3B bore out to 82.5mm?

[steve briance]

hello all,

My 7A block at the machinist at the moment is being measured for wear prior to being rebuilt and the bloke tells me it is on the limit for out of round figures but is going to give the worst bore a light hone, remeasure it and then give me a final verdict...

Re-bore may be on the cards but oversize pistons for 7A seem to be very few and far between from my research. Wiseco list a set but that is about it.

I have an idea which i would appreciate a steer on by one of the much more experienced folk on here who have done many more adventurous engine machining projects than i have...

Can a 3B be bored out to 82.51mm? That way i can re-use my 7A pistons which are in fine form and just get some standard size 7A rings which i know exist (though are still not that easy to get hold of)

I know folks go safely to 82mm as a 3B and I'm sure i have heard the basic casting is the same for the 7A and 3B other than turbo oil feed return and a core plug with a coolant outlet.

cheers

[Thuppu]

Sure it can be bored to 82.5mm no problem. 7A pistons may be weaker than the 3B pistons, but they will be ok to ~500hp. Have been proven to hold at that level but can be close to their limit. Was tested by a finnish audi enthusiast, machined down a bit to drop the CR though.

[Error404]

3B will be fine to bore out to 82.5
I'm surprised your 7A is near the wear limit, they are normally well within, albeit with some ovality even on high milers. The early 20v blocks seem to wear much better than the later ABY & AAN blocks (before the accountants got involved in running the shop I guess)

[steve briance]

Thanks both!
thuppu, i dont think i need to worry about the 500bhp limit, I'm building the 7a as a 7a I'd love to have a turbo under the bonnet but i like my cq20v and just want it to be as factory fresh as possible, thanks for.confirming my idea :-)

error, to be fair he has only commented in the ovality at the moment so actually perhaps the bore is ok. I just don't want to end up with another freshly built engine which only has 9.7bar compression. At least they are actually bang on the same for all five but i can't help thinking an oval bore is going to leak power...

[gnutz2]

I run 82.5mm on my aby block, the problem I have is lack of steel head gaskets, all the gaskets I've measured are 82mm so I resorted to machining the pistons so they clear the gasket.

If your running modest power levels then a 7a gasket will be fine.

[Albert]

Bore diameter: 83,50mm

Athena Cooper Ring gaskets, that are supplied with INOX steel ring

http://www.ebay.de/itm/AUDI-S2-RS2-2...-/111974920159

[Thuppu]

Reinz is about 82.25mm so it can be snurred out to fit 82.5mm. Cost ~ 30e.

[gnutz2]

I've been down the route of enlarging gaskets and it didn't create a seal.

The middle (0.1mm) gasket layer is folded back to create a fire ring around the cylinder and any machining detaches this part.

[SteveH]

Bore diameter: 83,50mm

Athena Cooper Ring gaskets, that are supplied with INOX steel ring

http://www.ebay.de/itm/AUDI-S2-RS2-2...-/111974920159

Nice, but rather expensive from memory!