AJS & Matchless Owners Club

Hi Folks, I'm needing help (absorbant shoulder to cry on) again... The G80CS that I've just got back on the road has really bad vibes throughout the revs, nasty at 50mph, unbearable at 60. It has cracked the brazing I did on the oil tank so I took the head and barrel off for a quick butchers.
The piston I am using is a Hepolite +20 which weighs 530 grams. Would this cause such bad vibes given that the original pistons were 486gms so i have read.
I am loth to remove the crank without definite need it was refurbished but i can't remember if it was balanced!
As ever, any help gratefully accepted, Dave.

Hi Dave, I have a wire-wound original in my G80, and a high compression JP piston (supposed to be heavy but I've never weighed it) in my 18S and the vibration is very similar for the two bikes. I do not think that the difference in weight between your two pistons would on its own account for horrendous vibes. If you have access to a lathe you could skim a bit from inside the lower end of the skirt and taper the inside ends of the gudgeon pin to knock a bit of weight off. Hopefully someone wiser than me will offer their thoughts! Andy

Dave. In your very first post you said

Dave T wrote:HELLO, I'm new to the club and forum. I've just "invested" in a G80C '54 vintage, bought semi dismantled and now repainted, engine refurbished( thank you T&L) and under process of remantling.

You cannot remember if the crank was balanced. What did T&L do to 'refurbish'? Did you supply the con rod and piston to T&L? Don't you have an invoice from them detailing the work? Have you (or T&L) replaced a 486gm piston with the heavier Hepolite or was the Hepolite with the'engine when you got it?
Waddever. Since you've not ridden it before the rebuild you don't know if it vibrated previously and it's difficult to guess the cause of the vibration without more detail. It could be due to many things (loose engine or gearbox or frame bolts/studs, steering head, wheel/tyre balance, badly adjusted chains.....) but it might help if you can be a bit more specific about where you feel the vibration (handlebars, feet?), whether it is high frequency (buzzing/rattling/tingling) or low frequency (thumpy/pulsing) and the work done by T&L.

Thanks for the replies.
The vibration is general, in the bars, footrests and up the seat! Literally, if I close my teeth together gently they buzz! I've never had vibes so bad. When I looked at the oil return pipe in the tank and blipped the throttle to encourage the flow a bit the whole tank blurred.
I kept the +20 piston in the bore which has about 6 thou gap (half worn?) and T&L renewed the big end (it was loose!) and the mains. I don't think they balanced the crank because I didn't rebore the engine, I have heard bad reports with JP pistons and wanted to hang on to the Hepolite! I am still looking for the T&L bill.
The engine is tight in the frame, really as tight as I dare.
It's not wheel balance, it's too high frequency and I've not gone that fast, 65 after a rush of blood, anything above 45 really buzzes.

I'm really annoyed, I thought I would have a nice old plodder for pootling round the lanes here but it's just too uncomfortable. It starts well, handles nicely and brakes when it wants to!

Thanks for the extra info. I'm at a loss to suggest much except that since you said the bike (and engine?) was 'part dismantled' when you got it, could it be that the piston and flywheel assembly balance factor were already totally unmatched and that was why the P.O. had partly stripped th'engine, intending to sort it 'one day'? As you do (or don't).
I don't think it will ever 'tootle around' like say a 350 or 500 touring model with lower compression and softer cams would but it really shouldn't rattle yer dentures like that. 'Bad' vibration is a subjective judgement so maybe your expectations are too high?
As a last resort you could try fitting weights to each end of the 'bars to shift the vibration frequency up or down the engine/road speed. Many modern bikes have these bar end weights. It probably wouldn't stop brackets/tanks cracking though.
Just a later thought: You have fitted the rocker box stay/heady steady I suppose?

Head steady shimmed to good tight fit on frame, I'm too old for those shennanegins! I've surely missed something even more obvious!

I say pottering about but I do like a bike to be "brisk" when I want but the tradeoff with this C is a bit much.

More head scratching and wallet opening required I think!

Thanks for the info! Dave.

Just a thought, could valve timing affect vibes? My inlet cam is on old one with a dot, no numbers, which I fitted with the timing wheel at "I forget where"°.

It's all getting complicated now, I prefer simples!

Think I'll take it all to little pieces and start from scratch.

All to little bits now, I found this:
and they weigh:
So I now wonder where I can find a piston weighing 70gms less than my Hepolite. The flywheels say G9.
Am I faced with rebore , then get crank drilled to balance the new piston?
I don't think I'm courageous enough but I could probably get better than 70gms!
At least now I know where the vibes come from.

I just got a +40 piston for my G80CS from PML Covers in the USA. It weighs 463 grams bare (no pin, rings or circlips). Are we to believe that your bike was running with magnets stuck to the flywheel? The silvery ones look like Hard Drive magnets. They must be bloody strong. No wonder it shook. TTFN Jack

It's strange what finds it's way inside an engine isn't it?
My piston with rings, clips and gudgeon pin weighs 510gms ergo, if it is 70gms overweight as it is a replacement +20, the original one must have weighed 440gms, are pistons of that weight available?