Balancing Pistons - How Much Weight

amskeptic · Post by **amskeptic** » Sat Dec 14, 2002 3:54 pm

Get this verified with one of the Masters,
but I think you can take weight off the boss at the small end of the Type 4 rods connected to your three heaviest pistons to match the light one. You have only 5 or 6 grams to get rid of. To keep your rods in balance, you have to also remove some weight at the lower end of your light rod to match the net loss of weight they endure from matching up your piston ends. Again, if anyone can verify this, I might be leading you off a short pier. . .
Colin

super72 · Post by **super72** » Sat Dec 14, 2002 6:52 pm

Check the weight or your wrist pins too and see if you can mix them to make the weight closer. .6g isnt bad though, if i remember stock were with in 5g. Hope this helps, TJ

jasonb · Post by **jasonb** » Sat Dec 14, 2002 9:38 pm

Yeah, but these pistons our about .4 ounces out of balance. That's like, over 11 grams.

I tried moving the wrist pins around a little and got no change in the weight. I think that I'm out of luck and that I'm going to have to buy a whole new set of P&C's.

After removing .2 ounces of material from the first piston, I still had .4 to go and there didn't seem to be anywhere left to remove metal. I suppose that if I had a die grinder that I might be able to cut into the back side of the crown but I don't have access to those tools. All I have is a dremel to work with.

super72 · Post by **super72** » Sat Dec 14, 2002 10:30 pm

Thats only 6/10ths of a gram range, where is the 11 coming from. Stock vws were within 5 whole grams. What is your application, engine size? You should be fine with those pistons unless you are trying to turn 10,000+ rpm, then i might have them perfectly balanced. TJ

------------------
TJ Brabham
'72 Super Beetle
'71 Super Beetle (parts)
'60 Baja
Aol IM- miniU2

jasonb · Post by **jasonb** » Sat Dec 14, 2002 11:40 pm

I just bought a single piston and cylinder to replace the one that got eaten by a screw.

When I weighed all 4 pistons I got:
20oz, 19.8, 19.8 and the new one weighed 19.4oz

It does not appear that there is as much as .6oz of material to remove from a piston to bring these in balance. I have managed to remove .2oz from the heaviest piston to bring it to 19.8 but I'm already nervous about where I had to remove some of the material.

Am I screwed? Do I have any other options other than buying a whole new set?

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Sat Dec 14, 2002 11:53 pm

The most I allow between 4 pistons on any engine is 1/2 Gram. Most of the time switching and indexing wrist pins will make the differences work out well.

I have been seeing some mahle cylinder and piston sets that were a whopping 23 grams out per set of 4,thats why we have to buy atleast 6 sets at a time and then flip flop all the pistons around to equal it out between them all.

Pistons will gather carbon and be out of balance after a short while,but if you run decent compression and run it hard enough you'll keep them blasted clean !!

jasonb · Post by **jasonb** » Sun Dec 15, 2002 7:10 am

<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:</font><HR>Originally posted by super72:
Thats only 6/10ths of a gram range, where is the 11 coming from. <HR></BLOCKQUOTE>

Just a misunderstanding. There's not a .6 gram differance, there's a .6 ounce differance. If there's 28.35 grams per ounce, then .6 ounce is 17 grams. After working a while on the heaviest piston, I now have 3 pistons that are 19.8 ounces and one (the new one) that's 19.4 ounces. That's an 11 gram differance.

I wanted to balance them to within .1 ounce (2 grams) but I can't find enough material to remove. What's the most weight that someone has removed from a set of Mahle pistons to balance them?

Post by **Marc** » Sun Dec 15, 2002 8:57 am

I've never had to actually do it (always had enough wrist pins around to play with) but it seems feasible to me that a wristpin could be lightened a bit by hogging out the ID. So long as the ends weren't molested and any stress risers were polished out it shouldn't cause any major loss of strength, should it?

super72 · Post by **super72** » Sun Dec 15, 2002 10:26 am

My mistake. I read your all your weights as grams for some reason. Sorry if i have appeared ignorant, got to learn somehow to read better, lol. TJ

jasonb · Post by **jasonb** » Sun Dec 15, 2002 12:58 pm

I think that I might try boring out the insides of the wrist pins like Marc suggests. It's not a high HP engine so I can't imagine a significant chance of failure resulting.

It's sure preferable to buying a new set of P&C's after I've already invested in the single.

RoWaMe · Post by **RoWaMe** » Tue Dec 17, 2002 9:54 am

Jason: a machinist friend on another VW Forum tried modifying his wrist pins- they are very tough steel and hard to modify/cut at all unless you have excellent equipment. Maybe ask advice at a good local VW shop- like Painter's Grinding in Golden?? They would certainly be able to help with the balancing, or at least point out where to take more material off the pistons so that you could do it yourself (save $).

jasonb · Post by **jasonb** » Sat Dec 21, 2002 2:36 pm

What I've learned from my experience is that you can remove about 6-8 grams of material from a piston to balance it. Any more than that and it gets a bit shady.
Also, the wrist pins are made of pretty hard material, so lightening them is unlikely.

I had to buy a new set of P&C's. One of the pistons in the set was .4 ounces (11 grams) lighter than the rest. I swapped in the single piston that I bought earlier and the set was perfectly balanced within .1 ounces. So I got lucky there.

So the moral of the story is, be careful when buying a single piston and cylinder. You might end up with a set that's impossible to balance. Also, a new set of P&C's can be unexceptably out of balance straight out of the box.

MASSIVE TYPE IV · Post by **MASSIVE TYPE IV** » Sun Dec 22, 2002 8:22 am

I never modify wrist pins for weigh imbalance I swap them in the set with other pistons and even index them to one piston and then cut the piston for the difference..

Lately Mahle sets have been getting really crappy, having to buy 3 sets of pistons just to get equal weights is not uncommon these days, they suck more and more all the time.

RT · Post by RT » Sun Dec 22, 2002 9:49 am

My last set of 85.5 forged Mahle's had 2 pistons 4 grams out from the other two. The pins were identical so moving them around didn't help. I found out that even 4 grams is a lot of material to remove. I managed to do it on the skirts (inside radius) but thats about the max weight I would try. A shop I used on some 90.5's balanced them by flycutting some material on the underside of the crown. They didn't go deep though but it made me kind of nervous. They boasted that these were done to 1/20th of a gram. Kind of pointless for my rpm application but it didn't affect the price which was $35.00 some years ago (for just the pistons).

392HemiTCoupe · Post by **392HemiTCoupe** » Sun Dec 22, 2002 7:28 pm

Just a thought, but why don't you have weld put on the inside the light one? I don't mean your neighbor, but someone that Helli-arc's all the time. I have high tech Helli-arcing done all the time, I go to two different places in the twin cities, one near me charges 25 bucks no mater what I bring him, and the other charges me 5 bucks for the same thing and the same quality, just 15 miles away!
cut pieces of the wire till you have the same amount weight you need to add and and mark a rod at the right lenght and tell them where to weld it and till they just pass the mark on the rod, so that you can remove if you need to.
Just a thought! Pat

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