matLT1

This is related to a thread I have going on the LT1 forum regarding a very low quench. I am looking to do heads and cam over this winter on an engine I had rebuilt 10k miles ago. After pulling the heads off, I measured how far down in the bore the pistons are only to find them at 0.045" blow deck height. So my question is, can I exchange pistons of similar weight to avoid extensive engine work, and if so how close do they need to be? I have found a set that is advertised to be within 3 grams.

Thanks,
-Matt

Rice Killer87

You already have to pull the motor and take it apart to replace the pistons (except the crank) your half assing it by not having it rebalanced...I would never try that I dont care what the pistons weight.

Why risk all the $$$$ you have into the motor only to skimp a small amount more on a re-balance man? I wouldnt risk it, but thats just me.

Stephen 87 IROC

3 grams doesn't sound like much but at high rpm, that translates into increased pounds. Using the same wrist pins also? A batch of wrist pins all for the same piston can have different weights also.

Injuneer

Asking if you don't need to balance an engine is not really "Advanced Tech". Its really just asking if you can be sloppy in your engine build.

matLT1

I tend to disagree with that statement. Since most engine balancers are only capable of getting a rotating assembly within 40g-cm of imbalance, it is most definantley a valid question. If the difference in weight will cause a negligible imbalance (i.e. significanly less than the 40 g-cm) then why redo it? My intention was not to half a-s, but more so to make sure I was not wasting money. These are two entirely different things.

Kevin Blown 95 TA

If I were in your position, I would ask the guys that previously balanced your engine if that would work, and maybe they will say to get the pistons and they will make all the pistons the same weight as the lightest one if they think it will work.

You say within 3 grams - is that lighter or heavier? Anyway I would ask the machine shop what you should do if you trust them and are still on good terms with them.

sbs

Keep in mind that this is a machine shop that built him a new engine with pistons 0.045 in the hole.

97WS6Pilot

Yes I would definitely rebalance. Nothing is harder on a motor than to be unbalanced. I took the balancing weight (very small) off my flywheel and it nearly shook my teeth out. I know yours will be internally balanced but this is just an example.

rskrause

It partly depends on how the engine was balanced in the first place. But in general, if it was balanced to the usual "50%" using a piston that is up to maybe 5% lighter is fine, in fact, there are some engine builders that prefer a little "overbalance". Heavier can be a problem, but a few grams does not matter.

Rich

MachinistOne

Like Rich said, it depends upon how the motor was rebuilt last time - if it was built as stock and not rebalanced because OEM weight pistons were used then you have some wiggle room with the piston weight due to the factory overbalance. If it was zero balanced by your builder then you need to use pistons exactly the same weight or lighter up to a predetermined percentage of your total bobweight.

rskrause

I was reading M1's post and re-reading my post and wanted to clarify something. "Up to 5%" doesn't mean 5% of bob weight. Normally, if you elect for overbalance, it is 1-2% of the bob weight. This could translate into up to 4-5% of piston weight. Example - bob weight is 1750gms, piston is 500gms. You elect for a 1% overbalance. 1% of 1750g is 17.5gms, which is 3.5% of 500gms.

Rich

matLT1

I appreciate you guys sharing your knowledge and experiences with me. I will take all things into consideration.

BTW,

This is the piston that was used for the build:
http://store.summitracing.com/partde...0&autoview=sku
I called Federal Mogul and they said the weight with pin is 695 +/- 7 grams.

Kevin Blown 95 TA

Good point. Wonder how that happened, cause they probably decked it too.

rskrause

I think you will have would have no issues if you install pistons that are up to 20-25gms lighter. More than that and it will need balancing.

Rich

blown94

Rich, just to clarify your statement, when you say 20-25 gms, you ARE talking each individ. pistons' weight, or the whole set compared to what was in there?