Exhaust valve seat again G11CS

Information relating to the Matchless G11 or AJS Model 30 600cc twin
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Re: Exhaust valve seat again G11CS

Postby clive » Tue Dec 02, 2014 8:57 am

Les P H wrote:Would it be possible to get an oversize seat made up for you they are usually made of aluminium bronze?..Les

I looked into this with the first head. Depth of my valve seat non standard (for the company that supplies most of the valve seat replacements to the Trade). They could do the work but quoted 250 plus VAT for one head. Assuming the second head can be fixed in the same way I will take my risk with it, bodge or not.
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Re: Exhaust valve seat again G11CS

Postby Rob Harknett » Tue Dec 02, 2014 10:28 am

Duncan wrote:Rob

One of my failures was down to a previous owner peening the seats in, personally I would only ever recommend it as a get you home fix as in my experience it will eventually fail again.

This was a get you home job Duncan. The member had struggle through the night alone 200 + miles getting to the rally. Meningitis had sent him stone deaf, a bit difficult to communicate with him, although one section members learnt sign language to help. Being deaf he never knew he had engine noise etc. He looked so sad and lonely at the thought of being grounded for the whole relay, then AA relay home. So Malcolm Saggers said said lets fix his engine while he's away. Due to his illness he also had problems with balance, hence the switch to a combo. Nearly 20 years ago now, and I do not recall him ever saying he had swapped or repaired the head with valve seats peenned in.

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Re: Exhaust valve seat again G11CS

Postby clive » Tue Dec 02, 2014 4:58 pm

Quick update. The head is off and yes the valve seat had rotated, but of course it is an enlarged inlet valve seat not exhaust that moved. It was last time too, just poor memory. Not sure if this makes any difference on the advice. Also I forgot to mention that when I got the bike someone had skimmed the heads to the extent that they would not bolt down onto the gasket because the spigot on the barrel locked against the head before the gasket was even touched. We had to machine a bigger recess in each head to accept the spigot. This will have upped the compression ratio. The timing is spot on 3/8 before TDC on both cylinders (I think the slip ring must have been replaced during a mag overhaul). Maybe the increased compression is the reason for the slightly hot running?
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Re: Exhaust valve seat again G11CS

Postby dave16mct » Tue Dec 02, 2014 6:17 pm

I presume G11CS heads will be a bit thin on the ground, but have you thought about 1959 G12 heads? I see quite a few of these about and I think they may be similar enough to your heads to make the swap. The exhaust ports are 5/8" but perhaps you could sleeve the pipes. The '59 g12 and the 600 piston tops look the same shape, so combustion chambers will be similar. I have some I could measure the spigot diameter etc. It might be a cost effective way forward?
Cheers Dave.

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Re: Exhaust valve seat again G11CS

Postby 1608 » Fri Dec 05, 2014 12:12 pm

Depending on just how much material has been removed from the heads, have you considered de-compression plates under the barrels. This would also ensure you have sufficient clearance btn piston tops and valve heads. The correct compression ratio restored would offer a smoother and quieter running engine too.

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Re: Exhaust valve seat again G11CS

Postby clive » Fri Dec 05, 2014 5:56 pm

1608 wrote:Depending on just how much material has been removed from the heads, have you considered de-compression plates under the barrels. This would also ensure you have sufficient clearance btn piston tops and valve heads. The correct compression ratio restored would offer a smoother and quieter running engine too.

Would smoother and quieter also mean less performance? I am rather enjoying the current performance although I suspect it is making the engine more fragile. Not sure if these heads are standard for the 1958 CS or if they have been worked on to increase inlet port size. Its a standard size inlet valve butseems to be seating at the edge of the valve head. Opinion at the last section clubnight was that you could see from the way it had rotated that the seat had been machined in rather than cast in. So perhaps it had already been replaced?
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Re: Exhaust valve seat again G11CS

Postby 1608 » Sat Dec 06, 2014 2:10 am

I don't think a decompression plate the same thickness as the material removed from the head would make much difference to the performance. But it will be a bit kinder to the 50 year old motor, and may help with the hot running you described. I'm no expert on models as such, but if the CS indicates that it has higher comp. pistons ( does it?) fitted anyway why up the comp. even more.

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Re: Exhaust valve seat again G11CS

Postby clive » Sat Dec 06, 2014 6:26 am

So the next question is where to get such a decompression plate. I have 2 for singles but never seen one for a twin. I think my engine has standard twin pistons, at least they look the same as standard ones. Off to Kempton now so I will take a break and look around.
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Re: Exhaust valve seat again G11CS

Postby Les P H » Sat Dec 06, 2014 8:19 am

Before using a compression plate make sure you have sufficient tappet adjustment left as it's the same as shortening the push rods, in fact it could cause the the rocker contact point on the valve end to be off centre at worst as the rocker angle will be steeper to take up the adjustment. As far as getting a plate, you could trace out the shape onto some alloy sheet from a spare base gasket...drill the stud holes first and then cut out the two round cylinder holes... the outside shape does not have to be too perfect, but I would sandwich the plate with a gasket both sides....Les

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Exhaust valve seat again G11CS

Postby Samuel » Sat Dec 06, 2014 1:08 pm

Rather than one inflict one bodge to fix the effects of another, it would be better to continue to seek the correct 600cc cylinder heads!
Sam

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