Snapped valve - G2

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
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mn757
Posts: 30
Joined: Sun Mar 22, 2009 10:24 am
Location: WEST MIDLANDS UK

Snapped valve - G2

Post by mn757 »

Hi all,

About three weeks ago my G2 cut out whilst I was riding and upon attempting to re-start it there was no compression at all. This meant a six mile push home, and after that I couldn't face looking at it for a few weeks.!

However, stripped it down this weekend and found a snapped exhaust valve, the valve guide has been pushed out of the head, and the valve has hit the piston.

http://www.martin-north.co.uk/matchless/g2_valve.jpg

http://www.martin-north.co.uk/matchless ... _valve.jpg

My questions are;

1). The valve has snapped quite cleanly. Is this a common issue, or could it be just metal fatigue? Would you propose the valve snapped first, or is it more likely it hit the piston and snapped ?

2). Assuming the cylinder head is ok, how easy is it to refit a new valve and valve guide? Do I need any special tools to do this ?

3). When i get it built back together again - how does the exhaust fit in the head? Is it just an interference fit ?

If the head's damaged, would it be fairly easy to find another one ?
I'll also be replacing the barrel and the piston as well, I assume these are fairly easy to source ?

many thanks

Martin.


itma
Posts: 7721
Joined: Tue Nov 22, 2005 6:01 pm
Location: UK

Snapped valve - G2

Post by itma »

). The valve has snapped quite cleanly. Is this a common issue, or could it be just metal fatigue? Would you propose the valve snapped first, or is it more likely it hit the piston and snapped ?

well thats a first I have never seen that before; could be a duff part, or some damage caused by hamfisted assembly

2). Assuming the cylinder head is ok, how easy is it to refit a new valve and valve guide? Do I need any special tools to do this ?

easy enough to do but as you are having to ask this question I suggest you seek some advice from someone who has done the job or get a professional to do it


3). When i get it built back together again - how does the exhaust fit in the head? Is it just an interference fit ?

yes,

If the head's damaged, would it be fairly easy to find another one ?
I'll also be replacing the barrel and the piston as well, I assume these are fairly easy to source

Plenty of G2 spares about, but assume nothing as regard to supply of spares on a 50 year old bike; get in touch with Tony Surbey at AMC spares or start looking on ebay
wilko
Posts: 2337
Joined: Wed Oct 12, 2005 10:45 pm
Location: victoria AUSTRALIA

Snapped valve - G2

Post by wilko »

Possibly the valve collets were dodgy or not fitted correctly.
Don Madden
Member
Posts: 2882
Joined: Mon Jan 01, 1990 12:00 am
Location: USA USA

Snapped valve - G2

Post by Don Madden »

It was common engineering practice on British engines for the valve stem to have a slight taper with collets to match so that they gripped the stem directly instead of putting the load on the shoulder at the end of the stem. The sharp radius from the groove to the end shown indicates that it was not intended to be highly loaded and that is where your valve failed. You may have incorrect valve design, collets, or both.

All this is conjecture on my part so can anyone closer to the factory than myself shed more light on the subject?

Cheers, Don.
Eric
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Posts: 1221
Joined: Fri Jan 01, 1993 12:00 am
Location: WEST MIDLANDS UK

Snapped valve - G2

Post by Eric »

My theory for what its worth is that the valve was unable to return when it should have done.

No 1 thought is that it seized in its open position quite solidly in the valve guide. Thats why when the piston hit the valve it pushed the valve and the guide up through the head as far as it could at the time also bending the valve stem at the other end. The travel of the valve and guide upward was resticted by the position of the rocker, push rod and cam as these were not yet in the fully closed position, so the next weakest point was the valve stem below the collet where it seems to have spapped off.

All this assumes the two pictures are of dirrent ends of the same valve.Edited by - Eric on 30 Aug 2009 6:13:29 PM
mn757
Posts: 30
Joined: Sun Mar 22, 2009 10:24 am
Location: WEST MIDLANDS UK

Snapped valve - G2

Post by mn757 »

Thanks for all your replies. I'm waiting on some prices from Tony at AMC spares, however in the meantime i've found a cylinder head on ebay but it has completely different valve spring design - Would this be suitable for my own engine ?

http://cgi.ebay.co.uk/AJS-MODEL-14-MATC ... 286.c0.m14

many thanks,

Martin.
Don Madden
Member
Posts: 2882
Joined: Mon Jan 01, 1990 12:00 am
Location: USA USA

Snapped valve - G2

Post by Don Madden »

It was common to convert hairpin springs to coil, especially for racing, & some of my CS engines had this. But there is a good reason for the hairpins, as they keep the coil portion away from the hot areas of the head.

Cheers, Don.
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