I'll be changing the pistons on my 360/426 stroker

-
N

Nat

Well-Known Member
Joined
Jun 11, 2019
Messages
974
Reaction score
620
Location
Canada
going from 23cc dish pistons to 13cc dish pistons to bring compression up a bit . Am I going to have to balance everything again ? Or can I just slap it togetjer and hope for the best ? that is what happens when you don't think about everything when putting together an engine , just took what they offered as the flat tops would bring compression over 12.0 so just accepted the 23cc dish , without thinking about asking for custom dish pistons , until after I assembled things and realized only way to get the compression up was new pistons .
Thanks
 
For me, if it's more than a few grams..'like 9+grams heavy' I have it balanced.
You can grind/lighten the pistons around the pin boss underside , but after 4 or so grams you end up weakening the piston and create a potential for failure.
I've changed a single out a couple times and ground 5 grams out to bring it within 2 grams of the rest and it runs great with no vibration.
But at a point... what's the focus, saving money ? or a great running engine that lasts...
What's the weight diff?
Without that info there nothing to base an answer on.
 
Exactly. Look at the weights of the old versus new pistons + pins + rings. If they are the same within a tolerance, then you are good to go. If they are different, then there are ways to make them work without a complete re-balance... within practical limits.

Do you have part numbers? Any specs?
 
nm9stheham said:
Exactly. Look at the weights of the old versus new pistons + pins + rings. If they are the same within a tolerance, then you are good to go. If they are different, then there are ways to make them work without a complete re-balance... within practical limits.

Do you have part numbers? Any specs?
Click to expand...
NO , I'd have to go to the shop to get the specs , pistons are exactly the same with the new being 13cc dish the old 23cc dish so a touch heavier , guess I'll balabce it again even if is only + 4 or 5 grams a piston
 
You can get 4 to 5 grams out of a piston without a ton of work. And you can get some out of the small end of the rod. If you can take the weight all out of the new piston + the small end of the rod, then you are done. No need to touch the big end of the rod or the crank.

If the piston is heavier, and you do not take enough weight out of the piston and small end of the rod, then you can take it out out the big end of the rod and get to the previous bobweight. Then you still don't need to touch the crank.

So the change in piston + pin weight is important and worth the effort to find out. You can save a lot of work if you plan it right. (And we are assuming that the same ring pack is used, so that there is no weight difference there.)
 
famous bob said:
Being me , I`d do my damdest to make them the same weight ! then no rebalance.
Click to expand...

I dropped the compression in my RB stroker from 12.2 to 1 down to 10.3 to 1 using a Ross custom, reverse dome piston designed with the guidance of Dwayne Porter. The pistons were light to begin with at 524 grams, and the new pistons were spec'd at the exact same weight.

That being said, Smokie Yunick is said to have swapped in pistons in his race engines to suit track conditions that were nearly 100 grams different than what he pulled out of his motors with no vibration. However, all the pistons were of equal weight.

I spent the money to get them the same weight as the pistons I was replacing, as I only wanted to build the motor once time and didn't want to chance it. Turns out according to Ross, it is a bit of an engineering challenge to get those big pistons down to 524 grams, but they did it.
 
THANKS GUYS I will weigh the pistons see if there is a difference between the new and old old bob weight was 1800 or so hope i don't have to rebalance . old days just swapped pistons , ogf course back then paid less for my car than the engine i built now , it will hurt if it grenades because i cheaped out .
All the new pistons are the same weight +/- 1 gram
 
Last edited:
Nat said:
THANKS GUYS I will weigh the pistons see if there is a difference between the new and old old bob weight was 1800 or so hope i don't have to rebalance . old days just swapped pistons , ogf course back then paid less for my car than the engine i built now , it will hurt if it grenades because i cheaped out
Click to expand...

One more thing I forgot to mention. It wouldn't hurt to consult with a knowledgeable cam guru to optimize your cam spec's to match the lower compression ratio. I can highly recommend Dwayne Porter, AndyF or Dave Hughes. There are others as well, but getting it right requires both knowledge and experience, which these guys have a lot of.
 
jbc426 said:
One more thing I forgot to mention. It wouldn't hurt to consult with a knowledgeable cam guru to optimize your cam spec's to match the lower compression ratio. I can highly recommend Dwayne Porter, AndyF or Dave Hughes. There are others as well, but getting it right requires both knowledge and experience, which these guys have a lot of.
Click to expand...
it's higher compression goingg from 10.5 with 23cc dish to 11.4 or so with the 13cc dish . think the cam i have now will like the higher compression
 
jbc426 said:
That being said, Smokie Yunick is said to have swapped in pistons in his race engines to suit track conditions that were nearly 100 grams different than what he pulled out of his motors with no vibration. However, all the pistons were of equal weight.

I spent the money to get them the same weight as the pistons I was replacing, as I only wanted to build the motor once time and didn't want to chance it. Turns out according to Ross, it is a bit of an engineering challenge to get those big pistons down to 524 grams, but they did it.
Click to expand...
FWIW.. The piston & rod assemblies being the same bobweight is done for the reason of minimizing the main secondary engine vibration. Secondary means it happens twice per rotation.

The use of the resulting bobweight to balance the crank is to minimize the primary vibration, which is once per rotation.

So if you get the piston&rod bobweights equalized across all 8, you can minimize the secondary vibration but a different equalized bobweight still be off on the primary balance. The OP's objective is to get back to the same original bobweight so that the crank is still in primary balance. It can be done with pretty direct methods as suggested if not too far off.
 
You must log in or register to reply here.
-

Back
Top