novaspyder87

Hi guys. so there we are. i want to know a little bit more about pistons and rods.

I might change mine because im loosing compression a little bit in one cylinder due to a piston that is scratched(spark plug cause that).anyway...


1- theres H rods...and there I rods...whats the difference???

2-86mm(stock bore)...86,5...87mm bore. whats does bigger one do?? more power?lower rpm range? does it make your car going to a 2,1 litre or even a 2,2 litre?

give me some info on that please...

ps: BTW my comrpession test on that cylinder was 125 compare to others all had 140-145...do I lost a lot of power....like a lot?????

UT_EvoX

I'm going to be helpful with this post, but preface with the fact that you need to do about another 6 months of reading before you start trying to wrap your head around planning an engine rebuild. Unless you want to entrust a shop to make all of the decisions for you.

First off, your piston isn't "scratched" - you're probably missing a piece of the ring land where your top compression ring pinched and caused pressure against the top ring land/crown which caused a chunk to fracture, melt due to the lack of a protective boundary layer, and then get pushed out your exhaust (and potentially sucked into other cylinders on their intake strokes, due to valve overlap).

If you smashed a spark plug ground strap and you're now low on compression, that's what happened.

1.) H-beam rods are generally a weaker design than more expensive aftermarket I-beam rods. Do a Google search to find out more on this subject - you will find a WEALTH of information on H-beam vs. I-beam rods on the Internet. There are valid cases to choose a cheaper H-beam rod over a more expensive I-beam rod, but I won't venture into those here.


2.) When speaking of a 0.5 or 1.0 mm overbore, the only purpose thereof is to correct a distorted cylinder bore or to correct wear/scoring/scratching/gouging that is present on the cylinder walls. Such a small overbore makes little difference in displacement, but will raise your compression ratio (CR) slightly (we're talking 9.1 CR vs. the stock 9.0 CR).

On the compression test numbers, 125 is still serviceable. You may or may not have a chipped piston, but it's for certain something is worn or broken. Will there be a noticeable or measurable power difference? Likely not, but whatever is causing the lower compression can definitely lead to a catastrophic engine failure.

novaspyder87

tks but yes my piston is really scratched....ivey did a boroscope test...and i was there to watch it...there was no chip gone or nothing except some very very small scratches on the top of that pistons.....due to part of spark plus that was moving around in that cylinder.

ok so ill go with a stock bore...tks a lot man :-)