Cylinder scratch cause poor running? Pic attached

[robmaxli]

Hello,

I've been trying to narrow down a poor running engine and found a scratch in the cylinder wall further down in the bore. I can feel it with my fingernail but its not too obvious.
Could this little scratch be the reason why i run so poorly?

Leakdown test was good
Compression : 90psi - cold w/ a little oil in cylinder. 67 psi cold w/ no oil

 

[Rivets]

Personally I don’t think that it is causing the problem. Looks to be below the piston ring travel. You can check this by doing these steps. First measure the distance from the top of the cylinder to the scratch. Second measure the distance from the top of the piston to the oil ring. Install the piston and rotate it to BDC. No need to have the rings installed to do this. Measure the distance from the top of the cylinder to the top of the piston. Add this measurement to the distance of the oil ring. If this distance is shorter that the measurement of the first distance, then the oil ring is above the scratch, and should not creat a problem. If it is longer then you have a problem.

 

[robmaxli]

Personally I don’t think that it is causing the problem. Looks to be below the piston ring travel. You can check this by doing these steps. First measure the distance from the top of the cylinder to the scratch. Second measure the distance from the top of the piston to the oil ring. Install the piston and rotate it to BDC. No need to have the rings installed to do this. Measure the distance from the top of the cylinder to the top of the piston. Add this measurement to the distance of the oil ring. If this distance is shorter that the measurement of the first distance, then the oil ring is above the scratch, and should not creat a problem. If it is longer then you have a problem.

thank you - I have a photo of when the piston was installed and when its at BDC, the top of the piston is below the scratch by about a 1/2 inch.
So the rings are going over the scratch during travel.
Is this enough to make it run poorly?
.

 

[Rivets]

Yes, I would definitely say you will be loosing compression because of that scratch. This will cause a lack of power, if that is what you are calling “runs poorly “.

 

[bertsmobile1]

That is not a scratch, that is a corrosion groove
There will be corresponding damage to the rings
Lots of wear to that bore from dust / grit / rust
Doubt that it will clean up even with a rebore
With cast iron bores you can some times get away by polishing the hole so the edges are smooth if it is that far down the bore
Aluminium bores tend to crumble in use regardless of what you do to them.

So long & short, try to polish it smooth then pop a new set of rings in after a light hone .
Not expensive to do & you have nothing to loose.

I had a gudgeon pin clip fall out of motorcycle engine at one time .
It ran OK up to peak torque ( around 3000 rpm ) after which it did an auto oil change via the breather.
Had too much else going on so I just lived with it as I was intending to fit an oversized piston
Finally there was a "death knock" so I pulled the engine down
The gudgeon pin had shifted m gouged a deep groove the length of the bore , right through the liner and into the barrel material till the pin ground down so short & shifted so far to one side that the piston fell off the con rod & broke up.
So you might get away with it, you have nothing to loose .

 

[StarTech]

Finally someone saw what I saw. It might clean up at 0.020 over but that is iffy. Must use a ridge hone and not a flex hone.

 

[robmaxli]

The scratch looks worse in the photos than it feels by the finger. The majority of that line is smooth but somewhere right in the middle is the only place I can feel it - and it is subtle.

 

[rgrottk]

I also doubt that is causing your issue I had one that had a groove 3 times that size the only thing it caused was a bit more oil usage. Most poor running issues are carb related, pull the plug ground it chk the spark if snapping good work on fuel delivery and carb.

 

[Rivets]

To you it might only be a scratch, but your compression test results indicate it is deeper that you think. Doesn’t take much to affect loss of compression. Depending on much you wish to stick into it, I suggest you follow Bert’s advice if you decide to save this engine.

 

[robmaxli]

Thanks for the replies. If I honed it out, can I use the same size of rings and piston?