Gudgeon Pin stopper

[pete.collings]

I am helping a club member rebuild his bike after nasty noises came from the bottom end. It turned out that the end of the crankpin had sheered off c/w nut, allowing the crank to flex. Amongst other faults caused by this, the gudgeon pin had hammered against the circlips, and enlarged the circip grooves. Square section segar circlips were fitted in this HC piston (made by Omega in the 60s), so they did not dislodge. After cleaning the pin bores, they appear to be servicable, and the pin is a good fit in the piston.

This piston is virtually irreplacable, and apart from some marking on the inside where the loose crankpin nut had bounced around, the exterior looks OK for reuse.

Doing a bit of research (in Radco's Vintage Motorcyclists Workshop publication), I see that an earlier method of locating the pin is to use End Pads made of brass or aluminium, fitting into the inner bore of the pin and extending to allow a little clearance from the cylinder wall. This would allow reuse of the piston.
Does anybody have experience of using these pads, and any info on what clearances to allow, and how long the pads should go inside the pin? In fact, any source of information on these pads (including specifications of materials to use)?

[robin.parker]

I think I have heard that VAG have used (and still list) such pads made from a material similar to PTFE, that may be worthy of investigation.

[pete.collings]

Thanks for the info, I presume VAG refers to the Volkswagen group. A preliminary search using VAG is a bit too non specific, if you have any further info that could narrow the search, that would be much appreciated eg. any name or part number that might reveal a specification for the material used.

I have some nylon rod that might be suitable, but don't know what temperature it would be able to withstand.

[chaz.wheeler]

Pete, the chap in the next unit to me drives and races Frazer Nash's and hill climbs a Becke power plus. all his pistons run PTFE mushrooms. have to be the corrrect grade as I gave him some ertecetal to play with but although looks the same doesnot have the correct results. the NAsh he races 1100cc runs in the top 5 at races using his own designed Maseratti blower running on methanol. so temperature is a big factor. his rods are special order carillo and pistons custom JP.
how desperate are you I might be able to see him somewhen this week.

[robin.parker]

Sorry Pete, I came across the info somewhere on a motorcycle forum, can't remember where, I'm afraid. VAG is indeed the Volkswagen/Audi group, I'd guess a friendly storeman, if you can still find them, would be a starting point. The ptfe pads may be referred to as buttons.

[nevhunter]

original Scott pistons use brass pads as do many earlier bikes Pre-30. A firm tap fit for about 3/8" is enough to hold them and vent one of them with a 1/16th hole. radius the outer face to approx the cylinder radius and allow about .040" less than bore dia for clearance. I have been using them for years . Aluminium is ok but expands more so may become loose after time. The 1' pin Ariels used them originally (Al)... Nev

[pete.collings]

Thanks for the reply Chaz, I would be grateful if you can get any additional info, especially on the exact grade of PTFE and where to get it. The bottom end has to be rebuilt first, so I am not desperate for info on the above, if you see your contact sometime in the next week or so that would be great.
Thanks also to Nev, that is the kind of info I need for turning up some pads.

[keith.hodgenia]

I've got a vintage bike with a villiers engine. This uses aluminium pads. When I stripped the engine there was a bad grove up the side of the bore caused by one of the pads. If other components in the crank assembly are worn the pin wants to move one way or the other. I would only use this method as a very last resort. Keith.

[Richard Kal]

My 1927 Model A has aluminium buttons in the "free floating" gudgeon; this was the standard factory design.


Regards,
Richard

[Danny.Lee]

I just noticed that holes in the pins on both my pistons have a shallow taper, this would affect whatever buttons or mushrooms you make

Regards

Danny.lee