Xu7/ Xu10J4Rs Hybrid Piston Choice

[lynmorgan 1]

I am planing on starting the engine build that i have been thinking about and gathering parts for some time but i have a few issues.

 

For the bottom end i will be using xu7jp4 alloy block with 88mm hdi crank and was planning on using forged rods with off the shelf 85mm mi16 wossner high compression pistons inside custom liners. The plan is then to put a gti6 head top of this, but am i right in thinking that the exhaust valve cut outs in the pistons will need machining as they are designed for a mi16 head which has smaller exhaust valves ?

[Vili 18]

You're building "my" engine I'm very interested how this goes on. If it performs well maybe I'll build my version too. So far it has been done only in my thoughts.

About the valve cut outs: how big is the difference between the valves? Probably there is some extra clearance in the pistons anyway.

[dcc 855]

im sure meirion will pop his head in here anyway

 

but I think he tried a 88mm crank and it wouldnt turn in the block. the weights fouled the casing.

 

But worth noting with bigger liners comes more restricted flow of coolant.

 

if your buying custom pistons, then why not just get them made to spec?

[lynmorgan 1]

from looking at other threads the gti6 exhaust valve seems to be 1.1mm bigger than the mi16. The reason i wanted to use the mi16 wossner pistons is that they are stocked in 85mm and a lot cheaper and readily available than custom ones, but the only potential downfall i can see is that the valve cut outs may not have enough room for the bigger gti6 exhaust valve. As for the 88mm crank not turning in the block, ill have to look in to that.

[welshpug 1,666]

it was a 92mm stroke 2.1 crank that I tried would have given 1998cc on the standard 83mm bore IIRC.

 

it wouldn't need much material removing to turn, can't imagine 88mm being a problem but then again you can't account for minor casting differences.

 

 

you could have the cutouts enlarged easily enough, I had the ones in my gti6 pistons made deeper, so either colin+sandy or CRF450 could sort that for you as they are both tooled up for that operation.

[lynmorgan 1]

ill give colin a call to find out costing on getting the cut outs done, guess it will still work out cheaper buying the pistons and having them machined rather than going a custom piston route.

[welshpug 1,666]

depends how much you are paying for the pistons, but I doubt you will be far off the price of a set specified to suit.

[lynmorgan 1]

piston and rod combo from pec are a pretty good deal so thinking of going with them. yet to hear a bad report about pec rods

[welshpug 1,666]

speak to sandy then

Edited January 16, 2013 by welshpug

[JeffR 51]

If you're going to use 85mm custom liners, won't you need to have the block machined to fit them? The stock liners are pretty thin walled where they locate into the block already.

[lynmorgan 1]

yep the block will have to be machined out for fitment of the thicker wall liners. oh thats not good news mei, i was thinking the pec rods wouldnt be as light or strong as arrow or robson rods but was hoping they would be up to the job.