Intake valve hits piston (2011)

In the end I couldn't resist changing 2 variables at once, I deepened and widened the intake valve pockets already existing in the pistons, and changed the valve clearance to Combat standards instead of Commando standards. Looking at the head (RH10) to cylinder interface, the fins are closer together than the rest of the fins. Didn't I read somewhere here that was the mark of the Combat? I'm left wondering if the PO had the head shaved in an attempt to raise compression, and was running right on the edge of valve to piston contact.

marston rhode said:

As one who was a member of the engineering staff at the time, I can state quite categorically that road-going production pistons, either 750 or 850, did not protrude from the top of the cylinder. I've built, and rebuilt, both enough times to be absolutely certain of that!
Any engine with pistons above the deck either have shaved barrels or non-standard pistons.

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Good on you. The Pistons in my photo are Omegas 10.25 to 1.

hobot said:

Ugh, right off the top of your head too. Will have to save that to files for easy reference.
Ok then do other Commando's pistons stick up above the barrel like my 2 Combats? If not why not? . I'm confused again.

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Is it because of the squish band on the standard head ? Protruding pistons would hit the head?

Cash

cyclegeezer said:

In the end I couldn't resist changing 2 variables at once, I deepened and widened the intake valve pockets already existing in the pistons, and changed the valve clearance to Combat standards instead of Commando standards. Looking at the head (RH10) to cylinder interface, the fins are closer together than the rest of the fins. Didn't I read somewhere here that was the mark of the Combat? I'm left wondering if the PO had the head shaved in an attempt to raise compression, and was running right on the edge of valve to piston contact.

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Sounds likely that the head has been milled. That was a fairly common mod on the 850s. It's also possible the cylinders have been shortened. I used to do that on 850s to get more CR, instead of milling the head, and sometimes a little of each. I wanted to leave material on the heads, which had expensive porting work, so I could true them up later. Stock cylinders at that time were readily avaialble and cheap, so it seemed better to me to modify them instead of the heads.

Right after the 850 came out, Norton issued a service release titled "1973 850 MODEL COMMANDO HIGH PERFORMANCE MODIFICATION INSTRUCTIONS", which detailed a modification package that included installing the SS cam, opening up the valve pockets in the pistons, milling .075" off the head and reshaping the ports a bit, shortening the pushrods, and fitting velocity stacks to the carbs. This mod was supposed to bring the CR up to 10:1. Maybe the PO did some of the mods, but not all.

They also had another service release for road racing mods that included taking .090" - .095" off the head, using the 4S cam, again opening up the valve pockets in the pistons, more pushrod shortening, fitting 36 mm Amal carbs, and using a racing exhaust with reverse cone megaphones. That was supposed to take the CR up to 10.5:1.

A while back I posted the service releases on the INOA web site, and they are probably still there, if you are interested. I'd post them here too, but they are in Adobe Acrobat , not a photo format, so I can't post them to photobucket and link to here.

If you have a way to conveniently cc your head, you can determine how much, if any, it has been milled. A stock Mk2 850 head has a combustion chamber of 51.8 cc. Milling it .060" takes that down to around 43.3 cc, give or take a tenth or so. It's a fairly linear process, so removing .040" would put it somewhere around 46.1 cc, and .080" would be somewhere around 40.5 cc. That should at least give you an idea of what you have.

Ken

Any engine with pistons above the deck either have shaved barrels or non-standard pistons.

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I've 2 Combats, piston .050" proud of bare barrel surface, one an early March issue the other mid run Sept. issue and both came with the comma oil slotted pistons. I know niether of these were machined prior as both had the innate .003" barrel top mis cut and CHO heads, I corrected barrel top in what became Ms Peel but left Trixie with that minor wart. Ken has Ms Peel head so knows it wasn't miiled thinner and Trixie CHO head is bone factory. So I have remaining doubts while feeling a bit stupid to be contesting such basic construction with seasoned experts. Why does only hobot find proud pistons in factory Combats? Are non Combat head milled relieved around the bore rim for piston space?

The various factory and non factory manuals on Commando had instructions on bringing non Combat up to Combat configuration but have not pulled em out to detail yet.

A further identifying mark perhaps? The underside of the head had an 'L' stamped into it.

Hi all the 750 I had played with had piston protruding of 1mm (ish) and the 850 pistons had their crown just at the level of the barrels , and in my mind that's why there is a CR difference between them ???

Hi all the 750 I had played with had piston protruding of 1mm (ish) and the 850 pistons had their crown just at the level of the barrels , and in my mind that's why there is a CR difference between them ???

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Its hard to go by racer builders as they have nil concern but stay within race rules
inspections and hardly ever deal with a plain Jane like the rest of us.

Thank you for another in long list of annoying hobot reality checks, [nah nah na na nah]

*So what's causes the difference then between flush TDC 850's and sticking up 750's construction?* Btw, 1 mm = 0.03937007874015748 inches so .050" = ~1.27 mm. I'm not smart enough - yet - to detail out or measure the various factory numbers given on pistons for definitive answer.

Those who have tried to detune Combats by just putting in standard cam had detonation issues. Apparently the 2S over lap bleed off at lower rpms protects them some till the rev's build to pack it in with piston speeds out running the flame front pressure heating shock wave.

I want to avoid this problem with a 1973 750 I am assembling. The cylinder deck has been milled down approximately 0.05" such that it is pretty much flush with the top fin at the rear.
Does anyone know what issues I will encounter with this? Assuming all else is standard.