.com/watch?v=5-k1GqIFP1g

Doesn't seem to have bobweights on this one, hmmm....

 

[IJ.]

As I said the inherent balance is pretty good, you wouldn't use bob weights unless you were making a big change in mass attached to it, ie: a Rod or Piston change.

 

[CyFi6]

Supra crankshaft:- 6M
http://www.youtube.com/watch?v=5-k1GqIFP1g

Doesn't seem to have bobweights on this one, hmmm....

That's the crankshaft that's in my car right now . Took it to John in Costa Mesa a couple years ago.

So then it seems bob weights on an inline crankshaft are not necessary but probably used for very high performance builds?

 

[supraguy@aol]

And what rods/ pistons are you running?

 

[jdmfreak]

Ok so i have another related question...when the scratches were honed out i was barely within spec with stock pistons ... i recently read somewhere(that i cand find now) that forged pistons have tighter clearances.... are they bigger diameter than the stock pistons or am i gonna run into issues if i stay with 20 over?

 

[Dan_Gyoba]

I know that the Probe forged pistons are slightly over stock size (something like .006") for a "hone to fit" sizing. The rest all were at 0.5mm (0.020") over.