318 crank into 340? pitfalls

[Jadaharabi]

New pistons should have new pins.

Show us a picture of your balancer.

Basically you're building a stock 340 that's blueprinted. The original 340 s did not come with forged pistons. You can make 400 horsepower with a cast piston.

 

[nm9stheham]

my crank has been very lightend already,old pistons were ross that had been worked and weighed 430 grams I believe.rods had been worked also.super light pins.so, will have to ad lots of weight back in, but I think it,s the way to go after what most people here are saying.

Yes, you will have to add lots of weight back in! Stock 340 pistons are 719 grams with a 154 gram pin....

The H405CP is what I would go for... Ooops...one problem; they don't have pin lock grooves in them. Probably the same for the Silvolites. Sorry to not see that before.

Back to something like the ICON FHR 9975 to keep the CR down below 9.0. 606 grams with 118 gram pin. These are forged, but I agree, forged is not necessary for this build level.

 

[nm9stheham]

damper does not say for 340 only.are this h405 cp pistons forged,seems I need to stay with a forged,I will get out to my shop in alittle and ck. but I think the # have been removed with wight removal.

OK, so an early 340 with the forged crank.

 

[j.d.duggan]

there are no # visable on rods,True up(resize,recondition,I thought that was standarn practice on rods?only thing on harmonic balancer is N15

 

[nm9stheham]

Gotcha.... I just had not heard the term 'true up'. Not sure what the rods are now.....the casting numbers are on the side between the 'top' and 'bottom' of the I-beam shape; did they grind there too? Put a caliper to measure the pin bore and make sure it is the SBM .984" diameter, and not a Chevy pin! With all the mods, it is hard to say.

 

[j.d.duggan]

Gotcha.... I just had not heard the term 'true up'. Not sure what the rods are now.....the casting numbers are on the side between the 'top' and 'bottom' of the I-beam shape; did they grind there too? Put a caliper to measure the pin bore and make sure it is the SBM .984" diameter, and not a Chevy pin! With all the mods, it is hard to say.

 

[j.d.duggan]

thanks for the info on pin size ck,nm9stheham.thats something I hadn't thought of.with everthing that had been done to this thing you could be right.I have a full set of 307 chevy rods that I took out of my nova when I put in a GM create.Is that something that gets done a lot?any probs getting pistons to correct bore and wrist pin size?

 

[Jadaharabi]

Yes, you will have to add lots of weight back in! Stock 340 pistons are 719 grams with a 154 gram pin....

The H405CP is what I would go for... Ooops...one problem; they don't have pin lock grooves in them. Probably the same for the Silvolites. Sorry to not see that before.

Back to something like the ICON FHR 9975 to keep the CR down below 9.0. 606 grams with 118 gram pin. These are forged, but I agree, forged is not necessary for this build level.

.
Isn't the 405 piston for a 360 not a 340?

 

[nm9stheham]

.
Isn't the 405 piston for a 360 not a 340?

Duuuh.. you are 100% right sir! Thanks for keeping things straight.

thanks for the info on pin size ck,nm9stheham.thats something I hadn't thought of.with everthing that had been done to this thing you could be right.I have a full set of 307 chevy rods that I took out of my nova when I put in a GM create.Is that something that gets done a lot?any probs getting pistons to correct bore and wrist pin size?

Don't know that it is done a lot, but with all the mods here, anything is possible.

As for the right parts, look up the ICON IC9975's..... they are for 340 dimensions, have a lower CR like you need, are forged, have grooves for pin locks, and are in the lower cost FHR product line.