Piston Weight Matching

[brewsy]

Hi All,
As you all know with our hobby there is no thing as a simple fix.

My engine is a stock SP1300 and has developed an exhaust leak/blow so needs changing.
That's currently on backorder as there seem to be none in the UK so I decided that I may as well pull the motor to make it easier to fit the new exhaust (thats my excuse) and then whilst Im there I'll do small 'stock ish' performance upgrade. No cam changes, no new valves (unless they are toast and certainly no larger), no rocker arms and stock (ish) heads, carburettor and rev limit.

• Swap B+P's to 1600
• Install the slightly odd 4 dowel pin 12lb lightened flywheel that I own
• Check and carbon clean the preheater tube on the manifold
• DIY port the SP heads
• Install and re-jet a slightly bigger Solex carb
• Install a Mexican style oil pump with spin on filter

As Im not splitting the cases Ive decided to try and do all this myself and using second hand parts to keep the costs as low as possible.

First purchase was some eBay 1600 B+P's which turned out not to be 85.5mm but first oversize at 86mm and with 2x of them having the pin retaining grooves worn out 
I managed to source 2x more 86's with good grooves however it turned out I now have 2x KolbenShmit and 2x Mahle pistons. Didnt think that this would be as issues as they are both OEM suppliers (I think?) and should therefore be pretty well matched.
Turns out I was wrong as after a good degrease and clean the weights seem waaayyyy off:
2x are 422g
2x are 407g

Ive had a look at 'how to match piston weight' etc all over the web and does seem a normal thing to do however most people usually only need to take 1-2g maximum out of their pistons as they are starting with a matched set.

Questions:
What's the safest stock mismatch anyone has seen? Are there specs from VW?
Can I safely take 15g from a stock cast piston?
If I cant take that much from the piston how much could I take could I then find the rest in the piston(gudgeon) pin?

I probably should have just bought some new 85.5 or 87mm pistons and be done but Im too stubborn at this point and want to try (as hard as possible) to make it work.
Besides which I like the fact that the 86mm pistons give EXACTLY 1600cc!! 

Cheers
Marc

[Martin S.]

Take the weight off the pins. You can use a drill that is just over the inside diameter.
Take the valves out and check the guides fit. Then you can hand grind the seats if you have the tool and get new stainless valves, get someone to grind them, or at the very least hand lap them.

[brewsy]

Take the weight off the pins. You can use a drill that is just over the inside diameter.
Take the valves out and check the guides fit. Then you can hand grind the seats if you have the tool and get new stainless valves, get someone to grind them, or at the very least hand lap them.

Thanks Martin

[Bruce]

Take the weight off the pins. You can use a drill that is just over the inside diameter.

A solid carbide drill big enough is going to cost over a hundred $.

[alex d]

I would just run one pair on 1-3 and the other on 2-4

[Jesse Wens]

I would just run one pair on 1-3 and the other on 2-4

this is exactly what I was thinking too. Just match the opposing pistons and call it a day for your level of performance.

[Martin S.]

Take the weight off the pins. You can use a drill that is just over the inside diameter.

A solid carbide drill big enough is going to cost over a hundred $.

How much with a 5-finger discount Bruce? 

[brewsy]

I would just run one pair on 1-3 and the other on 2-4

Cheers Alex.
That had crossed my mind too.

Didnt want to say anything first as I didnt want to miss any potential ideas.. 
May still see if I can get some aluminium from under the crown AND dremel the pins.
Would like weights to be somewhat close so as to keep the rocking couple down to a minimum.

Now the question is...
Heavier pair on 1+3 OR 2+4 

Im thinking 1+3 so as to keep the rocking closer to the centre of the car but arent there more bearings at the 2+4 end?
Or am I just overthinking that??

Cheers
Marc

[mikko k]

Factory info is max 5g weight difference between pistons. I would go max 1g diffrence.

15g is a lot! I wouldn’t even start to balance these.

But everybody do what they want to do.

[brewsy]

Here's some pictures.

KS vs Mahle
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Underside Mahle
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Underside KS
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Looks like way more 'meat' on the KS....

Headed to the garage and it seems that my two scrap pistons are KS.
Got the daughter to help and a few minutes later we had...
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[brewsy]

And then measured at some spots
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So compared to the Mahle there is much more meat (obviously) and even if I remove all the 'ribbing' the crown still looks to be about 8mm of ally.
Been on eBay and ordered one of those corded drill to pillar drill things, £15 well spent I reckon!!
Will drill the F out of them and I might actually get within the 5g VW difference

[Martin S.]

Interesting to see the cross-section, thanks!
The crown of the piston is most crucial when it comes to strength which is a factor in dishing pistons.
That’s likely the reason pros balance using the pins not the pistons.
Another reason is that pistons also have a inherent balance to them across the pin axis to keep the “snap” force to a minimum.
And also keep in mind that aluminum is half the mass of steel so you would need to remove twice as much.
None of this would matter with a STF engine so no big deal. 

[PPRMicke]

info Vw had a limit of 5 grams on a std engine Replacement piston was 10 grams but I would never dare to put in 10 grams I want to be closer to 1 gram both on piston and rod (the smaller the difference the better) It will be ( tonnes) in force when there is a weight difference
We who play with the 1600 challenge in Sweden use Mitsubishi which is 86 mm (85.95) o piston pin of 22mm But if you think the effect is in the piston should probably think once to Na the engines are around 130-168 hp and turbo around 300 hp
/// M

[brewsy]

Interesting to see the cross-section, thanks!
The crown of the piston is most crucial when it comes to strength which is a factor in dishing pistons.
That’s likely the reason pros balance using the pins not the pistons.
Another reason is that pistons also have a inherent balance to them across the pin axis to keep the “snap” force to a minimum.
And also keep in mind that aluminum is half the mass of steel so you would need to remove twice as much.
None of this would matter with a STF engine so no big deal. 

Cheers Martin,
One question.. What does STF stand for?? 

Marc

[brewsy]

info Vw had a limit of 5 grams on a std engine Replacement piston was 10 grams but I would never dare to put in 10 grams I want to be closer to 1 gram both on piston and rod (the smaller the difference the better) It will be ( tonnes) in force when there is a weight difference
We who play with the 1600 challenge in Sweden use Mitsubishi which is 86 mm (85.95) o piston pin of 22mm But if you think the effect is in the piston should probably think once to Na the engines are around 130-168 hp and turbo around 300 hp
/// M

Micke,
Thanks for that but this if this build hits 55-60BHP at the flywheel Ill be happy!
Im staying with stock manifold, carb and exhaust so RPM range will be pretty much stock
Cheers

[Martin S.]

Sweep The Floor. Using parts leftover or on hand like what you’re doing.

[brewsy]

Sweep The Floor. Using parts leftover or on hand like what you’re doing.

AHA.
Like it 

[Neil Davies]

Sweep The Floor. Using parts leftover or on hand like what you’re doing.

Jim Ratto was the first one I heard using that phrase, about 25 years ago now! He'd got some great stories about STF motors!