'70 750S cylinder spigots too low ?

Highflyer

Highflyer

New member
Location
Aigion, Greece
Hi all,

I was measuring squish clearance, when I noticed the spigot height above the barrels, with a used head gasket, was much less than the depth of the recess on the head.
It measures about 1,5 mm less.
Now, having a ring with a 1,5 by 2,3 mm section ending the squish band, spells detonation and/or unburnt mixture to me.
My first thought was to skim the head, but I'm very reluctant to take so much metal off.
I hope I make sence.
Please advise.

Regards

Angelo
 
I should imagine you'd want what you call the spigot to be less than the depth of the recess in the head to avoid the head sitting on the spigot. That would be why the copper ring in the head gasket is called a flame ring?

Paul
 
"cylinder spigot(s)" might be a wrong term for the upper 2-3mm of the liner that protrude above the cylinder block and locate the head gasket and head, but you get the idea.

Of course, otherwise the head gasket wouldn't compress and seal,
Problem is, it leaves a 1,5mm gap in which mixture is traped and might detonate or not burn at all.
It functions in the exact opposite way to that of a squish band.





 
The 750 combustion chamber is so old-fashioned, a little extra space around the squish band isn't going to make much difference in overall performance or efficiency.  Much of this space will be quickly filled with carbon anyway.  Paul is correct, the gasket flame ring will handle the heat and pressure that squeezes through the gap.

It's not rocket surgery...

piet
 
I see..
Better leave it alone then.
Funny you mention it Piet, the old-fashioned combustion chamber.
2000's V11 Guzzis have a similar chamber and some of them ping even under moderate  load at 4500-5000 RPM.
Installing differently contoured crown Hi Comp. pistons cures that - no retarding the ignition advance needed.

 
Highflyer said:
I see..
Better leave it alone then.
Funny you mention it Piet, the old-fashioned combustion chamber.
2000's V11 Guzzis have a similar chamber and some of them ping even under moderate  load at 4500-5000 RPM.
Installing differently contoured crown Hi Comp. pistons cures that - no retarding the ignition advance needed.
Click to expand...

Guzzis ain't exactly brain science either... ::)

piet
 
Highflyer said:
I see..
Better leave it alone then.
Funny you mention it Piet, the old-fashioned combustion chamber.
2000's V11 Guzzis have a similar chamber and some of them ping even under moderate  load at 4500-5000 RPM.
Installing differently contoured crown Hi Comp. pistons cures that - no retarding the ignition advance needed.
Click to expand...

I'd probably want to see accurate before/after compression ratio calcs before I went with that. Certainly interested to know what alteration in crown shape would have that effect without any reduction in the previous C/R. Interested also because I'm going with a different crown shape in an SF motor I'm building, 880cc Ross Racing hi-comp pistons.

I previously used std 750 Mondial pistons, upped C/R to 10.25:1 by removing 1.8mm from the effective deck height (.8mm thinner base gasket, 1mm removed from head after getting a measured 8.5:1 with std setup). Had to remove 1mm from the swish band to retain clearance. Ran std full advance but raised the spring tension on the advance unit to retard at lower RPM. never had major pinging issues, but top ring lands on cast Mondials don't like that level of compression ratio. In hindsight I should definitely have run less full advance (maybe 32 deg vs std 40 deg), but was many decades ago and I was on a learning curve.
 
What was the advance on your learning curve?

Are you still mucking about with points, springs and centrifugal bob-weights? Playing with advance curves is all much easier now with Ignitech's fully programmable ignitions.
 
Indeed Cam! And two lovely Ignitech systems I have all ready to go onto bike one and bike two.

I must say I did away with points in about 1978 when Japanese bikes started coming out with very reliable ND igniter boxes and magnetic induction pickups. I converted one from a GS Suzuki with little trouble but back then they still used centrf advance units so I retained the original and Loctited the elec trigger over a machined down points cam.

And my learning curve seems to be finally approaching full advance 😼 Actually that?s bullsh*t. You never get to full advance! Especially doing some of the crazy stuff I?m now doing to the big custom project!
 
Piranha Brother 2 said:
I'd probably want to see accurate before/after compression ratio calcs before I went with that. Certainly interested to know what alteration in crown shape would have that effect without any reduction in the previous C/R. Interested also because I'm going with a different crown shape in an SF motor I'm building, 880cc Ross Racing hi-comp pistons.
Click to expand...
Sorry, no info on that, only reports from members of https://www.v11lemans.com/forums/
You should contact Mike Rich, as I haven't be able to obtain a set myself.
He is a very knowledgeable and friendly guy in the States .
http://www.mikerichmotorsports.com/products.html half way down the page the pistons are listed.

I presume, if you up the compression, increase turbulence and eliminate gas pockets in the chamber, you get faster burning rates, so less chance of detonation.
How M.R. does that, I don't know, but I recall an article in "Cycle"  magazine, back when Honda 3-valve 400cc twins first came out, where they used silicon rubber to take a mould of the combustion chamber at TDC and then designed a crown to match.
Same thing with a tuner in UK with a 4-valve KZ750 that wouln't respond to any of the usual tuning mods, until the chamber shape was fixed.
 
Highflyer said:
How M.R. does that, I don't know, but I recall an article in "Cycle"  magazine, back when Honda 3-valve 400cc twins first came out, where they used silicon rubber to take a mould of the combustion chamber at TDC and then designed a crown to match.
Same thing with a tuner in UK with a 4-valve KZ750 that wouln't respond to any of the usual tuning mods, until the chamber shape was fixed.
Click to expand...

Ross Racing also have that facility - you send them a silicon cast of your chamber and they produce a piston to match. I'm pretty sure Red sent them a CNC readout of the SF chamber and they designed the OS pistons based on that. Originally I asked them for a std 74 SFC piston with a wider swish band (to accommodate the larger bore), but these pistons went into another bike Red was doing with a looming deadline and the redesigned ones came my way in exchange - don't ask him about how much fun it was getting that done to our satisfaction!  :-\
 
Dellortoman said:
Sounds interesting. Next time I'm in Melbourne I'll have to come and have a look at your workshop projects.
Click to expand...

Sure thing, Cam. Be warned, you won't find a pristine workshop environment. Things can get messy when I'm knee deep in various project mods!
 
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