Acceptable Piston to Valve Clearance?

Nourish

Joined
Dec 2, 2009
Messages
93
Country flag
I'm reading quite a variation on what people are recomending for the Piston to Valve clearance on the 850.
Jim Comstock if I read it right recommended 0.020". The Norton release notes on tuning the 850 to Combat spec 0.030 - 0.040 " but Mick Hemmings on the AN Technical Notes a massive 0.060" Things I imagine would alter the recomdations would be Conrod material an length, Piston weight and expected RPM.
I'm building a pretty standard 850 with a combat cam for the street!
 
There is a topic on this forum about shortening pushrods to accomodate the Combat cam. I bought one, however decided to not go down that road. The upside might be defeated by the downside. I am really impressed by my 850 motor - it is not far removed from normal. It just needs to be used better than it usually is. The only thing wrong with a standard 850 motor is the crank balance factor.
 
Minimum P/V clearance depends on whether you are considering whether it is an intake or exhaust valve.

Since minimum P/V clearance of the intake valve occurs on opening of the valve, float is not an issue like it would be for the exhaust valve which occurs as the valve is closing.
 
I used a minimum of 40 thou on my 500 race bike reving to around 7500. But Mike Hemmings knew what he was doing so I'd go with him if you wanted to be certain. If the pistons have valve pockets machined in them also check carefully for side clearance. I had valves touch at the side of the pocket. 10 or 20 thou should be enough sideways.

It's also very important to check clearance to valve spring coil bind. Some people recomend over 100 thou although I used 60 thou and never had any issues.
 
Nourish said:
I'm reading quite a variation on what people are recomending for the Piston to Valve clearance on the 850.
Jim Comstock if I read it right recommended 0.020". The Norton release notes on tuning the 850 to Combat spec 0.030 - 0.040 " but Mick Hemmings on the AN Technical Notes a massive 0.060" Things I imagine would alter the recomdations would be Conrod material an length, Piston weight and expected RPM.
I'm building a pretty standard 850 with a combat cam for the street!
Click to expand...
I think Mick's figure is a bit conservative, 0.050" is reasonable clearance in most circumstances. Bear in mind that certain valve spring set up notes say 0.50" from coil bound, so it seems wise to accommodate the valve floating to coil bound! That may not be realistic, but it seems to work fine. I am pretty sure I have not completed a build with less than 0.050"

Yes you can go tighter, your choice of the above limits, but you are talking valve to piston, not piston to head. So a decent clearance can be achieved in the pockets without compromising compression much. You should be guided by the pistons and rods you use.

Talking to Norman White, who had used Titanium rods at one point, it was clear that material would allow you to close up the piston to head clearance to below 0.010". But the rod is controlling the piston, it is not controlling the valve, so you would still need to maintain a good clearance piston to valve.
 
When I built my 850 Featherbed back in the early 80s using a 2S cam profile, my head was shaved as it was warped forget how much we shaved off, did it on a milling machine at work, when I was putting it together just made sure the pistons didn't hit the valves, didn't shorten the pushrod and to this day still running the original pushrods and valves, I set my valves in between the stock clearance and Combat clearance, been doing this for 42 years now and have had no problems at all and I running 40 oversize flat top Hepolite pistons with tight clearance from final hone from piston to bore when rebored.

Ashley
 
ashman said:
When I built my 850 Featherbed back in the early 80s using a 2S cam profile, my head was shaved as it was warped forget how much we shaved off, did it on a milling machine at work, when I was putting it together just made sure the pistons didn't hit the valves, didn't shorten the pushrod and to this day still running the original pushrods and valves, I set my valves in between the stock clearance and Combat clearance, been doing this for 42 years now and have had no problems at all and I running 40 oversize flat top Hepolite pistons with tight clearance from final hone from piston to bore when rebored.

Ashley
Click to expand...
Pushrod length will not affect P/V clearance unless it results in more valve lash.
 
Dan1950 said:
Pushrod length will not affect P/V clearance unless it results in more valve lash.
Click to expand...
Indeed, it affects the rocker position and angle compared to valve stem, but piston to valve clearance is a function of valve lift, driven by camshaft lift etc.

It's geometry mate!
 
acotrel said:
There is a topic on this forum about shortening pushrods to accomodate the Combat cam. I bought one, however decided to not go down that road. The upside might be defeated by the downside. I am really impressed by my 850 motor - it is not far removed from normal. It just needs to be used better than it usually is. The only thing wrong with a standard 850 motor is the crank balance factor.
Click to expand...
The 850 crank balance factor is just fine in a Commando, but not when rigidly mounted.
 
marshg246 said:
Those mentioning valve pockets - he has an 850 - no pockets!
Click to expand...
The OP mentions tuning to Combat spec, which implies a 2S cam and 0.040" off the head for a 10:1 is compression.....if you want the engine to turn over without a collision I am pretty sure you will need pockets cut in standard pistons!
 
Last edited:
But how deep to machine the piston pockets? I'd need to know the piston to valve clearance, I might not need to machine the head to increase the compression if I could minimise the gaskets!
I'm thinking somewhere in the region of 9 to 9.5 CR would be ideal for road use. In the UK 5 star fuel was available when the Combats were sold - sadly not so available now.
 
SteveA said:
The OP mentions tuning to Combat spec, which implies a 2S cam and 0.040" off the head for a 10:1 is compression.....if you want the engine to turn over without a collision I am pretty sure you will need pockets cut in standard pistons!
Click to expand...
Do standard 850 pistons have enough thickness to cut pockets? The 750 pistons have quite deep pockets - are you saying to cut 850 pistons like 750 pistons? BTW, 750 pistons are the same for Combat and non-Combat.
 
I've measured the piston to valve clearance that I have - 0.040" with a base gasket but no head gasket. JSMotorsport is saying that adding 0.040" to the headgasket reduces the CR by approx 1 and vice a versa.
The 850 is supposedly has a CR of 8.5:1. the base gasket measures 0.030" and a standard copper headgasket 0.040" = 0.070" gasket combined thickness that can be varied, so on a standard 850 you could reduce the combined gasket thicknesses to just 0.030" and have a theoretical CR of 9.5:1!
Now I'm using a different cam and have to keep a valve to piston clearance. If I exchange the base gasket for a 0.030" head gasket I still have the clearance but I've reduced the combined gasket thickness of a Standard 850 set up by 0.040" (0.070 - 0.030) so theoreticaly I'd have a CR of 9.5:1
Now this all needs CCing and if the clearance for the valves requires pockets machined then it would'nt be very much. But the qustion was would 0.040" be enough piston to valve clearance?
 
The Drivle on the Factory Hot up spec , states ' the W&S valvesping , raised redline to 7.500 rpm ' . I fink they say 30 thou. minimum . Rather Very Carefully checked .

Wotcher do is Wind in the adj. ONE turn . From Closed . 0.040 " . And turn it over , Plugs OUT . Very Judiciously . If its TIGHT asinit dosnt turn very freely . Fingertip
style . So you can STOP if it goes CHING . Before it STARTS to CHING . Or itll CHANG . Then its BENT . And youll have to staqrt again . From Scratch .

Actually , useually youd just have the inner springs init then . AFTER having checked , at FULL LIFT , THEY are 0.040 Off Coil Bound . 30 is a bit tight , but you may get away with it .

WHAT you Wanna Watch , is the VALVES THEMSELVES dont collide. Tangle . If theyre oversize . WHICH is where the first said check is indicitive they wont .

It Think thats WHERE they mention ROD STRETCH , which reduces the Squish Band Clearance from 0.040. to 0.008 , as in 32 thou. ' take up ' in bearing slack , crank bow , and rod elasticity .

So If you hold it at 8.000 continuous , at 30 cleance , theyre only hitting by 2 thou .

Cheap Nasty cl;apped out valve springs and Youre Doomed , if you try doing that . Theyll meet the pistons half way . Which will be a compromise . Mechanically . In the space time continuem .

As in its not helthy for two objects to occupy the same space at the same time , no matter how briefly . Theyre apt to become overly familiar with the rest . Then. Simultaeneously .

Throw a degree wheel on it , while your at it . And get a note book & pencil .The best one for NORTONS is three foot diameter . You can then read 1/10th Degree clearly .
1/60th of a degree being One minute . '
1/360th of a degree being One Second . "
denoted .

Acceptable Piston to Valve Clearance?

Nazty chapz . getting synchronized .
 
Last edited:
marshg246 said:
Do standard 850 pistons have enough thickness to cut pockets? The 750 pistons have quite deep pockets - are you saying to cut 850 pistons like 750 pistons? BTW, 750 pistons are the same for Combat and non-Combat.
Click to expand...
Well, I think so, but we might have to question what is a standard piston?

here again is the factory Stage 1 info, with pockets..... https://commando.yolasite.com/performance-mods.php

Mick Hemmings supplied me with a set of Hepolite +0.040" 850 pistons with pockets cut in, to use with a head skimmed 0.040" and barrels skimmed 0.060" and a standard copper gasket. Don't get too excited, because the head had hemispherical combustion chambers, so it was still only about 10.25:1 compression. Camshaft was 4S lift. This was in '77/'78 and to be honest I haven't bought 'standard' pistons since then.

Clearly this was a compromise and only done after a valve embedded itself into one of the Omega pistons and the combustion chamber. It didn't turn over on first assembly and to clear the head I had to have a 45 degree chamfer cut around the periphery. Zero squish effect, and I remember the motor didn't rev quite as hard as it had with the ex works semi domed pistons. But needs must.
 
Last edited:
You must log in or register to reply here.
Back
Top