Combat heads questions

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Roadrash

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My 71 project bike came with combat heads and a non-combat engine. I would appreciate any advice on properly mating them.
I’m getting conflicting advice on what needs to be done to lower the compression ratio to the stock 9:1.
1) One well known parts supplier said I need to put a shim under the cylinders to lower the compression because the big difference between combat heads and stock is the combat deck has been shaved and that is what gives those heads their high compression.
Another experienced Norton mechanic has told me that the heads are the same (except for the inlet port sizes) and the high compression comes from combat pistons being crowned (taller), Therefore, by using the new flat pistons I’ve already purchased I’ve effectively lowered the compression back to stock.
Who is right? Or are they both right?

2) I’ve noticed shorter pushrods are sold (Old Britts) for combat engines, but there is nothing called out in the shop manual as different for combat engines. Any idea which ones should be used when mating combat heads with a non-combat engine and using the stock cam?
Thanks for any advice
 
#1 is correct, the head is shaved and has larger ports.

Shim the barrel or use a thicker head gasket.

The pushrods would be a little shorter. Standard ones can be shortened by pulling off the end and trimming the tube.
 
Hi, the heads on the "C" head was planed off 40 thou. the pushrods were not altered, so causing the springs to coil bound. Sometimes the insulating washer was omitted as well. Returning to a standard height, and replacing the insulators in theory puts it right. There are theories that the rockers were different, but not in the parts list. The only safe way is to to do a dry run to check that the valve springs on full lift are .050" short of coil binding. The cam also gives more lift, so shortening the push rods by 40 thou. will give an easier life to the valve gear. Lots of info on NOC.ORG. have fun, Paul
 
Roadrash said:
Another experienced Norton mechanic has told me that the heads are the same (except for the inlet port sizes) and the high compression comes from combat pistons being crowned (taller), Therefore, by using the new flat pistons I’ve already purchased I’ve effectively lowered the compression back to stock.
Who is right? Or are they both right?
Click to expand...

I'd be concerned with someone who told me this.

If it were me I'd shim under the barrel.
 
If you want to keep it simple, get an oversize head gasket .062" from JS motorsports. This is .020" over standard. I little more compression on a 750 won't hurt.

I have a .030" base spacer right here in my hand. I used a .030" because I would go with a gasket on both side.

My 750 ran real good with the Combat head even though it had a std.cam. Twin 30mm Amals and you will be good to go.

A detuned Combat head (base spacer and 30mm carbs) will work great for you.
 
Dang if it was mine I'd keep the Combat head's high CR and stick in a 2S cam to prevent detonation and get the Combat off idle beyond redlined spunk. If so then would want to get the shortened push rods to center the rockers on the stems. W/o the hi CR and the more aggressive 2S cam the CHO head is over ported so may end up with ho hum response till upper rpms but w/o the torque rise of 2S packing mix in.

No difference in standard or Combat pistons, both flat topped and in my Combats they stick up proud of deck by .050", if no base plate under the cylinder.
 
hobot said:
Dang if it was mine I'd keep the Combat head's high CR and stick in a 2S cam to prevent detonation and get the Combat off idle beyond redlined spunk. If so then would want to get the shortened push rods to center the rockers on the stems. W/o the hi CR and the more aggressive 2S cam the CHO head is over ported so may end up with ho hum response till upper rpms but w/o the torque rise of 2S packing mix in.

No difference in standard or Combat pistons, both flat topped and in my Combats they stick up proud of deck by .050", if no base plate under the cylinder.
Click to expand...

godamm
a whole post and no mention of Trixie. Dixie, pixies, peel or the Isle of man :shock:
Can't understand what is being said, but I did do a word search for these words to check if they were included. :D
Must go to bed and sleep I have waited a long time for this post! LOL
Yawn
JohnT
 
swooshdave said:
Roadrash said:
Another experienced Norton mechanic has told me that the heads are the same (except for the inlet port sizes) and the high compression comes from combat pistons being crowned (taller), Therefore, by using the new flat pistons I’ve already purchased I’ve effectively lowered the compression back to stock.
Who is right? Or are they both right?
Click to expand...

I'd be concerned with someone who told me this.

If it were me I'd shim under the barrel.
Click to expand...

Ouch...experienced Norton mechanic, huh. Maybe not with Commandos.
 
Well with all the same answers and with such conviction, I’m convinced. Thanks everyone. I’ve ordered a spacer for under the cylinders and a set of stock length pushrods. I’ll make sure the valve springs have enough clearance before firing her up. Not sure why the mechanic was so insistent. One look at his shop and you know the guy has a long history of vintage bike knowledge. Getting old and crotchety I guess. Like ME :wink:
 
I've run on to a shop owner or two that were long on opinion, but short on knowledge.
 
JimC said:
I've run on to a shop owner or two that were long on opinion, but short on knowledge.
Click to expand...


+1
That's why we have NENO tech sessions. The stuff we have come in for autopsy and the associated story is sometimes down right hilarious. :mrgreen:
 
+2 I cant believe some of the advice on this one from the ex spurts on here

dynodave said:
+1
That's why we have NENO tech sessions. The stuff we have come in for autopsy and the associated story is sometimes down right hilarious. :mrgreen:
Click to expand...
 
bill said:
+2 I cant believe some of the advice on this one from the ex spurts on here
Click to expand...

I think it would be more helpful to all concerned if you said what the advice was that you disagree with.
 
case # 1 there are at least 3 things wrong in this post. planing the head and and not altering the pushrods has NO effect on coil bind. there is NO difference in rocker arms. has no clue about the insulators. do I need to point out more???

jagracer47 said:
Hi, the heads on the "C" head was planed off 40 thou. the pushrods were not altered, so causing the springs to coil bound. Sometimes the insulating washer was omitted as well. Returning to a standard height, and replacing the insulators in theory puts it right. There are theories that the rockers were different, but not in the parts list. The only safe way is to to do a dry run to check that the valve springs on full lift are .050" short of coil binding. The cam also gives more lift, so shortening the push rods by 40 thou. will give an easier life to the valve gear. Lots of info on NOC.ORG. have fun, Paul
Click to expand...
 
case #2 IMHO more bad advice. why would you want to add another joint and another gasket to the cyl. base to give trouble. I MUCH prefer to machine the piston tops to lower the deck height and not even use a base gasket.

pvisseriii said:
I have a .030" base spacer right here in my hand. I used a .030" because I would go with a gasket on both side.



A detuned Combat head (base spacer and 30mm carbs) will work great for you.
Click to expand...
 
We're not worthy, we're not worthy.

For Christ sake bill, not everyone has your expertise and a lathe in their basement.
The poster simply needed some information. I had experience in this particular situation and what I did work well for me, period. A gasket on either side of the spacer is not a sin, neither religiously nor mechanically.

I can offer you and others like you some advise: Offer your experience, advise, and wisdom rather than wait for others to offer what they have only to have you swoop in and criticise.

If you can help a person out, then help a person out. If you feel you are protecting the individual by being a pompous ass, it isn't working. It only goes to show that, in fact, you are a pompous ass.

Sorry L.A.B.
 
Best Solutions 100 Octane , or Av Gas . :P

but , as a example ( decompression plates were common to BSAs centuries ago . )

BSA unit single decompression spacer

Combat heads questions


" To place under the barrel to change the characteristics of your engines' power delivery.
Available in thickness' from 1mm to 15mm. "

http://www.progressiveclassicproducts.co.uk/index.html
 
I have a combat with shortened push rods, and the stock 2S cam.. It is a joy, and is much ( much) much more peppy than my friend's 850. Burns regular gas and is fast and tractable.

I'd recommend getting the 2S cam and running it as a Combat. Combat push rods were the same as the none Combat and according to Norton Notes cause unspecified problems with valve train geometry. Or maybe it was in the performance chapter of the Clymer manual.

Is there any way of uploading articles? I did a short write up on Combats in general and mine in particular.

Cheers,
Eric
(Who had a bit of a get off at the San Jose Clubman's morning after ride.)
 
Hey Ho Eric. I of course 2nd your summary. Much much eh, hehe mine does that too but on 87-91 octane sans base gasket or corrected push rods. Maybe in Mexico a Combat spec engine would detonate on low grade pump gas but I did a survey a few years ago to decide on Peel CR and really only got old stories of past Combats detuned by standard cam that detonated and some racers over doing beyond hard hearted and a few with tuning issues to correct, but thats it.

On the other hand Peel's best configuration that drove me over the edge was with a plain jane standard head not the CHO.
 
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