Cylinder Bore Discussion
 

I am to the point that I will be purchasing pistons soon, and am thinking of going to either a 85 mm or 85.5 mm overbore.

Flyin Miata keeps their stroker kit to what I believe is 84 mm. Maraha punches it out to 85.5mm.

On this motor, I am planning to run a stock stroke and stock rod length, but will have at least 11:1 compression with nitrous.

The question I have is if cylinder wall failure is common (other than weak rods punching holes in them) when there have been a mixture of large overbores and boost.

I've only seen a few blown out walls in my research over the years, and if I recall they were stock bore. Maybe go 85mm just for that tiny margin of safety.

.5mm on each cylinder isn't going to be where you really net power. Its either in more rpm or more NO2. I'd agree keep it with 85mm and keep some safety in the build.

I went with 84 mostly so I can use a stock HG. I also had a rod try to escape on that block due an F-up at the machine shop. Even at 84 mm, the sleeve was starting to get thin on the sides of the block. I don't think it will necessarily cause a failure, but personally, I don't think I'll go bigger than that unless I go absolutely all out, sleeves and all.

You could also use a block filler to strengthen the cylinders. On many engines you can fill the block about 1/2 way up the bores with the special cement to good effect. You have to investigate the coolant flow and sometimes change the procedure a bit. Some engines need flow at the bottom of the bores, this is achievable with filling too.

My last motor had an 84 mm bore. I have one cylinder with a scratch, so I will have to go larger. I want to save my spare for my dream motor after I grenade this one.

I have considered sleeves.

One of the reasons for a larger overbore was I figured it would maximize cylinder head output. I am seriously considering a valve larger than the 1mm over I have now, at least on the intake side.

Well if you must go larger than 84, go all out. You have to buy the expensive HG anyway. You will just have to get a new block if something does happen to this one.

Custom soft copper HGs are not very expensive and can be reused again.

I will likely go with an 85 mm bore.

I went about as far as I can go with cylinder head porting without spending a fortune, and am specing a cam to match the work done on the cylinder head. I can only spin it so far with the parts I have and can afford.

As far as the nitrous is concerned, I am using a single stage fogger with polly lines. The polly lines are limited to about 150 hp, but are much easier to work with than the hard lines that can flow much more power.

Foggers are underneath the intake, and changing jets would be a living hell with the hard lines.

I am looking for power in every nook and cranny, and think a large bore is good place to get it, even if it is a little risky.

The larger overbore eats up your margin for wall thickness variation. The best thing to do is have the shop run a sonic wall thickness check on the cylinders so you know for sure you have good wall continuity. There are plenty of gasket shops that can make you a custom MLS head gasket. I got a custom composite made at Lubbock Gasket and they did a fine job- discussed the order before I shipped it in, and actually called me when they received the pieces to let me know and confirm my order. You send them the OE gasket and the specs/changes for the new gasket. They send you back your junk and you bolt it together. :D

if motorworx on m.net can do 81.5 on a 1.6 with 20 psi, then 85 should be easy sauce on a 1.8

I personally sonic checked a 2001+ block recently that had been bored out to 85.5 and there were places the walls were less than .060. Wayyyyyyyyyyyyyyyyyy too thin for performance use.

The main bad spots seemed to be the forward part of the cylinder wall just below the deck surface.

Yikes! :eek5: