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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

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.

Rivets said:

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.

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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?
.

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 “.

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 .

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.

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.

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.

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.

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

Firstly and this is important,
IT IS NOT A SCRATCH.
Scratches go along the bore not around it
It is a corrosion pit
It will be a lot deeper than it looks & feels but most importantly the rings will also have been damaged .
And this is verified by the big difference in the wet & dry compression readings
The piston grooves could also been damaged , they usually are .
As to weather the crack is the cause of your poor running that is one of those how long is a piece of string questions.
The wear & vertical scratched in the bore will not make the engine run better and that crack will not improve the running either.
It could be allowing excessive oil into the cylinder fouling the spark plug for example .
SO the first thing is to grind out the crack & polish the edges then new rings.
Have a good look at the air induction system as you are getting dust into that engine

Better would be to get the cylinder bored if you can get oversized pistons .
However if that is a cool bore, all aluminium engine then it can not be bored out
he alloy they use is an aluminium silicon copper casting alloy with high iron impurities & usually high chrome , nickle or titanium.
The last 3 come from remelting scrap and cause finner grain size .
The iron causes an iron silliside to to form on the wall of the casting, particularly is the core for the bore was a chill.
The depth of this high iron phase is very shallow but it provides a harder wear surface for the rings to run against.
This is std practice for model aircraft engines and basically the bore is just honed after casting.
Now I have never sectioned the barrel of an all aluminium engine so can not confirm this but I have sections a lot of model aircraft engines and made tons of casting ingots ( sticks like solder actually ) for factories making the castings .
And note Joed , because Australian business is lazy & we are stupid, we send the castings over to Italy to be turned into complete engines thus do the hard work to give a competitor an advantage over the locally made product .

Now getting back to your engine
If it is a cool bore the depth of the iron rich region may not be big enough to allow the engine to be bored out.
Thus the new bore will wear very rapidly in use because the rubbing surface will be too soft .
So if the engine maker does not supply oversized pistons that is why & the engine can not be rebored oversize .

Only you can answer that question. You are going to have to measure the cylinder, top middle and bottom. At each point you take one measurement and then take a second measurement 90 degrees to the first. The reason you need two measurements at each point, is to find out if the cylinder is out of round. You need six measurements. Then go to the engine specs for your engine and compare. If your measurements are greater than specs, you will need to rebore to the next available piston and ring size. If you decide to rebore, make sure that the proper size piston and rings are available. Carefully reread Bert’s post about reboring before you start, might not be able to rebore, depending on the cylinder.

bertsmobile1 said:

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 .

Click to expand...

That is insane but awesome

Main point is that damage can seen and I doubt anyone see .0025" of wear (.005 diameter wear). A caliper must be used. Here there is both the damage in that spot along with linear scratches. The rings will also have both damage the area seen standing out and other wear so their end gaps will be over .030" if placed in the cylinder.

Also without the engine's model, type, and date code we have no idea if it even have a .020" OS piston and rings available. Also the cylinder must be honed using a rigid hone as it will also be ovalled and tapered which a flex hone only makes worst.

thanks for the replies. Ok - the engine is a Rato R210 on my Ryobi # 80544 (bought in 2017)

Now I’m getting very confused as to what you have for an engine. One website I checked lists it as a Predator engine, but when I go to another site it gives Kawasaki part numbers. Second I believe you have two threads listing problems with the same engine model number, but giving us different descriptions. What’s going on here? Are they the same engine or different?

same engine my friend. Predator may be a clone or vice versa in my research - not sure. Yes it took me a while to find any documentation on this Rato R210- but I finally did : https://www.manualslib.com/manual/1314179/Rato-R160.html#manual

So why do you have two threads trying to solve the same problem? The bad cylinder is probably causing the stumbling problem.

Since this engine is an oddball (Rato) I'm thinking they probably don't have larger piston and ring options. I'll look into buying another engine (maybe the Predator).
thanks for all the help

On the Rato R210 only standard size piston and rings are available. Also the cylinder service limit is .0065" over the standard so you should not hone beyond .005 over.

Based on the photos you are missing a cylinder head, this will also cause poor running. My advice is to find a good used engine that will fit and buy it.

Joed756 said:

Based on the photos you are missing a cylinder head, this will also cause poor running. My advice is to find a good used engine that will fit and buy it.

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You think? Maybe that is why it will not run now...

Besides it was removed so the cylinder could be looked at and the pictures taken. Come on use your head for something besides a hat rack.

StarTech said:

You think? Maybe that is why it will not run now...

Besides it was removed so the cylinder could be looked at and the pictures taken. Come on use your head for something besides a hat rack.

Click to expand...

It's called humor.