Advice re piston for ‘55 G3LS

[G3L1946]

Hi all,

It looks like the Matchless will need a rebore. I’ve spoken to my local, and knowledgeable, engineering company and sorted that end out. But I’m not sure which piston to purchase to replace the old, wire wound, one. Will the JP ringed piston be suitable as on club spares #800313/4 ) The present piston, see attached, is 20 thou oversized so a +40 will probably be required ( I await confirmation from the engineers ). A replacement wire wound, I guess, is not on the cards ( unless there’s one out there in a shed! )
Any advice, help or suggestions much appreciated.

Steve

[g80csp11]

the 800313/4 will fit 350 WD G3/16M Piston 0.040" 1947 - 1955 , however the clearance you need to run will be for a non - wire wound piston
Id recommend 0.0015" per inch of bore , so 4 - 4.5 thou clearance at biggest diameter on piston shirt
your rebore shop should do this for you when you get the rebore done

[G3L1946]

Hi ,

Thanks for your reply. I understand that 800313/4 will fit my ‘55 G3LS. I think what you’re saying is that a ringed piston requires a different clearance than a wired one? I’m putting my hands up to general ignorance on this matter!
I assume the rebore shop will understand all this and factor this in automatically when I turn up with the piston and cylinder? They are engine specialists so I’m assuming so.

Regards,

Steve

[Joker_Bones]

I fitted a JP piston to a '59 G3.
It came with fitting instructions, clearance given.

IMG_20230822_223043.jpg

IMG_20230822_223105.jpg

[Joker_Bones]

It was heavier than the Hepolite that came out.
I can't remember the numbers but appreciably so... perhaps 20% or 25%.
I machined some material away from the inside to get it closer to the weight of the Hepolite. I'm not saying it was necessary, I just decided to.

[Joker_Bones]

The wrist pin supplied with the JP was thicker walled than the original and was therefore also heavier.

IMG_20230822_224430.jpg

Also it was 0.875" outside diameter. The original was 0.874".
The little end bush may need reaming to suit.

[Joker_Bones]

Screenshot .jpg

[Joker_Bones]

That said... It has been okay since I fitted it.

[Expat]

Hi Russell, I’m going through the exact same procedure as you at the mo except at +0.020 oversize bore on my 1960 G3. I have the piston and ring kit from club spares too and like you, have found that the gudgeon pin is larger than the original, which slid out easily when the piston was warmed slightly. I was hoping the new one will slide into the small end bush after a spell in the freezer. It’s a slide fit in the new piston obviously.
Did you need to ream the bush after all?
Thanks, Steve

[G3L1946]

Hi Russell,

Thank you for that absolute wealth of information! I’ll digest all of it and discuss it with the guys down at the rebore shop. Likewise, interested to know if you needed to attend to the wrist pin.
Very much appreciated.

Regards,

Steve