850 Cylinder head Skimmed

Hi I have an 850 Cylinder head that is perfect in every way New valves guides and great threads However the head has been skimmed down to completely eliminate the built up area for the head gasket As a result the combustion chambers are very shallow. I am concerned that the valves may not clear and the compression will be quite high. I would appreciate any thoughts on this. I thought I might double up on base gaskets and I'm not sure if there are thicker head gaskets that could give me more clearance.
Thx Drummer 99

Drummer99 said:

Hi I have an 850 Cylinder head that is perfect in every way New valves guides and great threads However the head has been skimmed down to completely eliminate the built up area for the head gasket As a result the combustion chambers are very shallow. I am concerned that the valves may not clear and the compression will be quite high. I would appreciate any thoughts on this. I thought I might double up on base gaskets and I'm not sure if there are thicker head gaskets that could give me more clearance.
Thx Drummer 99

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Hi, first check the valves vs piston clearance with any plasticine (any model clay) , and see what you have , then Jim Schmidt have some ticker copper head gasket and /or "misnamed" compression plate under barrel .......more will chime in !

marinatlas said:

Hi, first check the valves vs piston clearance with any plasticine (any model clay) , and see what you have , then Jim Schmidt have some ticker copper head gasket and /or "misnamed" compression plate under barrel .......more will chime in !

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Yes , J.S. Motorsports for you.

I'd go with thicker cylinder base gaskets from J S too rather than head gaskets.

You biggest issue maybe push rod length. Have a good look at how the rocker contacts the valve. You may need to shorten the push rods.

johnm said:

I'd go with thicker cylinder base gaskets from J S too rather than head gaskets.

You biggest issue maybe push rod length. Have a good look at how the rocker contacts the valve. You may need to shorten the push rods.

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Why thicker base gasket instead of head gasket?

ILLF8ED said:

Why thicker base gasket instead of head gasket?

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Thicker head gasket is more likely to blow

I rebuilt a 750 combat for a mate of mine that wanted the bike detuned a little
I fitted a 1mm steel plate under the barrel,the bike runs spot on

What a bunch of girls more compression more bang for your buck, no only joking, I am running my 850 with hot cam and shaved head, yes it has higher compression but that's great on my bike and with the Joe Hunt maggie and longer kick starter the extra compression is no worries for me, but then I have been kicking my Norton for over 46 years now and the hot motor since 1982, I am still running the original valves and push rods and 40th over Hepelite flat top pistons no problems with valves hitting the pistons.

Ashley

Drummer,
I have the same head.At one time it was shaved .030 and when Comstock rebuilt the head he took off another.008 to make the head flat. I run a flame-ring head gasket (stock) and just have to go through the whole re-torquing process after install. I don’t know how much you head is shaved but I have no problems with mine with stock head and base gaskets.
Mike

YING said:

Drummer,
I have the same head.At one time it was shaved .030 and when Comstock rebuilt the head he took off another.008 to make the head flat. I run a flame-ring head gasket (stock) and just have to go through the whole re-torquing process after install. I don’t know how much you head is shaved but I have no problems with mine with stock head and base gaskets.
Mike

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The way he described it with no head gasket outline, it has been skimmed more than 0.040" possibly 0.060" More than that and you have very little of the squish left and will be machining pistons for some clearance.

A compression plate under the barrel is a good way to go with this problem, I bought a steel one some years ago just in case I got to this eventuality! Hasn't happened yet.

The only downside would be an increased likelihood of the barrels moving around. You certainly would not want to use two paper gaskets because that is pretty much guaranteed to move around.

Use Threebond 1184 each side.

I would need to measure it, but I know it is thicker than JSM copper ones.

I used a plate under the cylinders when I had the same problem, also check your pushrod lengths and valve geometry

ashman said:

What a bunch of girls more compression more bang for your buck, no only joking, I am running my 850 with hot cam and shaved head, yes it has higher compression but that's great on my bike and with the Joe Hunt maggie and longer kick starter the extra compression is no worries for me, but then I have been kicking my Norton for over 46 years now and the hot motor since 1982, I am still running the original valves and push rods and 40th over Hepelite flat top pistons no problems with valves hitting the pistons.

Ashley

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The "hot cam" lowers cylinder pressure at low RPM. My 86 Cu in Panhead stroker was 10.5:1 static compression ratio but I could start it with my hand due to overlap in the valve timing. While I knew how to start it with my hand and one kick start it with my foot, others had difficulty starting it because they would try to feel the static compression resistance of the stock engine that did not exist. It would also run on regular unleaded in a pinch as long as I did not lug the engine.

Measure the chamber volume with a burette, using a perspex plate with a hole in it. or fit the head with a standard gasket and tilt the engine so the plug hole is vertical,piston at TDC then measure using a burette. Work out the compression you have then you will know how much to lift the barrel to get back to a sane compression ratio. Any thing else is guesswork. Lifting the barrel would be my preferred method to reduce the CR. Around 50cc combustion chamber volume is approximate what it should be as standard.

For reliability its best to eliminate the base gasket - just use sealer. Then use whatever thickness head gasket you need for the compression you want. Changing the HG by a thickness of .040" usually changes the compression from 9 to 10:1 or visa versa etc.

jseng1 said:

For reliability its best to eliminate the base gasket - just use sealer. Then use whatever thickness head gasket you need for the compression you want. Changing the HG by a thickness of .040" usually changes the compression from 9 to 10:1 or visa versa etc.

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How thick would you advise to go with a head gasket?

In order for me to start my little Norton 750 stone cold by hand I'd have to be able to curl about 300lbs and bench 600. Fully hot it might be possible, but not at my age. My arm might come off. 2S, shaved head, flat top pistons. No valve interference.

+ Jim Schmidt is very helpful

If dry assembly and doing some measurements was mentioned, I second, third, or fourth it.

Here is a formula to calculate either the compression ratio change when adding a shim, or the height of a shim to obtain a desired compression ratio:

h/s =(CR1-CR2)/((CR1-1)*(CR2-1))

where:
h= height of shim
s = stroke
CR1 = compression ratio without shim
CR2 = Compression Ratio with shim

Keep the dimensional units the same; that is, height of shim and stroke must be mm or inches, or furlongs, but not mixed!

Example:

If 10.5 CR pistons are fitted and it is desired to lower compression ratio to 9.0 using the same pistons, and stroke is 89 mm, then the shim thickness required is:

h (mm)/89 mm = (10.5 - 9.0)/((10.5-1)*(9.0-1)) = 1.5/(9.5*8) = 1.5/76 = 0.0197
h (mm) = 0.0197 *89 = 1.76 mm
NOTE: the thickness of any gaskets used, in excess of the original configuration, contribute to shim thickness.

Also note, shaving (skimming) a head is equivalent to a negative shim height. What would the resulting CR be for the example above, if the head were shaved 1 mm?

Ans:
-1/89 = 10.5 -CR2/((10.5-1)*(CR2-1))
-0.0112 = 10.5 -CR2/((9.5)*(CR2-1))
-0.0112 * (9.5*CR2-1) = 10.5-CR2
-0.1066 *CR2 + 0.0112 = 10.5 - CR2,
gathering terms,
CR2 - 0.1066 CR2 = 10.5- 0.0112
0.893 CR2 = 10.489
CR2 = 11.74

Slick

baz said:

How thick would you advise to go with a head gasket?

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Stock composite and copper head gaskets average around .040"/1mm in thickness and this is the most popular for matching stock compression (practicality).

I prefer copper head gaskets because they don't crush like composites and a crushed head gasket will vary in thickness and might contribute to head warpage. And copper needs less re-torquing.

Copper doesn't always seal perfectly unless you coat them with sealer - pliobond is the best option for that.

Street bike customers with milled heads usually choose a thicker copper head gasket if the head has been milled too much and they want to return to stock compression to avoid pinging .

I've gone to a thinner HG on mine to bring up the C.R. a half point which works fine with stock cam and street premium gas.

So from my experience .040" copper with pliobond and no base gasket is the most reliable for stock set up.

Some go 1/2 point higher C.R. with a thinner (.020 or .030") H.G. Others use a .040" head gasket and a .020" thick base gasket to keep compression down for easier starting (especially 850s & up).

Once settled in the copper seems to last forever whereas composites have always failed me after several years and lots of miles.

Personally - I also do without the base gasket.

May be of some help .


Technically , YOU MATCH THE PUSHROD LENGTH , to it .











In for a Penny , in for a Pound .





And , So Long as you Do this , Youll Be Right .



Just kidding . Think the Clymer miss quote was 1 7/8 O.D. , which is 1 3/4 " bore tube .

Below . F-750s ran 32 concentrics bored to 33 m.m. . AS WE ALL KNOW , 40 m.m. on
S S Bevel Ducatis arnt a bad idea . So 36 m.m. is about the mi9nimum for a Concentric on a Worked 850 . !



These are the Dave Rawlins Norton bulliten mods . Factory issued . U S Noise Laws prevented a Combat 850 being marketable .



Below is a subverseive video , Dave Rawlins & the Anti Jap League .

CLEARANCE :

Theres about three ways ,

One Pre assemble , nipped down . Wind the Tappets in to zero ( set ) Then in one turn . = .040 in. ish . Gingerly turn it over . CAREFULLY . Plugs Out .

Should be no 'ping ' ' click ' or ' Tang ' . WHICH IS WHERE YOU STOP . ( some leave off the piston rings to reduce drag ) . Wind in another half turn & try .
If you got a hit then , Id try 1/8th turns to find the true clearance . Valve to piston . 50 thou. is good. 40 thou is fine. Less isnt .

Two . Same trip . Set to ZERO ( tappets ) , plasticene , blue tack or somesuch clay , turn thru , remove head . Scalple cut clay & measure .

Three . Assemble . Leave the 40 thou. head gasket out . Set tappets to zero . Turn thru carefully .

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boils down to the Octane of the gas redilly available . Check the chamber volume & go from there .
Any sane twit works dust free . And turns at each stage to check drag / binding / clearance etc . Anyway . So nothing to it . But Fingertips . Not Fred Flintstone on a 3/4 inch socket bar .