NA8 rebore

alanwil · Postby **alanwil** » Mon Mar 28, 2016 8:21 pm

I know this would have been posted before but I have looked at various threads and can't find an answer. I have also googled and all I seem to find is garble that confuses more than informs. I have stripped down my 1996 MX5 NA8 engine. The cylinders aren't a pretty sight. The walls are rust pitted - the engine has obviously sat around with water in the bores at some stage. There are also horizontal wear marks in the bore. The engine is original and has dimensions of 83mm. I will need to get the block rebored. Question - what is the safest I can go to with a rebore? Not sure if it makes any difference but I intend to install pistons giving around 11:1 compression. Grateful for any advice.

sailaholic · Postby **sailaholic** » Tue Mar 29, 2016 7:06 am

Safest would be 83.5. You can go out to 85 or 85.5 for memory but would need to get your block sonicly tested. Many fail the test leaving you at about 84.5 for memory.

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noobee · Postby **noobee** » Tue Mar 29, 2016 7:51 am

first take it to your machinist to see how much it needs to be bored.no point buying 83.5mm pistons if it needs to be taken out larger to clean up the rust pits,they can go deep.
Mick

project.r.racing · Postby **project.r.racing** » Tue Mar 29, 2016 8:05 am

+1 for what noobee posted.

alanwil · Postby **alanwil** » Tue Mar 29, 2016 3:54 pm

Thanks for the help. I haven't ordered the pistons yet, some machine shops won't bore the block without the pistons. I wanted the machine shop to look at the block first and wanted to know what my options were for a rebore. The cylinders weren't too flash. I was a bit surprised today when I was told I would get away with 84mm. Fingers crossed, we'll see how we go.

hks_kansei · Postby **hks_kansei** » Tue Mar 29, 2016 4:02 pm

All pistons have manufacturing tolerances and are slightly different sizes, so most good machine shops will insist on having the pistons first, then bore each cylinder to suit each piston (and of course, you then need to assemble it properly and put piston 1 in bore 1 etc)

"blueprinted" engines are all about making these tolerances as low as possible (not just pistons, but basically all the moving parts)

They should be able to measure the damage though and give you an idea on how oversized you need to go (they say we'll need to take 0.9mm out, you can then but 1mm oversized pistons and provide them to the shop before they start cutting)

if it were my engine, and I had a programmable ECU, i'd just get it bored out as far as it can safely go, or see if the BP block can be sleeved.

madjak · Postby **madjak** » Tue Mar 29, 2016 4:49 pm

I run 84mm Wiseco 10.5:1 pistons with a head deck to up the compression a bit more. They are a good middle size, and easy enough to get rings etc for. It's not hard to get another block if the pitting is too bad, but it would have to be horrendous to require more than a 84mm bore.