Gudgeon Pin Fit

Information relating to the Matchless G80 or AJS Model 18 500cc Heavyweight.
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Gudgeon Pin Fit

Postby Halfwhelk » Fri Aug 02, 2019 4:09 pm

Following my previous post about pistons. I eventually got a new-old-stock original wirewound piston in the US, (thanks to Dave T for the Ebay link). Its +.020" but I don't care especially as I found some new Grant +.020" rings at the same time.
I got a new gudgeon pin too. Its spot on .875" (7/8") as per spec but I'm going to have to heat the piston to get it in.
So my question(s)...
I believe it should be a hand push fit in the piston and small end. is that correct?
If its a new factory piston and a new gudgeon pin, why doesn't it fit?
Should I heat it to get it in or relieve the holes in the piston to get a sliding fit while cold?
Thanks

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Re: Gudgeon Pin Fit

Postby Mick D » Fri Aug 02, 2019 4:23 pm

Hi

In my opinion the gudgeon pin should be a good sliding fit in the small end - you should be able to slide it in without heat, (just lube it).

The piston should be heated to around 80 degrees and the lubricated pin should then slide in with just finger pressure - I use a hot air paint stripper to achieve this.

I wouldn't consider relieving anything without further testing and investigation.

Regards Mick

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Re: Gudgeon Pin Fit

Postby cbranni » Fri Aug 02, 2019 4:35 pm

Ditto what Mick said.
only dead fish go with the flow

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Re: Gudgeon Pin Fit

Postby SPRIDDLER » Fri Aug 02, 2019 5:59 pm

Ditto; heat the piston but the oiled pin should fall through the little end slowly under its own weight.
Anyway, the point of my reply is to caution you to make sure you have the correct circlips. There are the round (wire) type and the stamped out types. You must check the shape of their annular grooves in the piston and use the correct ones.
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Re: Gudgeon Pin Fit

Postby clive » Fri Aug 02, 2019 7:24 pm

Sprids how does the pin fall through the little end under its own weight when it is horizontal lay the bike down perhaps? I agree it should be a sliding fit and no tendency to rock. I have always had to heat cold pistons to get the gudgeon pin in or out.
clive
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Re: Gudgeon Pin Fit

Postby Rob Harknett » Fri Aug 02, 2019 8:35 pm

I have only ever used rag soaked in hot water to heat a piston, to get a pin in a piston. Small end needed no heating.
I did have a bottom end fitted with new bearings and big end by Ken de Groom. He said bring the piston and barrel when you collect, I'll fit that for you. I had a nice looking + 40 barrel and piston I wanted to use. Ken could not get the pin in. He eventually used a blow torch to heat the piston. I was amazed. I never got far before the engine blew. There was not much left of the piston. Bits everywhere in the engine carb and exhaust. On telling the story at our section I heard two others remark, same thing happened to me with those Heplex with a diamond pistons. They were lucky thought. Their pistons only broke into two pieces. Below the oil ring was stuck in the bore above it still connected to the con rod.

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Re: Gudgeon Pin Fit

Postby SPRIDDLER » Fri Aug 02, 2019 9:48 pm

clive wrote:Sprids how does the pin fall through the little end under its own weight when it is horizontal lay the bike down perhaps?

:oops: Quite so, Clive. It's not a very practical suggestion as a way to check the fit when the engine's in situ. I edited my initial longer text before posting and it ended up as mix of two thoughts. :roll:
Last edited by SPRIDDLER on Sat Aug 03, 2019 11:41 am, edited 1 time in total.
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Re: Gudgeon Pin Fit

Postby Pharisee » Sat Aug 03, 2019 7:50 am

Are these pistons breaking because you're trying to 'mix and match' them with gudgeon pins that weren't supplied with the piston originally, possibly from a totally different manufacturer?

I'll admit, I've only ever used pins that were supplied with the piston and generally, these only needed the piston warming with a hot water soaked cloth to get them moving. I've never had to use a blow torch... that would be worrying indeed and I'm not at all surprised that the piston failed in service.
John

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Re: Gudgeon Pin Fit

Postby 1608 » Sat Aug 03, 2019 9:33 am

I have found that new GPM pistons don't need any heat to enable fitting the pin. I thought this strange and so contacted Gandini and was told that the latest materials used expand so little that no heat needed. I've had no problems so far.

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Re: Gudgeon Pin Fit

Postby SPRIDDLER » Sat Aug 03, 2019 10:07 am

1608 wrote:I thought this strange and so contacted Gandini ........

Surely he was assassinated in 1948?


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