AJS & Matchless Owners Club

hi, sorry for my bad English, i live in the Netherlands

We had, for a half year build this machine up, after it was disassemble for 5years.
Now after we had the Matchless G80 1952 checked by the authority , it has his license plate.

We did ride a 8km when suddenly the machine stops rapidly. Cool down and restart for 2km and again a heat seizure.
After disassemble the top end, the piston was too much damaged. ( at one side, ~ the fourth quarter)
We had measured the cylinder bore and piston diameter and found out
1 The temp cylinder was 284 Fahrenheit at a speed of 50 mph
2 that de clearance was to big. : Top Skirt : 0,441 mm / 0,017375328 inches.
We thought that that was the cause.
After buying the first oversize piston, and let it fit by the repair shop by the manufacturers specs .
We drove , and again a heat seizure. At 2 places at each side. Not so big , about 1/2 inch diameter .

Now we are confused about what the cause it is.
We fitted a brand new oil pump (with a greater capacity )
We replaced the big end needle's and aligned the crankcase by 2 dial indicators.
The crankcase covers are not from the same machine.
Both sides became new bearings , plain and ball.
new oil filter.
Now we are desperate , is it the oil circulation of to little gas oil supply?
Peter/ Yordie

Have you checked the plug? How does it read? If you're running lean it can cause excess heat and damage to the piston, valve seats, head dome, etc.

what carburetor and what jets are you running?, what slot is the needle on?

Main jet 300 and the needle on the largest setting.

We use a wassell carb.

Cfaber, we didn't get enough opportunities to check the plug

Not sure if something is being lost in translation? I understand that you thought the clearance between piston was TOO BIG. But this would cause piston rattle not an overheating seizure.
You then had it rebored to 20 thou over size bored TO MANUFACTURERS SPECIFICATION. But you don't say which manufacturer. Was it the manufacturer of the piston or did you go to the AMC records? In 1952 the AMC piston were wirewound and run to very tight tolerances. Modern piston cannot be run to these tolerances and the usual recommendation would be 0.0015 to 0.002 clearance per inch of diameter so not less than 0.005 on a 500? As you have seized the revote I wonder if timing is the issue. Try 3/8 before TDC.

PeterRozenveld wrote:Main jet 300 and the needle on the largest setting.

We use a wassell carb.

I am not familiar with that carb so can't help on that front I am afraid- hope you get it sorted though.

our first piston was indeed to small (to large tollerance) so we fitted a +020 JP piston with a manufacterers tollerance of 0.13mm specs where with the piston when we bought it. (dont know te inches tollerance )

we will try the 3/8 advance when i ran the bike on full advance the problem seems to occur sooner. thanks for the tip

when we fitted the larger main jet the engine was a lot cooler arround 100 degrees celcius. in stead of +-140

could it be that the fuel cap doesn't let enough air trough? if the bike runs stationair it runs great but on the road with 80km/h it seems to hold in and can't get enough fuel.

should we check the crank again for the oil channels? or do you think its a fuel problem?

we had been given an adres of a person who was an old mechanic and should have a lot of experience with these bike's so we let him look at the parts before we put the bike back together and try again. think that is a smart thing to do

PeterRozenveld wrote:our first piston was indeed to small (to large tollerance) so we fitted a +020 JP piston with a manufacterers tollerance of 0.13mm specs where with the piston when we bought it. (dont know te inches tollerance )

The clearance of 0.13mm is the same as 0.005" ('5 Thou') which is O.K.

we will try the 3/8 advance when i ran the bike on full advance the problem seems to occur sooner. thanks for the tip

The ignition timing must be set at 3/8" (9.50mm) Before Top Dead Centre with the magneto on full advance. If you don't do this the firing point will be too retarded (too late). If the timing is retarded you will get a good steady tickover but the engine will run very hot and hold back when being ridden. The first 20cm of the exhaust pipe may also turn blue/mauve due to overheating.

should we check the crank again for the oil channels? or do you think its a fuel problem?

If the piston is overheating and being damaged I would check the carb/fuel first. The oil flow in these engines is very low volume and low pressure and doesn't perform as much cooling as for example in a modern car. All oilways must be clear of course.

Which exact model is your Wassell carburettor? I believe it should be this 28mm bore unit ( One and one sixteenths inches).
Finally, as a Guest you have a limited time on here so if you haven't already done so it would a good idea to subscribe.

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