ok so i bought this rotating assembly off a forum member for a 355. now the kit came completely balanced with a flex plate and a balance card .

my question is can i remove the pressed on pistons ( hyperutectic ) and install some new forged pistons ?( i have a press )

will this throw off my balance to the point that i have to re balance ?

can i weigh all the hyper pistons and then weigh the forged pistons to see how close or far they are ?

if they are close can i remove some material if needed to get the close or are the hyper pistons hevier ?

i need some help here guys . thank you in advanced

 

If the weight is the same or damn close you will be OK.
You need a special guide to press the pins as not to get them cocked.
If you remove the material in the right places and not too deep you are fine otherwise you are looking for a failure.

My suggestion is take it too a shop that has the tools and experience.

 

You need a fixture to properly support the pistons/rods otherwise you can damage them.

The chances the piston weights are close enough are astronomically small, believe the goal on a set of pistons is just a couple grams variance to change type entirely and not need a rebalance would mean you need to spend you money on lottery tickets rather than parts.

 

i have the fixture at work . we never use it since now in days bmw doesnt make you rebuild anything .but i just used it to rebuild a ford escort .

ok so if i weigh all pistons and get them close to 1g from each other . will this throw off my original balncing ?

oh and 96capricemgr i always read your post at the impala forum , but god dam dont make it so complicated to read . im a jar head so keep it grunt proof.. you know like gun go boom ....lol

thanks for you help.

 

one more question i cant seem to find forged pistons for a LT1 on summit . can i use gen 1 pistons ?

 

Pistons up to 20-30 grams lighter would not be a problem. You don't want them much heavier though.

Rich

 

Rich must have already opened the window by the time you posted otherwise he would have answered.

Yes the LT1 is fine with Gen 1 pistons, those are what you should be looking at.

 

thanks guys you and rich are very helpfull on here .

 

I learned a lot more than I knew about balancing recently. Basically, the convention of balancing the whole piston/rings/pin weight and 50% the rod weight is somewhat arbitrary. Different shops have slightly different ways of balancing. If you don't specify exactly what you want, many will deliberately "overbalance" by 1-2%. Overbalance means there is more counterweight than the traditional rod+50%. Some engine builders are convinced that overbalancing makes for smoother running at high rpm with a neglible increase in low rpm vibration.

I had a situation when assembling my race motor this year where this came up. After the shortblock was already assembled, I found a piston to valve clearance issue that necessitated new pistons. The new pistons were 36g lighter than the old ones. My machinist, engine building mentor, and JE all said "no problem, it might even be better". In spite of hearing this from three reliable sources I was skeptical but went ahead as the time and cost for rebalancing was prohibitive. And what do you know, it runs smooth as silk to 7,500rpm. Noticibly smoother above 5,000 than with a 50% balance and it is now ~3% overbalanced! I am going to take a look at a main bearing this weekend and will report any probems. But from the way it runs, I doubt there will be any.

Rich

 

Sometimes "accidents" can be more informative than tests, thanks for sharing.

 

This is good to know! I had always wondered which side to "err" to if replacing pistons. Now I know.

 

BTW: I had to pull the pistons for an unrelated reason and the rod bearings look fine after ~50 1/4 mile passes shifting at 7,500rpm. Didn't look at the mains, though maybe I will pull a cap off just for a look see. Based on what I "knew" prior, I would have thought just running the thing under low load would lead to severe vibration and major damage.

Live and learn.

Rich