Small End Bush Worn Out?

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
Mick D
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Joined: Mon Mar 24, 2014 5:44 pm
Location: Aberdeenshire

Re: Small End Bush Worn Out?

Post by Mick D »

Hi

Looking at the impact damage, is it possible that the piston has been fitted 180 degrees out? - just a thought for others with more knowledge that I to mull over.

Regards Mick
alanengineer
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Posts: 1175
Joined: Wed Oct 12, 2005 10:45 pm
Location: KENT UK

Re: Small End Bush Worn Out?

Post by alanengineer »

Hi, i hant been up to The Kent section for a while, but i do keep in touch. Its an active section with many knowledgeable people, always pleased to help. They always make me feel very welcome when i go and visit. Give it a try.

To help you in any more detail, it would be helpful if you told us what the engine number is. People have started to guess and that will lead to all sorts of mis directed help.
Halfwhelk
Posts: 37
Joined: Wed Jan 09, 2019 6:27 pm
Location: Kent UK

Mating the Head and Barrel

Post by Halfwhelk »

After some reading and measuring, I think I'm going to have to resurface the mating face of the barrel. Someone has tried to do this with a file (see earlier picture) with the obvious awful results.
Reading the supplementary instructions the only information I can find that's relevant is " A ground joint is used on the cylinder head and barrel".
What I don't know is whether the liner can be ground flat to the mating surface or whether it has to sit proud. There is a cutaway in the cylinder head to take a protruding liner.

I blued the barrel to see how it mated aganst the head and basically it touches almost nowhere.

I can't grind the barrel, but if it can be made completely flat, I can put it in the mill and fly-cut it. If I have to leave the liner proud I could mount it in the lathe and turn it, avoiding the liner.
I think however I do it, I'll need to fit a head gasket to restore the clearances.

Damn. Yesterday this seemed like a straightforward job. Now its a bit of a nightmare.

The engine number is 55/181878C. It has an alloy barrel.
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1608
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Joined: Mon Oct 05, 2009 1:00 am
Location: ESSEX UK

Re: Small End Bush Worn Out?

Post by 1608 »

I should leave the spiggot ( top of the liner ) well alone, as this creates whats called a fire wall and helps with the sealing. You need to check both barrel top and head for flat. Any machining of either means an appropriate amount will need to be removed from the spiggot. Personally I would use a head gasket and use a compression plate under the barrel to compensate for material removed from the barrel. I believe the inlet valve head is a little larger than the exh valve ensure both have not been swapped over, it has been known. As Spriddler has alrrady mentioned the possible cause of the contacting btn piston and valve could well be a sticking valve caused by overheating or build up of carbon. Check for bent valve head and stem and push-rod.
Halfwhelk
Posts: 37
Joined: Wed Jan 09, 2019 6:27 pm
Location: Kent UK

Re: Small End Bush Worn Out?

Post by Halfwhelk »

I have now machined the top face of the barrel so that it's flat. I had to remove about 60 thou. To compensate for this I'm going to make a compression plate to fit under the barrel. (cut from 2mm ally sheet?)

Can anyone please advise the position of the piston (standard flat top) relative to the top of the liner at TDC? It seems sensible that the bottom edge of the chamfer on the piston crown should align with the top edge of the liner, but I'd like to be sure. As 1608 suggested, I'm going to fit a head gasket too. I can deal with compression ratios once I have the thing running properly.
Dave T_LAPSED
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Joined: Tue Mar 29, 2016 7:19 pm
Location: CALVADOS FRANCE

Re: Small End Bush Worn Out?

Post by Dave T_LAPSED »

GEDC1319.JPG
Here's a pic of my C piston in the barrel, it was a wirewound +20 piston NOS that I bought from the US of A and he had five he might have one left. Yours is the wrong piston and i think it's a bit tall.
You do not have the required permissions to view the files attached to this post.
Halfwhelk
Posts: 37
Joined: Wed Jan 09, 2019 6:27 pm
Location: Kent UK

Re: Small End Bush Worn Out?

Post by Halfwhelk »

Thanks Dave,
I have adjusted the height of the piston in the barrels by making a 2mm alloy base gasket so that the piston chamfer is aligned with the top of the liner. The exhaust valve no longer hits the piston.
I made a shim to remove 60 thou end play in the exhaust rocker shaft. i also replaced the small end which looks like it had been sized to fit the gudgeon pin with a finger wrapped in emery cloth ( it fitted in the middle but had a thou play at each end, so it rocked). There is only one tapping noise left now and I suspect thats inlet cam end float. Today I shall go to the cams!
Dave T_LAPSED
Posts: 157
Joined: Tue Mar 29, 2016 7:19 pm
Location: CALVADOS FRANCE

Re: Small End Bush Worn Out?

Post by Dave T_LAPSED »

Blimey you've had your work cut out haven't you?
End float on the rocker shafts can be removed by moving the inboard bronze bush I've read somewhere. Just saying!
Just been for a ride on my G80 this AM, bit cold! The new piston I put in, a Robbins NOS at 380 grammes to replace the modern GPM which weighed 440grm vibrates much less and is a great improvement. I look forward to spring & summer now! Hope you get your's running nicely for the "season", cheers, Dave.
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