1960 Matchless 350 piston replacement and engine inspection

[AFSMatchie61]

Hi All,
Following on from my ‘piston death' post this thread will chronicle the strip and rebuild. I will need help along the way as I am not very experienced when it comes to ‘bottom ends' ir indeed G3 singles so please do chime in and assume I know nothing!
So the Matchie was running like a champ a few weeks ago when I lost all power at about 50 mph and coasted to a halt, a strip down showed the piston had come apart, the top had Come off and hit the valves and parts of the skirt have gone into the cases. The damage to the piston is remarkable, big splits up the sides too.
The big end has no movement up and down which is good and nor does the little end. Last night I removed the piston and my next step will be to inspect and measure the bore.
I've read a bit about pistons and clearances and it's all as clear as mud! I have been offered NOS pistons and of course there are various new. Once I know what size I suppose I can narrow the field. I believe my old piston was a wire wound low pin (later) job with a split skirt
All thoughts welcome, pics to follow
Cheers
Dave

[AFSMatchie61]

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[AFSMatchie61]

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[alanengineer]

I think the vertical 'split' should be there. Also 350 pistons are still fairly common, so shouldnt cost the earth. Their are good strip down magazine articles to help with your rebuild. Best of luck

[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. If the barrel had been bored for a later type pattern piston and then the earlier wire wound one fitted the clearance would have been too great.
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 not much should 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 the 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.

[Mick D]

Hi

Difficult to be sure from the photos, but some of the skirt damage does not look that recent tending to support the fatigue theory.

Buy a good piston and have the bore sorted to suit, I wouldn't be tempted to try to fit a piston to the bore as is, (no matter how good it looks).

Regards Mick

[AFSMatchie61]

Thanks Chaps, yes lot's to think about, I agree the vertical split is part of the design, as to understanding the cause again that would be good, More research needed. One hypothesis is that it's seized and then the momentum of the flywheel has ripped the head off the piston, whether it was already weak is hard to say but it's entirely credible the crack was growing for some time. There was no piston slap, she was going like a train right up to the failure. I don't think there's an alternative to splitting the cases to remove swarf and inspect. What is critical, I'm told is that the new piston has the pin in the correct position, this changed depending on year of engine manufacture. I agree a rebore is the way forward but some basic forensics on it first will maybe help understand the Cause of failure.
Are there piston marks that will tell me if it is original or oversize?
Thanks
Dave

[Mick D]

Are there piston marks that will tell me if it is original or oversize?

Hi

If it is an oversized piston it will be marked on the crown: (probably), +0.020, +0.040 or +0.060 - twenty, forty or sixty thou over size.

I could offer a more informed diagnosis if you were to thoroughly clean all of the fractured surfaces with acetone, or similar, and a soft brush followed by blow drying.

Regards Mick

[AFSMatchie61]

Hi Mick
Cleaned up the crown. W142 I think is factory code for ww pistons perhaps?
BHB is the maker
Then there's K or is it M 60? Possibly 60 oversize? I dont know

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[AFSMatchie61]

A few more pics looks to have seizure marks front and back. The skirt damage Where chunks are missing at the back is recent as fresh metal Not sure about the cracks at the back will look again

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