Aluminium piston

jimmyw

Hi, is this normal for an aluminum piston and head to be pot marked like this?

The head gasket is gone I think.The oil started blowing out the breather badly and over revving. The engine is an intek 17.5 hp B&G lawnmower engine.

K.Flyer

Short answer... No.

It looks like something solid got sucked into the barrel and and has bounced around on top of the piston taking lumps out of it.
Any scoring on the barrell walls?
Maybe something has broken away and fell into the cylinder. You will need to check the valve's heads and seats for any bits missing, unless something got sucked in through a bad or missing air filter.
If the valve seats are damaged its not going to run right and the cost of repairs may not be worth it, same if the barrell is badly scored.

jimmyw

Wow! did you see the pic of the head I posted? It looks ok around the valves.The cylinder walls are grand and smooth.There is a thin piece broken off the head gasket, and its marked a bit on the gasket between the cylinder and where the pushrods go.This seems to be the only "damage"?

I thought I had more pics but they are not there.Will post some more tomorrow.

Thanks.

K.Flyer

The head looks like its taken a fair beating.
Tighten the spark plug back into the head and sit the head evenly on something with the inside facing up.
Carefully pour some petrol into the head and watch to and see if any petrol is getting past the valve seats, it will show up in the ports if there are any gaps.

jimmyw

Ahhhhhhhh yeah, it looks ok but a good test that, to see for sure, thanks.


Nothing fell out of the head or the cylinder during disassembly, but cant know what marked it.Apparently, there was some noise, (ringing sound, when at high revs,dont know didn't hear it myself) not a knocking sound like worn big ends.

By the way, there were no shells on the conrod big end, I assume there are not meant to be?

K.Flyer

Check in and the exhaust port on the head for small metal fragments.

CJhaughey

I don't think its valve related unless its a seat fallen out.
These engines suffered from undertorqued head bolts from the factory.
I'd say you could find internal damage inside if its run low on oil.
They burn oil until the engine goes bang and hopefully yours hasn't done so.
Was there much oil in the engine when you stripped it?

jimmyw

There is no scoring marks on the crank, only slight score marks where the crank exits the engine that would be going to the pulleys, blades etc.There is an oil pump on it and a filter with a drilling going out to the exit hole.When the pump is operated manually, oil is coming out this hole, so its not blocked or anything.There was a good amount of oil that came out of the sump, cant say exactly how much.

thanks, I will investigate further.

More pics to come.

jimmyw

The valves appear to be sealing as the fuel did not leak away. As I said previously,just light scoring on the exit hole.

I have found a small disc with the two holes in it that fell out of the head,but I can't figure out if it came from the engine,or if it dropped from something not related to the engine at all. It doesn't look like it's from the engine.

Does it look familiar?

jimmyw

After removing the oil control ring,the piston easily dropped into the cylinder without needing any compressor. So,either they are worn,or they are jammed into the groove.

gctest50

that disc is from the carb - the two screws that were holding it in place probably fell in and caused all the marks

jimmyw

gctest50 wrote: »

that disc is from the carb - the two screws that were holding it in place probably fell in and caused all the marks

You my friend are a lifesaver. You are bang on. I removed the carb and this is what I found. The choke valve doesn't look right either,but that's why the engine just kept speeding up. What I can't figure out is why didn't the disc itself not fall
into the engine,but just ended up on top with the valves?

I can't believe I didn't spot it earlier, but if I had taken off the carb, I would have. Just goes to show that you can't over look anything.

TeaBagMania

you know this is in the Gardening section

jimmyw

Yeah, hard to know where to put it,as it's not motors as such. (maybe it is?).I do agree it's only "loosely "gardening.

Mods can shift it if they want,I won't be offended ;-)

TeaBagMania

Nah, its fine. Just weird to see mechanical stuff in a place you expect to see questions about seeds, soil, and fertilization

jimmyw

The word. "lawnmower "is not mentioned anywhere else to my knowledge :-D lol.

TeaBagMania

Hey Jimmy, so what’s the plan, new head, piston, and carb? Id loctite those butterfly screws into the cross shaft upon reassembly.

jimmyw

I don't know really. The engine is belonging to the brother and he is just deciding.

I am still trying to figure out how the plate ended up around the valves. I assume that it was too big to get past the valves,but a bit confusing though. Would the screws alone cause this damage? Definitely was the cause of the over revving for sure.

The old rings are hard to take out of the piston, not enough of a gap to lever them out but if I take off the old oil control ring,I can drop down the piston in the cylinder with out any compressor. That can't be right.

If we do decide to reinstall the engine, a butterfly valve will be needed anyway.

TeaBagMania

Indeed the screws alone could cause that damage. This is why FOD (foreign object damage) searches are performed on aircraft carriers before operations begin. Even the smallest screw or rock will eat a jet engine.

As beat-up as the piston and head are you might not get full compression and I would definitely hone the cylinder.

You can buy a small honing attachment for your drill, this will clean up and small imperfections in the cylinder.

CJhaughey

jimmyw wrote: »

I don't know really. The engine is belonging to the brother and he is just deciding.

I am still trying to figure out how the plate ended up around the valves. I assume that it was too big to get past the valves,but a bit confusing though. Would the screws alone cause this damage? Definitely was the cause of the over revving for sure.

The old rings are hard to take out of the piston, not enough of a gap to lever them out but if I take off the old oil control ring,I can drop down the piston in the cylinder with out any compressor. That can't be right.
/QUOTE]
Your piston should be a sliding fit in the cylinder, don't forget that the aluminium piston will expand during use.
If there is no scoring in the cylinder walls then I would reuse the cylinder as is.
What is the ring end gap for the two compression rings in the swept area of the cylinder?

jimmyw

The rings are practically flush with the piston grooves so are not extending beyond it, so it will go into the cylinder without any compressor needed.There is no ring gap to speak of on the rings when you look at them on the piston, so removing them is very hard.

There is no scoring on the cylinder wall at all which is surprising given how much the head and piston were marked by the errant screws.

I have never heard of these screws coming undone before, but suppose there is always a first.

CJhaughey

jimmyw wrote: »

The rings are practically flush with the piston grooves so are not extending beyond it, so it will go into the cylinder without any compressor needed.There is no ring gap to speak of on the rings when you look at them on the piston, so removing them is very hard.

Sounds like the rings are locked into the grooves by varnish, soak it overnight in petrol and see if the rings can be carefully worked out of the grooves.
You might need a pick to start at the ring locating pin and once you get it moving it should start to move, heating it with a blowtorch might also help if you haven't time to soak it.

jimmyw

CJhaughey wrote: »

Sounds like the rings are locked into the grooves by varnish, soak it overnight in petrol and see if the rings can be carefully worked out of the grooves.
You might need a pick to start at the ring locating pin and once you get it moving it should start to move, heating it with a blowtorch might also help if you haven't time to soak it.

Cheers!

TeaBagMania

I would never attempt to reinstall the damaged piston and rings in my engine, bin the lot and get a new assembly