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