over bore questions
 

After rocking a signature that said something along the lines of "better yet get a 1.6" for a couple weeks, I have broken down and seen that there is no replacement for displacement. I am still going to run the 1.6 for an undetermined amount of time, but I see a 1.8 in the future. However I want more than 1800cc. I know flyinmiata has the 2.0L stroker kit that adds 4mm of vertical movement and 1.5mm larger pistons. Then there is the Maruha 2.1L which has a bore of 85.5mm (2.5mm overbore) and stroke of 89mm. While searching I came across a thread on m.net where someone said the max. over bore is 3mm putting me at an 86mm bore with the stock 85mm stroke (if my bore/stroke findings were correct, everywhere shows something different). I'm getting 1974cc's with the 86mm bore, can anyone verify this? Also is the 3mm over bore considered safe, albeit still a maximum, or should I go lower?

I did search, and found this thread with lots of good info from pat, which got me wondering about this.

https://www.miataturbo.net/forum/t17594/

86mm = 3.3858 inches. -> ~120.50 in^2 or 1.975L.

I wonder if BP can be bored that much... I'm gonna look into this FOR SURE. When I build an engine this summer, I planned to get the biggest pistons available. If a 86 will go, it will be done. :)


EDIT: And yeah, 1L = 1000cc, so your numbers were right. Link me to the thread you found that said 3mm was ok. Did they do it, or is this speculation? I'll have to talk to a couple machinist I know about how thick a cast iron cylinder needs to be. 3mm over is .118 inches over. That's a good bit.

it was all purely speculation on the m.net thread

MX-5 Miata Forum - Boring the engine - good result or just a bore?

One member says that 3mm is max, but 2mm is safe, then what is the millimeter in the middle classified? (if 3mm is truly too far, I would probably aim somewhere inside this pandora's box of a millimeter to push the boundries)

turning the boost controller up 2 psi for free > paying $5k for a stroker motor

I paid $2k for my built motor, I doubt the people with stroker cranks are having $5k more fun in fun.

For once....OMG I'm saying this....I agree with hustler.

Hustler is right...for the cost (considering you are going to turbo) there is no reason to put out the extra money.

I don't want the stroker, I just want more displacement for a lot less. If I can safely achieve 1975cc of displacement just by boring it over 3mm, then why not?

No reason not to if it's possible. Does anyone know what the bore spacing is on our engines? That's really what we need to know. I don't think I've ever seen this info posted before.

If this can be done, you'll need to get custom pistons made (no big deal) and a custom head gasket.

yeah I knew about the custom head gasket, and Wiseco custom makes orders anyways don't they?

have you ever had the head off? the bore spacing is pretty minimal...

I've had the head off my car more times than I'd like to mention. :) Never checked the bore spacing though. Hell, someone with a 1.8 head and a caliper can check it.

Also i've read that on turbo motors you don't want to overdue the overbore. N/A's can get away with hugh overbore. Not sure why but I would guess thinner cylinder walls, heat dissipation, etc... Anybody know why?

thinner cylinder walls = weaker cylinder walls

Thing is this is a cast iron block. Their fucking strong and rigid. But water jacket placement and bore spacing will dictate what's safe. Need some more info.

I found 85.5mm pistons and head gasket at REV9 when I was considering building a 1.8L. There is no price listed so I'm guessing you'd have to order them.

I believe the stroke is 85.5

I wouldnt go past that though.. Not turbo anyway..

I am going 84.5 and that +o.60

without any real numbers to back it up (my buddy used to have them on his computer, dunno if he still does). it is possible to get the 1.8 to almost exactly 2.0 with the stock crank. however, you would be boring the shit out of and sleeving the block....as others said, there isn't a whole lot of room to safely take out material.

after a bit of looking we found a set of nissan pistons that fit the bill pretty much perfectly.

but yeah, unless you had access to a machine shop i'd just buy a turbo kit.

I believe Solomiata used a crank from an F2 (Mazda B2200, 626, or MX6) to give their engine a 94 mm stroke. Cost? $25-$150 for the crank. Would still need rods and maybe pistons to make it all work but can certainly be done for much cheaper than the $5k kits. Of coarse this significanly increases average piston speed and will reduce high rpm ability. Truck owners find that the FE which has a slightly shorter stroke accelerates faster and will rev higher. I'm not sure how these compare to the stock Miata strokes though.

I have a buddy that bored his 1.8L out to a 2.6L and it was fully built and ready for boost

That sure is awesome, thank you for that.

I found this, it's for you and your buddy.

:eggplant::eggplant: