G80/85CS cylinder heads

Information relating to the Matchless G85 500cc Heavyweight, AJS 7R, Matchless G45 and Matchless G50
mdt-son
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Re: G80/85CS cylinder heads

Post by mdt-son »

No one has stepped forward to offer an explanation, so I will do my best. Others may second me.

The firing slot required the spark plug to be positiond in a retracted position. At the compression stroke, the cavity thus formed is filled with a lean gas mixture. As the burning rate of lean gas is much faster than rich gas and droplets left in the combustion chamber, a high pressure develops in the cavity with a "bang" and a jet of hot burning gas shoots into the combustion chanmber, meeting the piston top on it's way up, and diverts upwards again in all direction like an umbrella, subsequently igniting the main gas charge on a wide front and all way around, probably evaporation droplets at the lower charge level by radiation as the piston crown will run very hot.
The idea was probably adopted from diesel engines, which all had pre-chamber ignition at this time (direct ignition and ultra-high fuel pumps appeared in the late 80's).

The design promised the advantage of a quick burning rate, and it was a bold move, but I am guessing the objective wasn't met. The "pre-charge" is too small to offer a strong jet and thereby a wide flame front (the cavity will not fill up completely because of the slot). Upon firing, the desired jet produced will not be a real jet because the pressure is too low, and there will be a lot of turbulence by firing through an elliptical and rough slot. So, the burn rate will be rather slow and possibly slower than pre-57 engines with their off-center plug location. Furthermore, the very hot but weak jet will heat up the piston crown locally, and it may promote a hole in the piston. Another disadvantage ist that the firing jet causes erosion to the cavity and slot, strong heating, and peaky pressure pulses, causing material fatigue. It is rather common to find the firing slot cracked towards the valve seats. Strong heating of the material surrounding the slot may cause detonation of the entire gas charge and imposes another threat to the piston crown. I have seen samples of cylinder heads showing clear signs of detonation (pinging).

So, with all these assumed disadvantages in mind, Jack Williams reverted to a traditional spark plug design placed off-center, but still admirably close to center of the combustion chamber. With improved swirl (a moving squish edge is more effective that a static one) the burn rate of gas charges in the 1960 head design will be much higher than in the previous design.

-Knut

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ajscomboman
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Re: G80/85CS cylinder heads

Post by ajscomboman »

GOLDSTAR wrote: Fri Aug 27, 2021 11:31 am just as a matter of interest, what was the purpose of the 'firing slot'?
kind regards
To make the bugger harder to fire up! It worked a treat too as the bikes with those heads definitely are more tricky to start than the earlier or later conventional plug hole.
mdt-son
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Re: G80/85CS cylinder heads

Post by mdt-son »

Further to the saga of cylinder heads: Here is a sample of a cylinder head made by Keith Rippin. Getting the geometry and the oilways etc. right is quite an achievement. Has anyone used one of Keith's cylinder heads, and do valve seats stay put? Improved anchoring of valve seats was the last change by the factory in the development story of the G80CS head. (G12 heads underwent a similar alteration.)
Anyway, here is Keith Rippin's copy of the 1960 cylinder head. Plug location differs, and I wonder why Keith chose a deeply countersunk angled orientation rather than a vertical one. In terms of combustion the orientation shouldn't matter. Maybe the objective was to avoid erosion at the pug bore.

- Knut

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Last edited by mdt-son on Sun Jun 18, 2023 11:45 am, edited 2 times in total.
mdt-son
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Re: G80/85CS cylinder heads

Post by mdt-son »

Another elusive cylinder head is the one by Andy Lee, a late variant of the 1960 stock G80CS head, albeit with a straightened inlet. This is a first glimpse for me!
I am not sure how Andy made them. Although the inlet looks like a welded one-off, the number of heads produced for the Rickman scramblers would make welding awqward, so I think it's a casting, albeit a little crude. Note different shape of cooling ribs at the rear. Although looking weird, the exposed inlet manifold may have been intentional in an effort to keep the inlet charge as cool as possible. (The massive filling towards the rocker box land shown below looks like a later repair.)

The head was probably cast alongside the 4 factory standard heads at Stones at Charlton, an aluminum and magnesium alloy foundry in proximity to the AMC factory, at 669a Woolwich Rd, London SE7 8SL. The foundry was closed in 2019.
https://charltonchampion.co.uk/tag/stone-foundries/

Andy Lee's head is shown below.(Photos are by James Holland.)
Andy Lee cylhead-1.jpg
Andy Lee cylhead-4.jpg
Andy Lee cylhead-2.jpg
Andy Lee cylhead-3.jpg
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mdt-son
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Re: G80/85CS cylinder heads

Post by mdt-son »

Marty wrote: Sun Dec 02, 2018 10:22 am '64 and later had 1 3/8 inlet port standard and inlet valve has 5/16 stem also
Unless I have been misled, all 1960-on G80CS cylinder heads feature the 1-3/8" inlet port and an identical inlet valve (026028). Notable difference for 1964 is valve seats anchored in the head casting (a feature shared with the G12 cylinder heads). It seems the alloy is too porous for endurance racing, promoting a high CTE and valve seats dropping out or starting to rotate, hence the need for anchoring. Maybe AMC could have chosen a different alloy instead.
I wonder which alloy Keith Rippin used. Anyone know?
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