cylinder scoring question

[Boostedgmc]

this m20b25 is supposed to be freshly rebuilt, had an issue with the cylinder head and sent it out, but just found these marks in cylinder #5. cylinder has great compression, but was wondering if i have anything to worry about?


marks on the right in the picture, they barely grab fingernail and are about 1/4 inch long at top of wall

[pazi88]

I see sings of detonation on the piston. Those cylinder wall scratces are not end of the world. But maybe that cyl 5 runs lean and those are caysed by detonation also.

[digger]

that looks like something foreign got in there and kept getting squashed between piston and head. if i was rebuilding an engine id never use it like that if it was catching a nail as its right near the top ring. if it runs well with good compression there probably isn't much harm in leaving it except it might wear the top ring quicker

[e30davie]

"supposed to be rebuilt" let me guess, previous owner told you that ;)

As a general rule for mechanical things if you can feel the scratches with your finger nail its too deep and needs repairing. Your description sounds borderline I suppose.

Depends how many dollars you want to throw at it. If it ran good and has good compression it may give many years of service. If you have a spare few dollars, i would look at disassembling the bottom end also. Be aware that this can be a rabbit hole of dollars though...

[redlightpete]

Based on that photo, a more plausible story is “freshly replaced cylinder head after the timing belt broke and the motor ate a valve.” There are no cross hatch lines in the cylinder indicating that it was honed any time recently. But, as stated above, if it works you might as well keep running it.

[SkiFree]

Not even a hone job done. Look at the piston (upper left), something nasty had a party in there. I'm guessing it "was a really good deal".

[Boostedgmc]

yea i am having the same thoughts on the reliability of the bottom end being rebuilt, all the cylinders have good compression and look clean at least. i guess im gonna put the head on and see how it runs

[TobyB]

Yeah, something small and pointy bounced around in there for more than a few revs...
maybe when the belt broke, maybe some other time.

'freshly rebuilt' means 'I cleaned the carbon off the piston crowns with a wire wheel' in this case, methinks.

t

[jeffnhiscars]

My guess is someone was wise enough to raise that piston to TDC to clean the block off during a head install but was not smart enough to then lower the piston to remove the crap from the block that got wedged between the piston and upper cylinder. Doesnt take long to cause that kind of damage in that case

[95BMWIC]

yea i am having the same thoughts on the reliability of the bottom end being rebuilt, all the cylinders have good compression and look clean at least. i guess im gonna put the head on and see how it runs

Did you perform the compression check? The only reason I ask is the pic above looks to have a teaspoon of oil in the cylinder which will mask low compression. This is known as a wet compression test.

Of course, I'm just pointing it out - If you did a regular "dry" test and you got good numbers than what the hell - go for it..

You can also - bore and add a larger size piston set and a different crank from an "e" model - build yourself a stroker for a bit more HP..