How many grams does a OEM 3.2L piston weigh?

[gliding_serpent]

just wondering what an OEM 3.2L piston (9.5:1, 95mm) weighs new. This would be for the 1984-89 cars. I figure someone must have balanced an engine and got some numbers. Mine are no longer in my hands.

Thanks for any help. My searches came up blank.

[gliding_serpent]

never mind... searching "911 3.2 piston balancing" got me the answer to a level of accuracy that I am happy with.

Originally Posted by 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

[KTL]

Couple of things worth noting. Weight depends on which pistons you have, Mahle forged or KS cast, and which code group they belong to.

Taken from the 911 '84-'88 technical specifications booklet:



Also, I don't necessarily agree with Determined's approach of mixing and matching pistons & pins to balance the engine. If they were brand new pistons & pins? No problem mixing and matching. But once they've been run thousands of miles, I think it's wise to keep pins with their original piston. Porsche actually made a point of mentioning this on the the next page for Turbo pistons.



I have measured used pin diameters and piston pin bores. Pins were meas'd with a 0.0001" micrometer and the pin bores were meas'd with a dial indicator style small bore gauge. The pins were basically all the same diameter. Pin bores varied a bit. Since I had a set of pistons and pins that were not kept together, I had to do something to decide how to select which pin would go in which piston.

So I think it's just good practice to keep the pins where they came from & avoid uncertainty. To balance you can massage the underside of the crown or other non-structural areas to get the weight balanced. However, milling/grinding light aluminum alloy is one of those things where you grind a bit off, weigh it, still weighs the same. Grind some more off, weigh it again, still hardly lost any weight. Point being is you end up with a sizeable pile of aluminum shavings! Or you can hone the ID of the pins with a tiny grape hone.

Just my opinions, not an expert, so take that for what its worth.

[gliding_serpent]

I agree with you. I have new 3.4l pistons, i was trying to get an idea of the weight of new vs old pistons. Based on the factory numbers, it would seem that the mahle forged pistons are a good deal lighter than oem, independant of weight grouping. The 3.4l's are 490grams... So they are a big reduction in rotating mass.

[KTL]

Keep in mind those numbers in the spec book are with pins, clips and rings. So that's why they appear so heavy.

I previously had a set of Mahle 98mm non-Moritz pistons and they did indeed seem like they were pretty light. Unfortunately I hadn't put them on the scale to see how much they were. But with the rather small dome, large valve reliefs and minimal skirt, they looked to be pretty light.

http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/670722-98mm-mahle-motorsport-pistons.html

[gliding_serpent]

Mine are the wedge domed max moritz styled ones for single plugs with 9.8:1 comp ratio.



So i guess i am back to them being a touch heavier than oem, plus or minus. Not bad for the size increase.

[KTL]

Yep you have to accept the fact that with the bigger piston diameter, you're really hard-pressed to get them lighter than stock pistons. So I agree that it's a fair trade off to be using larger displacement pistons that weigh nearly the same as stock size pistons!

You'll want to smooth those sharp edges around the valve reliefs and at the relief-to-crown transitions. Thin metal like that is a site for hot spots. Also helps the piston move thru the air more easily.

[gliding_serpent]

Good tips. They were installed in the engine last week. I presume they were smoothed as the same was done to the heads.