850 piston - valve clearance

A quick question to the wise - on a standard 850 with standard flat top pistons, std head and gasket (0.040?), does anyone have a figure for the clearance between the valves & piston? I foolishly forgot to check this before whipping the head & barrels off last night.

Just doing some speculation on how much I could improve the C/R before pocketing the pistons becomes necessary. The pistons in the motor look unworn, but machining valve pockets is going to be difficult & probably expensive (i.e. it will be better to buy a set of high compression pistons rather then modify these). However, if a worthwhile increase in compression ratio can be had with the existing pistons.....

/Steve.

yes Steve , hi comp pistons would be the best option and not skim the head, just wonderin what your intentions are or why you want to raise the compression?

Hi Madass, I posted my aims last month in 850-mild-tuning-t23683.html

You could check the clearance without a head gasket and if its ok fit the 0.003" thick copper rings and wire kit sold by J&S that way nothing is machined or altered an so you could return to standard compression easily if for example pre-ignition (pinking or pinging some call it) becomes a problem or starting with older legs as time goes by. Be aware that any change in height of the the rockers in relation to the cam by removing metal or gaskets also adding to the height by using thicker gasket (for those who want lower compression) may affect the contact of the rocker on the valve. So changes to the pushrod length in some not all cases maybe required and best to check this as well as valve to piston clearance. Why is so little in life as simple as it first appears :?

Are higher compression pistons available for 850 Nortons for road use ?

I thought shaving the head was the usual way of achieving this, as say the Combat did.
And keeping the flat topped pistons keeps the combustion chamber shape reasonable.
Mit valve clearance cutaways...

The std 850 pistons have something like at least 40 thou (is it ?) valve to piston clearance .

When I first converted my 850 to the Featherbed in 1980 I rebuilt my motor with cam built up to 2S specks and 40thu over size pistons (flat tops) head shaved with stock valves but lots of port work and new springs am running the stock pushrods and have never had any problems with valve to piston clearance, I am still running this same set up after 35 years, at the time I never checked the clearances.

Ashley

In theory, a shaved head and 2S cam in an 850 should have had the valves touching the pistons, or getting mighty close.
And had less than good clearance in the cam tunnel and the cam followers for the extra lift and travlel...

The head was only shaved 15 thu as it had a 12 thu warp and being young at the time never gave it any throught on the clearances, just over 5 years ago did a complete rebuild and still running the orginal valves with a bit more work done to the head, the pistons had no marks on them and using the same pistons but of course new rings, when the rebore was done I went very tight clearance between the final hone and pistons (under the recondmened clearance) never had any problems with sezing was a very cold morning when I clocked up the first 100 miles (2 am in the middle of winter), it might have been diffrent if the head was shaved 40 thu.

Ashley

Increase compression out of necessity, not convenience. A lot of compression plate are sold because you cannot put the material back on the head. Changing the pistons back and forth may be less permanent, but still, high compression is a demon in disguise. Pinging is a killer and racing gas is $7 a gal. The gains of higher compression come nowhere near pains of high compression on the street.

Some cams seem to demand a higher compression to operate well, but be conservative. I suggest using JS head gaskets to raise it a bit, but taking a big swing at it is detrimental.

I am, as we speak, have my top end off and am adding a compression plate for these very reasons.

pete.v said:

Increase compression out of necessity, not convenience. A lot of compression plate are sold because you cannot put the material back on the head. Changing the pistons back and forth may be less permanent, but still, high compression is a demon in disguise. Pinging is a killer and racing gas is $7 a gal. The gains of higher compression come nowhere near pains of high compression on the street.

Some cams seem to demand a higher compression to operate well, but be conservative. I suggest using JS head gaskets to raise it a bit, but taking a big swing at it is detrimental.

I am, as we speak, have my top end off and am adding a compression plate for these very reasons.

Click to expand...

Not quite certain what you mean by this, but here in Denmark, 98 octane is normal road petrol, costs around $6 per us gallon and is reasonably widely available (most common though is 95 octane). Mind you, when we go to Spa in Belgium, the Shell service on the motorway nearby sells 100 octane!

According to my rough calculations, with 0.020 oversize flat top pistons, std head and gasket, the C/R will be around 8.4:1, using a JS thin gasket (0.003) will give about 9:1. Skimming 0.040 from the head will give 9.2:1 and 10:1 respectively, but I think this will require valve pockets in the piston, in which case it's easier for me to use high compression pistons.

Mind you, the head may require skimming anyway, depending on if there is a difference between the combustion chamber volumes, using the methods Comnoz posted in head-surfacing-t15756.html.
In this case, it would be very easy to take 0.040 off the head (to be adjusted to get around 10.5:1 or slightly less)

/Steve in Denmark

You are generally safe to remove up to about .040 from the head surface [from stock] and still get away without valve notches.
That is with a stock cam. I wouldn't go that far without checking clearances first. Jim

SteveBorland said:

pete.v said:

Increase compression out of necessity, not convenience. A lot of compression plate are sold because you cannot put the material back on the head. Changing the pistons back and forth may be less permanent, but still, high compression is a demon in disguise. Pinging is a killer and racing gas is $7 a gal. The gains of higher compression come nowhere near pains of high compression on the street.

Some cams seem to demand a higher compression to operate well, but be conservative. I suggest using JS head gaskets to raise it a bit, but taking a big swing at it is detrimental.

I am, as we speak, have my top end off and am adding a compression plate for these very reasons.

Click to expand...

Not quite certain what you mean by this, but here in Denmark, 98 octane is normal road petrol, costs around $6 per us gallon and is reasonably widely available (most common though is 95 octane). Mind you, when we go to Spa in Belgium, the Shell service on the motorway nearby sells 100 octane!

According to my rough calculations, with 0.020 oversize flat top pistons, std head and gasket, the C/R will be around 8.4:1, using a JS thin gasket (0.003) will give about 9:1. Skimming 0.040 from the head will give 9.2:1 and 10:1 respectively, but I think this will require valve pockets in the piston, in which case it's easier for me to use high compression pistons.

Mind you, the head may require skimming anyway, depending on if there is a difference between the combustion chamber volumes, using the methods Comnoz posted in head-surfacing-t15756.html.
In this case, it would be very easy to take 0.040 off the head (to be adjusted to get around 10.5:1 or slightly less)

/Steve in Denmark

Click to expand...

All I mean is, trying to gain performance by increasing compression to the point of just shy of interference is not worth it. Sure, there are preferred fuels around but no where near to the point of convenience and if you choose to bump your compression to the point to where the author suggests, you will be a slave to the octane. Not convenient.

For the past 7 years I have stored fuel at home in two 5 gal. containers of 50/50 110 racing fuel and 93 premium. It gets old after a while and quite frankly maxing the compression beyond a practical point just does not make sense. Bring it up a little if needed, but no more then practical. That's all I mean.

Steve, I think the. 003 wire sold by JS will give you about one full point higher compression if memory serves.
I made the switch from an. 040 copper to a. 025 flame ring composite gasket and this brought comp up by about a half point.
I can still run regular if out in the boonies and that's all that is available, but normally run midgrade. The engine does pull a little harder with the slightly increased compression, though it was decent before. The improvement is about 3 MPH increased speed on the local dyno test hill. Not bad for just a gasket.
Like you and many others here I've contemplated all sorts of performance mods however, the bike works so well overall now that I don't want to change anything for fear of messing things up. Someone here came up with the saying "the enemy of 'good is 'better' and that seems to be the case quite often.

Just some open pipes and old style Boyer , nothing to brag about on a MK3 that doesn't know it's supposed to be a slug!

Glen

Thanks for raising this topic. I have a 2s cam ready to install into my 850 motor. The usual technique isto put kid's plasticine on top of the piston below the valves and turn the motor over. Then section the plasticine with a razor blade to see how much clearance there is. My problem is that at present it is too hot in my shed to work in there. If I pull the motor apart and fit the cam, if there is insufficient clearance, I would need to either machine valve pockets or re-install the old cam - in the heat. I have never really been interested in increasing comp.ratio, however If It was fitting Jim Schmidt's long rods and light pistons, I'd opt for 12 to 1 comp. - Lovely with methanol.
I'd question the increase in performance you get by increasing comp. ratio. If your bike is jetted slightly rich and you increase the comp. ratio without re-jetting, you would expect an increase in performance. I've found that methanol fuel works well at comp. ratios as low as 7 to 1, as long as you jet lean enough to get the most out of it. Petrol jetting is more difficult because the jets are smallerany internal variations have about twice the effect on how lean you can run, however the considerations are the same. In the end the combustion temperature is the limitation.

I suggest that if you correctly jet both a low comp. motor, and the same motor at high comp. without changing the fuel, there is not much in it as far as power is concerned.

If you use the 4C's of engine tuning - Compression Carbs Cams Cubes - and fully develop it, more compression etc can give LOTS more hp.
As any race engine builder will tell you.

But road use needs a sensible approach.
How much do you really NEED ?
A stock engine goes pretty good.
And gives a good baseline to work from...