Depth of Fully hemisphered head

jseng1

jseng1

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Anyone have the depth of a fully hemisphered head as in the photo (Factory short stroke). The only measurement I have is 22.3mm but I think they vary. I'm not sure if the measurement in the photo is correct.

Depth of Fully hemisphered head
 
Probably 100% irrelevant, and not helpful at all... Oddly enough I get the same 22.3xmm if I subtract the squish band height from the total depth of the combustion chamber on a stock early Commando 750 head. I'm guessing that fully hemisphered means no squish band ledge. I can't tell if that is exactly the case from the image.
 
My copy of the 750 short stroke head drawing is getting a bit faded now, but I make the nominal depth 0.896” (22.75mm). Definitely not less than 0.887” (22.53mm).
 
You might need a burette, if you are calculating compression ratios. However the actual compression ratio should not matter as long as the two combustion chambers are the same. When the carburation and ignition timing are adjusted, they compensate for the compression ratio differences.
 
I had a customer with a 21.7mm deep chamber so I wanted to hear some more. I'm trying to design pistons for him. I assume his has been milled because those hemi heads are too low on compression. Below is the kind of piston these need. That lower corner of the taper is level with the top of the cylinder.
Depth of Fully hemisphered head
 
My fastest 650 Triumph had pistons like that, I machined the crowns of two 350cc Gold Star BSA pistons to fit closely into the combustion chamber as a squish band. The little end bushes needed to be machined for the larger gudgeon pins.
 
I made a plate with a stem and a draw bolt to hold the piston while the edges of the crowns were being machined to fit an aluminium template I had made.
 
jseng1 said:
I had a customer with a 21.7mm deep chamber so I wanted to hear some more. I'm trying to design pistons for him. I assume his has been milled because those hemi heads are too low on compression. Below is the kind of piston these need. That lower corner of the taper is level with the top of the cylinder.
Depth of Fully hemisphered head
Click to expand...
That'll make some compression. Tight like a tiger.
 
I measured three regular unmilled heads and they vary between 21.9 and 22.9 which seems weird. And several people have sent measurements of fully hemisphered heads in the same range.
 
Question from this Layman.
Why is the Short Stroke being referenced as a 'Fully' Hemisphered Head.
I thought all 'Heavy Twin' heads were Hemisphered.
What's the diff?
 
The 740 & 850 heads were derived from the 650 SS head with insignificant changes to machining of the combustion chamber because of the valve positions which were fixed to the original positions. Also the cylinder bores were moved rearward on the 750 & 850 resulting in the chamber not covering the whole of the bore (not “fully” hemisphered). On the short stroke 750 the chamber was increased in diameter and centred over the cylinder bores to cover the whole bore of the 850 barrels. The inlet valves were re-angled to position the valve head correctly in the enlarged chamber. Thus it became “fully” hemisphered.
I hope this explanation makes sense.
 
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