84.5mm pistons for BP
 

Going to refresh my old 84mm bore BP block with a small overbore and some 84.5mm slugs. Looks like 84.5 isnt a common size and only wiseco and JE make an off the shelf piston in this size. I had 84mm wisecos in the car previously and would like to run them again. Wiseco 84.5 lists the compression as 8.5-8.8....so Im not sure if its 8.5 or 8.8. They both have the same part #. Does anyone run 84.5 wiseco that can confirm? Thanks.

It's probably cheaper to get another junkyard block and run 84mm pistons again than to use 84.5s in your existing one.

--Ian

The 84.5 isnt any more expensive though, I could use the existing block out of my car but I already have the old block out and ready to take to get machined. I dont think itll cost any more to bore 84 - 84.5 vs 83 - 84 would it?

When I checked a few years ago, the Wiseco 84.5mm pistons cost around twice as much as the Supertech 84s. Don't know if that's still true or not.

--Ian

The price must have come down because I can get K1 rods and 84.5mm wisecos for under 900 shipped. The Wiseco's by themselves were I think 525 or so.

Reason it's not common or popular is because without properly checking/measuring the walls you're basically playing Russian roulette with your cylinder wall(s) breaking

Ironic. I thought that was for 85s?

Maybe. But I’d still be nervous going bigger than usual with boost when bp blocks are so cheap

I did read about that, I thought the general consensus was that 84.5 is ok but going to 85 with boost is too thin. I do have another block I could use instead if the walls end up being too thin. I can talk to my machine shop and have him measure the thickness to find out. My buddy years ago ran 85mm with 28psi on his drag miata, made over 450 at the wheels and it was perfect, but I would rather not take any chances.

the problem is the inconsistency between block castings. IIRC either TSE or 949 reported in the past seeing some blocks ok at that bore and others being super thin

I assume my machinist can check this for me before he goes with the overbore?

Only if he has the tools to sonic test it

https://www.miataturbo.net/engine-pe...g-boost-69187/

Seeing dozens of people get screwed by liar machine shops, I'd probably ask to see the machine they use instead of just having them tell you "yeah we tested it, it's ok" which usually means "we eyeballed it, nothing stood out, so pay us our money"

gotcha, luckily the machine shop I use is VERY VERY good. The guy that runs it has built my old motor, and countless others so I know I can trust him. I read that linked thread previously good info in there. I know FM uses 84.5 on their stroker builds so more than likely I think ill be ok. Im still going to have my block checked though, thanks for the help boys.