Question on Piston and Wrist Pin Balancing, 3.2L Carrera

[Determined]

Gentlemen,

I had the opportunity today to start measuring the weight of my pistons and wrist pins in order to try to balance my 3.2L engine. I first took measurements of each piston and wrist pin separately then created a spread sheet in Excel. I then made my 1st arrangements of pistons and wrist pins to try to get each assembly close in weight. My 2nd arrangement yielded better results with a smaller standard deviation and lower Min/Max Delta. I have attached my data below along with a comparison of what it would like in the engine. I did also weigh the circlips but I did not include this data yet. I can get them to balance out as well as my 2nd arrangement data.

My question now is:
Is this acceptable as is without removing material from any of my wrist pins or should I reduce more weight to get a smaller delta weight across cylinders? Across cylinders 1 and 4 I had a surprisingly 0.040 grams weight difference. Across 3 - 6 and 2 - 5 these both were the same, 0.870 grams. Any input is greatly appreciated!

Best Regards,

Alex

[ChrisBennet]

Until the experts wake up and weigh in, here's my 2 cents:

I can tell from the way you shaded your spread sheet that you are aware of the importance of side to side balancing but for other people reading this:

You want to "pair up" the pistons that are opposite each other. If say you have 2 pistons that weigh 589gr, 2 that weight 590 and 2 that weigh 591 your would arrange them like this (heaviest should go near flywheel):
#1,#4 589gr
#2,#5 590gr
#3,#6 591gr

I think it is more important to get the side to side weights balanced more than reducing the weight difference between all the pistons. In your first table they looked pretty well balanced:
#1,#4 .09gr difference
#2,#5 .19gr difference
#3,#6 .14gr difference

In your last table they were much more imbalanced:
#1,#4 .04gr difference
#2,#5 .87gr difference
#3,#6 .87gr difference

I could be wrong on this of course but it seems to me like your balancing efforts are taking you in the wrong direction.

I'm kinda curious about the the weights in the first table. They are arranged with the heaviest pistons pairs towards the pulley - the opposite of the way I thought they should be. Was this motor taken apart before?

-Chris

[Determined]

Oh no! Thanks for the information but this motor has not being taken apart, however, I did purchase the pistons and cylinders separately. I'm replacing my original pistons and cylinders with the ones above. I don't have the originals anymore. The pistons and cylinders I purchased were matched and labeled accordingly but maybe they were wrongly labeled. Would there be a problem rearranging them to place the heavier set towards the flywheel? These are Euro pistons and cylinders that were in great shape when I purchased them. Thanks for the clarification on balancing approach, I thought I had to balance across each opposite ends of each cylinder and not side by side.

Best Regards,

Alex

[cstreit]

Didn't look it all over, but the heavier matched sets go towards the flywheel.

[Determined]

When balancing, do I need to concentrate across cylinders for front to back or cylinders next to each other like 3, 2, and 1?

Another question for anyone out there, different subject:
1. Is loctite 242 (blue stuff) good for installing new ARP head studs? Or should I use the "red stuff" instead that is the maximum strength?

Thanks for your time,

Alex

[ChrisBennet]

Originally posted by Determined
When balancing, do I need to concentrate across cylinders for front to back or cylinders next to each other like 3, 2, and 1?

Another question for anyone out there, different subject:
1. Is loctite 242 (blue stuff) good for installing new ARP head studs? Or should I use the "red stuff" instead that is the maximum strength?

Thanks for your time,

Alex

The 911 is a "boxer" motor - the pistons immediately opposite each other mirror each other in motion i.e. when piston #1 is at the top of it's stroke, piston #4 is also at the top of it's stroke. The opposed pistons shoot out away from the crank at the same time and return at the same time. That's why you try to make opposing pistons match in weight.

-Chris

[BK911]

For a street engine I would put the heaviest piston with the lightest pin, then work down to the lightest piston with the heaviest pin. Then I would put the heaviest piston/pin assembly with the lightest rod. I usually get within a couple of grams total difference between lightest and heaviest total assembly with no material removal. This works fine for a street engine. For an engine that will see a lot of high RPM's, I would follow the same concept, but I would end balance the rods and tighten up the variance on the piston/pin assemblies.

[afterburn 549]

I get a lot of crap over this- but I always make the mall the same weight ( with in a 10th of a gram)...that way all should be happy. I used to say balanced to "0" grams and some tell me its not possible...well maybe not but on paper it looks better then a 1/2 a gram....

[Determined]

Thanks for everyone's feedback, in my scenario where my pistons may have been labeled incorrectly. Are there any problems if I rearrange them to move the heaviest towards the flywheel and lightest towards the alternator? I purchased these cylinders which were in individual bags labeled with a number on them that matched the factory number onto of the piston, but the actual weights of these pre-numbered pistons shows that the heaviest is at the alternator and lightest at the flywheel. Something does not seem right.

Thanks,

Alex

[ChrisBennet]

Keep the cylinder with the piston when you move them around.
-Chris

[bdonally]

I have a similar situation but I want to bring evrything to the same weight. That involves removing nearly 4 gram in the worst case (everything is within the same weight class but a couple of pistons are at the high end and the rest are near the low end.

Should I take it out of the pins, by grinding a chamfer on the inside at each end, or remove it from the pistons?If the piston, from where should the material be removed?
The heaviest pistons have noticably thicker webs.

Any advice appreciated.

[afterburn 549]

well I am no pro, but have put together a lot of diff engines including my 2.7. What i did- measure all the pins ( dia ) if they are all the same and probably are, you can swap them around to even the weight of the pistons out..( once you know the weight of everything individually ) ...after that you can remove weight from inside the pins and some off the piston skirts....works for me and it reg sees 7000 rpm

[bdonally]

Originally posted by afterburn 549
well I am no pro, but have put together a lot of diff engines including my 2.7. What i did- measure all the pins ( dia ) if they are all the same and probably are, you can swap them around to even the weight of the pistons out..( once you know the weight of everything individually )

I have done all of that

Originally posted by afterburn 549
...after that you can remove weight from inside the pins and some off the piston skirts....works for me and it reg sees 7000 rpm

Where on the skirt?

And is the skirt a better place than the vertical webs (which, as mentioned above, are thicker on some pistons)?

[afterburn 549]

I have been told I am a cowboy here so with that said ...the skirt keeps your piston in line with the bore.,the less skirt you have the more wiggly the piston tends to want to be..but its one of the most handy places to remove weight too...I have never broke one by removing it from that area. Do it so it all has a gentle radius and leave the skirt as long as possible..How many grams do you need off ??

[Determined]

So if I understand correctly, I can move my matched piston/cylinder sets around to better balance the engine? Sorry for repeating this but I want to be certain before I really mess anything up. Any other recommendations on what not to do?

Thanks,

Alex

[afterburn 549]

the cylinders have nothing to do with the weight...I would leave them where they were. I would just make all the pistons weigh in the same and put them back in the same hole...if you start shuffling cylinders around it could turn into a can of worms -there are diff grades (lengths ) yours are probably all the same but........you could get cam binding and stuffff

[ChrisBennet]

Just as there are different height groups for cylinders there are 4 different cylinder tolerance groups (diameter). I would keep the piston with the cylinder is was originally mated to and check the height groups of the cylinders - look for a 5 or 6 in a triangle at the base of the cylinder.
-Chris

[afterburn 549]

thanks Cris- I know you are more tuned up on this then I am

[911pcars]

As you can see, there are many variables at work here including:

Con rod small end-to-piston pin clearances
Piston pin-to-piston clearances
Cylinder heights on each bank
Deck height on each bank and on individual piston assemblies
Oil clearance on individual pistons and cylinders
Weight of con rod small end
Weight of piston pin
Overall weight of each con rod and piston assy.

It'd be easier if all component specs were identical. If not, them you mix and match within limits of compatibility and/or modify each/select components to arrive at your objective.

IMO, if all else is okay, I would send the reciprocating parts to be balanced with the goal of matching to the lightest con rod/piston assy. Don't forget the pulley. flywheel/ring gear and clutch assy. They should be balanced separately from the reciprocating parts.

Sherwood

[911pcars]

As you can see, there are many variables at work here including:

Con rod small end-to-piston pin clearances
Piston pin-to-piston clearances
Cylinder heights on each bank
Deck height on each bank and on individual piston assemblies
Oil clearance on individual pistons and cylinders
Weight of con rod small end
Weight of piston pin
Overall weight of each con rod and piston assy.

It'd be easier if all component specs were identical. If not, them you mix and match within limits of compatibility and/or modify each/select components to arrive at your objective.

IMO, if all else is okay, I would send the reciprocating parts to be balanced with the goal of matching to the lightest con rod/piston assy. Don't forget the pulley. flywheel/ring gear and clutch assy. They should be balanced separately from the reciprocating parts.

Sherwood