350 piston choice for an overbored cylinder

[Adz.F]

Hi,

Whilst digging through the boxes of parts in my WNG build, I have turned my attention to the cylinder, which apart from a small lip is in excellent condition externally. Unfortunately it measures at just over 73mm (not a bore mic but with a vernier) not including the minor wear.

As far as I can see it should be a 72mm piston.

I could get it sleeved and use a NOS WNG piston from the club, is this likely to cause issues due to decreased heat dissipation in your experience or perhaps wall thickness issues?

Is there a different piston choice i could use in a 73mm size range that would still work, with the right sized gudgeon pin and crown height?

Many thanks

Adam

[Simon.Gardiner]

Different piston in the 73mm range with the right gudgeon pin and compression height.....

Hmmmmm - how about a +040 Ariel 350 piston??

(Or maybe a +060 if you insist on a rebore!)

There's no problem with sleeving these barrels as long as you're not going racing (I think the Hepolite catalogues used to have a sleeve in the list of standard things they would supply - just saying that sleeves for these have been around for a very long time....)

SG

[will_curry]

A far worse thing to do to a cylinder than sleeving it is to powder coat it.
This really does limit the heat transfer away from the cylinder.

As Simon says sleeving has been going on for a long time. I ran a sleeved 500 for
three years, mostly two-up, and quite hard without anything nasty happening.

[Adz.F]

Thank you both for the information.

I'm not sure the +0.040 will do with the existing wear but I will do some more measuring this weekend.

Many thanks

Adam

[Vincent.vanGinneke]

Link to the Westwood Liners page showing the liner for the 350cc models :

https://westwoodcylinderliners.co.uk/pr ... ed-hunter/

[Adz.F]

So after waiting a very long time for my local shop to take a look, he's run a hone through it and it doesn't look great at +.040. Top limit but still has a visible lip at the top and a few small marks in the bore. I see a 350 'Slipper' piston at +0.060 in the spares scheme. Does anyone have experience of these in the WNG?

Thanks

Adam

[keith.mettam]

Hi Adam, what about this company they show +0.060" pistons in stock ? :-
https://www.classic-motorcycles.co.uk/c ... b_pid=5709
Good luck with it,
regards,
Keith.

[nevhunter]

The later style JP pistons are a vast improvement on the earlier pattern ones. The 3 piece stainless oil ring is common these days but a bit too effective in my view. My41 W/NG runs 3 comp rings and never shows smoke after wearing out 2 back tyres and a chain. The head has never been off since 1997. It doesn't leak oil either. (engine wise) Nev

[Simon.Gardiner]

Adam:
I'd smooth out the lip in the bore(so there's no chance of a top ring in a different piston hitting it), buy a +040 (if Allan still has one) and run with that while keeping an eye open for a NOS +060 or a replacement barrel that'll rebore to +040.
If I remember rightly those slipper pistons are extremely high compression - although that could be fun! Maybe have a word with Allan about it (also the spares list still dates from 2020 so stock may have altered).
I'm no big fan of JP pistons (they've not been kind to me and in my opinion they're a little too 'needy' ) - and experience has taught me that many engines run quite happily (if not perfectly) with all sorts of imperfections in the bores (my current VH has some appalling tram-lines down the barrel but you can still stand on the kick start - and yes, I am perpetually surprised by that!)

SG

[nevhunter]

It's only the area where the rings travel that affects compression. The "rest" can be pretty ordinary. The "lip" has to be removed or you can break a top ring. What then remains for consideration is the amount of taper in the bore. Check the ring gap( An old ring will do) at the most worn part and where it's not worn and divide the difference by 3 (close enough). Let's say it's .040 and .028 difference 12 so you have '.004" of taper which is about the safe limit or something to keep you going for a year or so till you can do it properly In these circumstances often the top ring groove may be worn and require a a wider ring and regrooving..Nev