ET: Engine (Twin) Foreign Object

[nigsey]

I had a peak inside the cylinder bores of my C Rap using my new bore scope and noticed a ‘foreign object’ on the head by the exhaust valve. Using a piece of wire with a dollop of grease on the end I managed to extricate said object through the spark plug hole. The object is metallic and sticks to a magnet, it’s only a couple of millimetres thick and 6 mil long, it snapped in half easily using my finger nails. My first thought is that it could be part of a piston ring, on the camera it looks like there might be some light scoring on the liner but no pitting on the piston or head. The engine has done 14,000 miles since it’s total rebuild (carried out by the previous owner) the engine doesn’t smoke and still has honing marks on the liner, piston is +10
I guess it would make sense to remove the head and barrel to do a proper inspection but wondering if anyone can suggest where this foreign object may have come from.
Thank you, Nigel

 

[oexing]

Could be some carbon deposit from piston top or head. Can you look around with the endoscope for thick deposits anywhere for likelyhood of deposit ?

Vic

 

[Alyson]

it could have come in through the carburator. Are you running filters ?

 

[davidd]

Broken rings usually stay in place. Over time the bits of ring widen the ring groove, but they still remain trapped in the piston.

I suppose it could be a small shard that made it by the piston. The best thing to do is a leak-down test, but you probably don't own a tester. A compression test should tell. When I did a leak-down test on the above cylinder it showed 60% of the air passing by the rings.

Fortunately, I had a Nicasil aluminum cylinder liner in the muff and it is still running today 13 years later.

David

 

[nigsey]

Could be some carbon deposit from piston top or head. Can you look around with the endoscope for thick deposits anywhere for likelyhood of deposit ?

Vic

I must admit I did suspect it was carbon, there is a buildup of carbon on the exhaust side of the valve seat, I’ll post a photo of it. But would carbon be magnetic, excuse my ignorance?

 

[nigsey]

Broken rings usually stay in place. Over time the bits of ring widen the ring groove, but they still remain trapped in the piston.

View attachment 60851
I suppose it could be a small shard that made it by the piston. The best thing to do is a leak-down test, but you probably don't own a tester. A compression test should tell. When I did a leak-down test on the above cylinder it showed 60% of the air passing by the rings.

Fortunately, I had a Nicasil aluminum cylinder liner in the muff and it is still running today 13 years later.

David

Thanks David, yes I do have a new leak down tester but haven’t used it in anger yet. Need to learn how to find TDC on the motor, it’s all good experience though.

 

[timetraveller]

Carbon is not magnetic. If there are some iron filings mixed up in the carbon then it might seem to be magnetic.

 

[nigsey]

it could have come in through the carburator. Are you running filters ?

Yes I’m running K&N filters.

 

[nigsey]

Photo of open exhaust valve showing carbon buildup, front cylinder.

 

[nigsey]

Carbon is not magnetic. If there are some iron filings mixed up in the carbon then it might seem to be magnetic.

Ok, that’s helpful thank you.