Raising CR on MK2a 850

Hello Gents,
This is my first post, after reading many for some time! I'm also new to Nortons, having been more of a Triumph man hitherto.
I recently bought a '74 MK2a 850 roadster that has been restored previously. I removed the top end just to have a look and everything looks good, so I've no desire to do a full strip at this stage.
I intend to do the following:
Replace the single 36mm Mikuni with 2x32mm JS PWKs
Replace the Boyer with Tri Spark and a double outlet 12v coil
Raise the CR slightly.
My question is re the CR, I'm looking to skim 0.040 off the head (or barrel) to get close to 9.5:1. Naively, I stripped the top end without first recording the valve to piston clearance!
Can you guys help me here: am I going to run out of clearance by skimming 0.040? Or is there bags of 'spare' clearance on these motors?
Also, any advice on the best double outlet coil to suit the Tri Spark? I used Dyna coils in the past and was very pleased, are these good in this application?
Thanks in advance!

Hi, welcome! I used a .020" head gasket to take me up a half a point. JFWIW

http://www.jsmotorsport.com/products.asp

Why raise the compression? Is the quality of gas in your area ever increasing? Are you getting a new cam that would benifit from a higher CR?
The performance increase will be barely negligable vs the possibility of ping and pre detonation.
The stock CR's are there for a reason and the torque of the 850 is hard beat. But if you have to, do what Concours suggests and don't be screwing up a good head. If you want to make a hotrod, get a 750.

pete.v said:

Why raise the compression? Is the quality of gas in your area ever increasing? Are you getting a new cam that would benifit from a higher CR?
The performance increase will be barely negligable vs the possibility of ping and pre detonation.
The stock CR's are there for a reason and the torque of the 850 is hard beat. But if you have to, do what Concours suggests and don't be screwing up a good head. If you want to make a hotrod, get a 750.

Click to expand...

Thanks for the welcome there Pete!
I've read all I can on this forum about this, and didn't realise I was being particlarly controversial. To answer your questions:
Why raise the compression? It seems the general concensus on here that it helps to improve performance a little. This is supported more generally too as all modern motors have higher compression wherever possible (even your average petrol driven family hatch back is around 12:1 here in Europe these days). Plus, I experimented with the bike by deliberately trying to get it to ping, riding in too high gear, at too low revs, and whilst I managed to stall it a few times, it is impossible to make it ping. All of this tells me there is clear scope to increase CR a little. Its easy, its cheap, and provided I don't over do it, I can't see a down side!
As to the quality of gas in my area... 97 octane is available everywhere and 99 octane is very easy to get too, so gas quality is actually OK.
No I'm not fitting a more sporty cam, I realise this would be a must at 10:1 or more, but I'm hoping for more like 9.2 to 9.3:1 which I didn't think was excessive.
Fair point ref screwing up a good head though. Clearly I can raise things somewhat without being so drastic with the mill!
If I could return to my question though, is there 'spare' valve to piston clearance in these motors, or am I going to see clearance issues?

Eddie, the link to JS products posted above also lists a. 003"thick copper head gasket. This will raise your compression by one full point. I dont know if it will ping at that point, however you should retard ignition timing if raising the compression, the two go together.
I did the same as Concours, raised my 850 by 1/2 point by using an. 020 gasket. It does feel a little sharper and doesnt ping on midgrade, tho I run premium to be sure.
The stock 850 is listed as having 8.5 to one cr, although various measurements seem to show it is a little lower, maybe 8.2 or 8.3 so going up a point should give you the number you would like to try.
My interpretation of the info on the JS page is that you do not need to worry about valve to piston clearance when using the. 003 gasket with stock pistons, but do need to deal with clearance when using higher compression domed top pistons.

Glen

The standard 9-to-1 750 pistons had valve pockets, which of course were indispensable on the 10-to-1 Combat engines with the high-lift camshafts. I'd suggest caution in seeking too much of a bump in compression with the standard flat-top pistons--a thinner head gasket with no base gasket might be just about the limit without changing the pistons



Tim Kraakevik
kraakevik@voyager.net

If you are shaving the head, changing pistons or using radical cam timings, it is good practice to put some plasticine in the valve pockets in the piston and rotate the motor over TDC on the valves open stroke. Then section the plasticine with a razor blade, and you can see what clearance you have.

Hi and welcome! Just wondering if you have confirmed the cr as it is presently and what cam it has. There's been 40 years for those things to have been altered from original specs.

Hi Eddie,
I skimmed 0.025" from the RH10 head on my MK2a & fitted a PW3 cam. I milled the piston valve pockets at the same time but can't remember by how much. The measured C.R. was 9to1 & it ran really well on BP 97 & Shell 98 octane .Even on the hottest days it didn't pink. Although this is not the same as your set up it will give you a good indication.
Martyn.

worntorn said:

Eddie, the link to JS products posted above also lists a. 003"thick copper head gasket. This will raise your compression by one full point. I dont know if it will ping at that point, however you should retard ignition timing if raising the compression, the two go together.
I did the same as Concours, raised my 850 by 1/2 point by using an. 020 gasket. It does feel a little sharper and doesnt ping on midgrade, tho I run premium to be sure.
The stock 850 is listed as having 8.5 to one cr, although various measurements seem to show it is a little lower, maybe 8.2 or 8.3 so going up a point should give you the number you would like to try.
My interpretation of the info on the JS page is that you do not need to worry about valve to piston clearance when using the. 003 gasket with stock pistons, but do need to deal with clearance when using higher compression domed top pistons.

Glen

Click to expand...

To add to it, Jim @ J&S claims the .003" head gasket is one of the most reliable as as far as sealing.

I never could get the. 040 copper head gasket to seal, tried three times. I did not take a chance on annealing, I bought a new gasket each time. For the first gasket I did not retorque as copper is not supposed to require it. When it started to show oil on the top cylinder fins at about 1,000 miles, I retorqued to no avail. For the next go, new. 040 copper gasket and a retorque at 100 miles. It still oozed oil. On the third I tried adding some permatex to yet another new copper gasket, no go.
Finally I tried a thinner. 020 " flame ring" composite gasket from one of the suppliers. It has been perfect to date, about 7,000 miles.

Glen

I run a CR of 9.8:1 on standard 95 octane fuel in the UK. I never hear it pinking and have had no problems.
I upped the CR at the same time as I increased the capacity from 840cc to 920cc, which has given me more power and upped fuel consumption from 45mpg to typically 64/5mpg (UK gallons, not US gallons). How much more power was down to the CR increase I don't know, but I'm pleased I did it.

My pistons have cut-outs for the valves although it is slightly higher lift cam than standard. IIRC valve to piston clearance should be = > than 0.060", and piston to squish band = > than 0.050"........I think that's correct.

To save shaving the barrels/head, you could always try with no base gasket, just a decent instant gasket or sealant such as wellseal for the barrel base and one of Jim Schmidts 0.020" copper head gaskets, that would take you towards increasing compression by nearly one point (can't remember the thickness of cylinder base gasket off hand). Build it like this and then see what you've got?

http://www.jsmotorsport.com/products.asp

Tuning Sheets #1 and #2 that the factory put out for owners to do their own Combat version of the 850 refer to flycutting the pistons for valve clearance if the head is milled or a high-lift cam is installed. So there is not a lot of spare clearance there...

It is also noteworthy that machining valve pockets in the flat top 850 pistons also increases the possibilities of hot spots promoting pinging to develop - is there an art to cutting valve-clearance pockets that won't cause any such problems ??

Reggie said:

My pistons have cut-outs for the valves although it is slightly higher lift cam than standard. IIRC valve to piston clearance should be = > than 0.060", and piston to squish band = > than 0.050"........I think that's correct.

Click to expand...

One person brought up squish clearance,it would of course be reduced if the cylinder head moves closer to the piston whatever route is taken to accomplish that.
1 mm (0.040") is pretty much the the norm for near anything,I doubt a Norton would be safe under that even with Ti con rods.
Although the Norton has nothing close to a pentroof chamber,most modern engines benefit from optimising the squish clearance even over the marginal compression increase,drivability.
Any one who builds engines knows that making the most of what you have can be an improvement even on something that is in near stock configuration. ??

Where did this talk of squish bands come from - Commando heads don't generally have too much squish designed into them ???

With a flat top piston, it and the head don't come remotely near each other - that is why the stock design is relatively free of pinking and detonation.... ?

head-surfacing-t15756.html

You might want to Google what squish is. :roll: :lol:

My Atlas head, which is identical ? to early Commando heads doesn't have any squish to it.

Did later Commando head versions come any different ??
850 head is kind of assembled, can't look.

There is no mention of squish bands in Tuning Sheet #1 or #2 (?) = building an 850 into a Combat spec motor, and beyond..

Throwing it together with the plugs out , winding in the tappet adjusters ONE turn from NIL clearance , gets the 40 thou bit .

Which it should have everywhere , so if anything else hits ( with a hot cam ) such as collets on valveguides or followers topping out ,
they need clearanceing too .

Turning it over very jdiciously till it hits :lol: :x tells you theyve hit !

With a degree wheel , both ways , gets you some figures to calculate . - For Valve Pocket depth ( machine pistons )

or back of by quater turns & then check . Unlikely to hit , but you ALWAYS check . Plasticine on pistons at cut out ,
carefully sliceing - removeing - and measureing , tells you clearance .

c.c. ing it BEFORE dismantleing . TDC , plugs out . lean till plug holes level . piston at TDC . use Burette of calibrated syringe to fill to
bottom of plug hole ( WHO allows plug internal volume in calc ? ) That ( about 60cc ? ) plus half actual capacity , divided by that , is it .

as in 828 / 2 = 414 + 41 = 455 odd for 10 : 1 , depending on bore oversize .

9.5 : 1 is plenty . 1 5/8 intake valves , and 36 carbs , or 34 Mk IIs , or 36 - 40 Dellortos would get it bogeying , with a cam & valve springs .
you can get the 1/8 over intakes in the std chambers . Likely out on the seats , so further into the chamber , raiseing C. R. so allow for that too .

The idea being to calculate the amount to skim head for correct required compression ratio . Someone ran 8.7 & 31 advance on the 750 ,
which was satisfactory , with the port mods . Combats were originally Interstates . They run longer pipes , which shift the power up the rpms a notch .
Also 72 mufflers were straight through glass pac . After a few years of everyday use with a bit of throttle , there wasnt a lot of fibre glass mat
left in them . Therefore they were straight through with perferated tube . Jetting biased rich , they get up in temp , underway in the powerband .
Any unburnt fuel incinerates in there , enhanceing the pulse wave extraction , induction therefore , and accoustically more stimulateing .
Though some might not agree with this last fact , though maybe they were always bad tempered and supressed the fact .

Commando head, squish band in view. I believe the Commando head also has ratioed rockers to give a bit more lift than Atlas?

Thanks Glen.
Did the squish band get discarded if larger valves are fitted, and re-angled ?