Rebuild

[Dusty1]

'68 340=== Nearing time for rebuilding the motor-- Do I toss the pistons and buy new? Is there much of an advantage in replacing them? Thanks

[Chryco Psycho]

typically the bores will be worn so by the time you get the bores straight again they will be too large to fit the old pistons so it is a must to get new pistons to fit the new bore size unless by some miracle your bores are not worn

[Dusty1]

Thank you!

[TreeFrog]

buy your pistons first
have your block bored to match the pistons

[72cudamaan]

buy your pistons first
have your block bored to match the pistons

Or even better, have your machinist measure the bores, then get the pistons.  Too many times I have seen people get their own pistons assuming that the block will size properly, before even measuring.

[Dusty1]

Sounds good, guys! Thanks

[Topcat]

piston CR, bore are only part of the equation. You also need to have the CC's in the head factored in. And that can't be properly done until the heads are done. For example; Tulip valves or flat faced can either add or take away a small percentage CR. 

  You need to really sit down and have an in depth discussion with your machinist on the complete package for everything on what you are wanting for the end result.

[Topcat]

This will help you get the calculations so you know up front what you will end up at.

http://www.csgnetwork.com/compcalc.html

[Dusty1]

Thanks Mike!  I'd like to see this '68 motor done right. You've been helpful!

[Chryco Psycho]

you need to know if you are using iron or alum heads , you can run close to 10:1 with iron & 11;1 with alum heads requiring different pistons usually unless the alum head have significantly smaller combustion chamber size , you also need to select a cam to work with the powerband, intake , torque converter so everything works together to make it perform at its max potential

[moper]

Another thought is pistons dont just wear against the bores. Which is what most guys look at. The rings wear the ring lands too. It's common for a piston to look great on the skirts but sstill be basically junk. The ring has to fit the groove properly because it seals on the bore plus the top or bottom surfaces of the ring lands in the course of it's cycles.

[Dusty1]

I'm using the iron heads that I have. 2.02 valves, hardened seats and the machinist says a beautiful port job. Cam will be in the 484 lift area. Sounds like new pistons are on the list. I thought this motor had 10.5 to 1 compression stock? Thanks guys, I appreciate the education. Just as long as it out runs my buddys AAR 340 6 pak!

[Chryco Psycho]

using the Lunati voodoo cams you have have low duration with good idle & vacuum & still have well over .500 lift , up into the 530-540 range , lift will not affect the idle but helps the power especially with ported heads

[Dusty1]

Sounds good! I've seen you mention those cams before. When they worked the heads, they installed springs good up to 500 lift. Spring change?

[Topcat]

  Wait on ordering Valve Springs until you get your installed Valve Spring height determined and/or corrected.

Basically, let your head specialist make the order on the valve springs once he has done his end of the work on the heads. He should know once the heads are done with the cam you are using what springs you will need to get.

http://www.fordmuscleforums.com/engine-articles/483498-installing-valve-springs.html