1960 Matchless 350 piston replacement and engine inspection

[Triumph-Legend]

They all do that sir. Nothing to worry about.

Sorry but could not resist saying it.

Give the sump a good clean out and then clean it out agian. blow out with air line to get rid of any bits that may be floating about and clean again.
Chuck a load of oil onto the big end and main bearing areas. You don't want to start with dry bearings do you.
Rebore & hone barrel to suit a nice new piston. I think the old piston is past it even with the rough bits filed off.
Good idea to remove and check to see there is no damage to the valves / guides. If parts are good and true not worn out and valves bejond a gentle refacing.
Grind valves in and refit.
Bung the top end bits back on and after a bit of running in it should be good for another fifty plus years.

[AFSMatchie61]

I like the cut of your jib Sir 😀 Have you done this clean out the bits thing with success on your own bike? Would the engine come out the frame or do you mean let it drain (has it got a sump plug!?)
Thanks
Dave

[Duncan]

What is the gold coloured diamond embedded in the back of the piston?

[clive]

Before you proceed with measuring and replacing the piston i would want to work out what happened or you risk repeating the problem.
Clearly the piston parted company around the oil ring groove which is the weakest spot because oil holes are drilled through it at this point. But why? Did the damage result in bits falling onto the flywheels and getting trapped under the piston skirt resulting in the damage to the skirt? Alternatively was the bore too worn for the piston resulting in piston slap when cold and resulting in damage to the skirt. The crack could then have worked its way up to the oil ring groove.
The long split as Alan says was part of the original design its the other jagged one i am talking about. Given the damage i would say you need to rebore the barrel to the correct tolerance for whatever piston you can source ensuring there is clearance between skirt and flywheel. You should also split the crankcases to clear out all the debris. I would check the oil lines to ensure nothing has got there. Personally i have got away without splitting the flywheels as the engine will have stopped pretty instantly and much should not have got into the big end. You might need to clean or replace the main bearings though.
Sorry to sound negative but in the 80s i lost the top of 3 pistons on a 500 single until i eventually worked out tge replacements i was buying had the expansion slot cut by disc and it was also cutting into the gudgeon pin boss. This allowed the crack to expand up to the oil ring groove. I was using it to pull a fully laden double adult sidecar at the time which may have contributed.

[clive]

I like the cut of your jib Sir 😀 Have you done this clean out the bits thing with success on your own bike? Would the engine come out the frame or do you mean let it drain (has it got a sump plug!?)
Thanks
Dave

Yes i have done it after one of my described piston failures but the destruction was nothing like as serious as your piston. It worked but in your case???

[56G80S]

Duncan is spot on with his question.

This is what I saw in the piston of the G80S following failure of the timing side bush. It really looks to me as if the safest course is to strip down as Clive advises. I'm not suggesting that it's the timing side bush in your case but brass / bronze pieces shouldn't be there, they're not part of the piston, barrtel and head structures. You've checked the small end (my current one is white metal but I understand some aren't).

So where's that from? Rocker box is very unlikely as there's no real entry point.

You need to know.

I would.

Johnny B

PS - edit - did you take a look at the residue in the engine oil?

[AFSMatchie61]

Thanks Chaps, that's actually a small wood chip from the chopping block I photographed the piston on, although it does appear very like bronze in the pic! No bits of bronze anywhere although a look in the cases does reveal tiny bits of silver metal, perhaps piston debris.

Still to drain the oil and inspect for metal filings, obviously I'm hopeful it will be clear as this would be good news for the big end in particular.

Based on the damage to the piston and various discussions here and elsewhere my view is that metal fatigue is the reason for failure.

Cheers
Dave

[56G80S]

Red face here!

Johnny B

[Triumph-Legend]

I like the cut of your jib Sir 😀 Have you done this clean out the bits thing with success on your own bike? Would the engine come out the frame or do you mean let it drain (has it got a sump plug!?)
Thanks
Dave

Yep got the tee shirt. plenty of parafin or petrol on the rag and push the rag round the crank case by rotating the flywheel. Do this quite a few times with clean rag until it comes perfectly clean. Check B/End play and put the gudgeon pin in to check rod straightness. Also a good idea to check crank rotation and feel for any odd free play or lack of smoothness.
Why strip down a complete motor for the sake of a knackered piston. Golden rule - Cleanliness beats all. With the case, flywheels and B/end all dried a good blowing out with an air line and a couple of final wipes round should see all clear and clean. Then a good soaking in engine oil to all moving parts before reassembly.

Has this been do recently. Hmmm. Good question. Yep, about five weeks ago. A friends Enfield let go with a bit of a clonk. Piston in bits, head a bit speckled, one valve head tweeked a bit. Good clean out, new piston, valves removed checked and one replaced. A bit of smoothing out of the C/head. Chuck it all back together and bobs your aunty. A bit of running in and all was fine and still is. I think the general consensus was that the cause was trying to get Yam R6 performance from a rather more mature single.

Try competing some two strokes and see how many pistons you get through. If you did a strip down every time a piston was knackered there would be no track time only work bench time.

[Triumph-Legend]

Thanks Chaps, that's actually a small wood chip from the chopping block I photographed the piston on, although it does appear very like bronze in the pic! No bits of bronze anywhere although a look in the cases does reveal tiny bits of silver metal, perhaps piston debris.

Still to drain the oil and inspect for metal filings, obviously I'm hopeful it will be clear as this would be good news for the big end in particular.

Based on the damage to the piston and various discussions here and elsewhere my view is that metal fatigue is the reason for failure.

Cheers
Dave

Dave, I pretty sure the wood chip is from the B/end bearing. A number of manufacturers used to use lignum vitae rollers in a softer boxwood cage.
I'm not sure of the P/Number, but I think Gary at club spares may be able to help on this.
I still think that most of your problems could be sorted with a set of rings and a bit of careful filing.

On a lighter note.
If the bore is not scored, don't waste time boring. See if a quick hone is all that is needed.
Also you will probalbly find that the new piston gudgeon pin will be over size. This from past experience. Pistons are OK, maybe a little bit heavier but not a lot. But the O/S pin is a pain as it means opening out the Small end bush. If possibble try to get a NOS Heplex piston of the correct size.

Stay Well,
Ady.