Combat head rebuild

Here goes my latest repair on my '72 Combat. I let the head get loose and blew the head gasket so when I took it apart to take out the aluminum spacer that no one recommends any more I decided to do a little porting and put in some bonze guides and stainless valves.
Here is how it started: (Well, I really thought I had loaded the as is fotos to Photo bucket but they will have to come later.)
Here is some of the progress:








The photos show that I'm not done.

What 'aluminium spacer' are we talking about here ?

Is that port work working to a plan, or just hogging out the ports...

Rohan said:

What 'aluminium spacer' are we talking about here ?

Is that port work working to a plan, or just hogging out the ports...

Click to expand...

I have an aluminum space/head gasket that is .080" thick. If replacing a copper head gasket that is .040" thick you get the .040" back that Norton milled off the combat head and it goes back where it was removed. The other "spacer" available goes at the cylinder base. I have not used this Al head gasket and I would think it wouldn't seal very well. Using a copper gasket on my combat. Working good so far, but only has about 150 miles on it.

Might be an idea to read the head thread here before taking much out of the ports. Combat ports are already too large for optimum power for a 750.

Glen

Too late. Can't go back on that kinda' hogging out. Try copper headgasket.

Youll check the pushrod length / rocker - valve contact etc . 40 thou off coil bound etc .

With electronic ign. the high comp shouldnt melt anything . The Flame ring gaskets less prone to weep .
Whichever , use sealant and have both surfaces deadly clean ( Lap on glass with grinding paste ) .

Dunno what all this fuss is about with COMBAT heads . whats wrong with 8.000 rpm . :twisted:

Keep a eye on the tacho , when youre acting irrascably . It aint gonna stop at 7.000 . You are resposable there .

The Aluminium head gasket goes in the bin or on the wall as a souvenir, in use it squeezes out on each re-torque as you try to maintain the right torque figures, no matter now many re-torques it always needs another.

Ugh an Al head gasket is what blew up my Trixie Combat during 3rd week of owning just tooling along no thought of testing its power or rpm just easing around on a runner after decade of shop storage, the gasket had ozzed into the bores so comma oil slot piston rubbed on it and one came apart to take apart engine about 50 mph and take a good while to recover from. An example of a lazy risk taker not taking a wonderful nice runner apart and spending before enjoying it. How lucky do ya feel punk? BANG! Aluminum is good for oil sealing washers better than the copper, ask any Brit Iron vender.

I plan to go back together with the stock thickness copper head gasket. I like the fact that it conducts heat from the head to the cylinders and vise versa. I had never heard Worntorn's theory that the Combat head has too much flow already for a 750!! All I ever heard was all the head flow testing stuff about how hard it is to get a Commando head to flow enough. Any way, it is what it is now and I think I have done a good job of making each port taper evenly toward the valve so the gasses will not be accelerating and decelerating and accelerating on their way in. Getting the guides out was messy because you can only clean them up on one side since they are within about 1/16th of an inch from the aluminum on the far side. Any way, I bought a .499 reamer to clean up the holes and the new bronze guides went in with 395 degrees of heat.





Next it will be on to the valve springs. I am told that the Kibblewhite "stock" set is a bit long and the valves need to be machined to raise the collets so the pressure wont be so high and coil bind wont happen. Now that is confusing! Kibblewhite made the valves and the springs and I have to modify one to work with the other???







Kincrome caliper bought in Papua New Guinea.

Here's the info on heads, Jim Comstock's flow testing journey.
head-flow-testing-t8640.html?hilit=head%20flow%20testing

This site by Dave Comeau is also a good read
http://atlanticgreen.com/nhth.htm

Not only are the 32 mm ports too big for a 750, they are also a bit large for an 850. As shown in the head thread, the RH10 30 mm head is best for the 850.

Glen

Combat head ports are too big for each 325 cc's inhaling velocity until close to the red zone then they do flow pretty good but about to run out of crank and value train rpm tolerance. Your nice springs are wise decision for running up hi enough to get on cam. Still its a Commando so shouldl be spunky responsive for ordinary traffic use below 6000+ if giving a bit extra gas and clutch slip not to bog down on traffic light starts. To avoid the speed up and slow down speed up, one should grind out wider sections next to the guides to maintain more constant cross section, similar was done on wasp wait jet fighers at the wing roots.

Have checked your rocker/valve/tappet geometry? Some Combat heads seemed to have been machine a little hastily. The guides being drilled a little off will cause the adjusters to land shy of the valve tip close to the edge. This causes accelerated wear on the tips.

I actually sent mine to Comnoz to see what he thought. Although not really confirm, it did seem obvious. This much used head would have needed much machining to bring it to a ideal, yet it would still be a very used head. Again, although it was usable, it was just not practical to restore.

I only say this because you are really do a great and thorough job and it would be ashame to have to redo guide and valve seats.

It shouldn't take but to slip a rocker and spindle partially in place and view it's landing on a seated valve tip to confirm this important factor.

kommando said:

The Aluminium head gasket goes in the bin or on the wall as a souvenir, in use it squeezes out on each re-torque as you try to maintain the right torque figures, no matter now many re-torques it always needs another.

Click to expand...

Amazing as that's exactly the location of the 0.080" thick Al gasket I have...on the tool peg board above my work bench.

I need some recommendations on how to shim the old springs or how to machine the valves for the kibblewhite springs. I don't like the idea of fully assembling the engine in the bike to measure the coil bind clearance. If any one knows a shim thickness for the stock spring shim to bring them to within .080 of coil bind or the exact recommended distance to move the collets up on the kibblewhite black diamond valves for the kibblewhite springs that would be cool. And, there is probably a way to calculate it without putting the head on the engine. I guess I could look up the lift of the Combat cam and then push the valve down that much and measure the spring gap. Are the head castings so different that each head would take a different shim thickness for the same result?

Here is how I am getting the quides cut to the proper size:






Still need to decide which set of valve guide seals to use. Any recommendations? The red ones are kibblewhite and the others seem to be good quality and have a spring.




The importance of not ordering 850 guides when you have a 750!!!

Vone Vay is to throw it on . Remember that cam timing is checked at 20 thou. clearance

BUT Testing Springbound , you bring Ea. tappet down to ZERO . Then in a turn - .040 . Somebodyll whine if the thread is not 1mm pitch .
Plugs out and primary absent , rather cautiously swinging it over - ready to STOP at the first contact - gingerly - with the finger tips - sober .

If nothing cathes , its fine . THIS is the final check test anyway . Depending on how far back the valve seats are cut as to fitted spring length .

A Cigarrette paper between coils here and there will show if theyve got close to it . Chucking a valve in , with fittings - or just inner spring
lets you measure the fitted length . Even if you trim a matchstick or toothpick to length ( the gap ) and measure that .

Youve got your rocker ratio tocalculate into overall lift at alve , incedently .


A Valve Spring centered in vice jaws between parralel packers ,wound shut ( gingerly :lol: ) with your fingertips , will establish its closed length.
Shims under the sping seat , to pack the fitted length to coilbind AT full lift Plus .040 means inirtia cant hurtle the valve a half inch extra into the piston .

Has the Good Ole COMBAT got the Good Ole COMBAT CAMSHAFT installed . 8)
with the overlap @ 3 deg. btc , it should start to haul at 3500 , and tear your arms out of your shoulders if you hold on tight from 5000.

How about just stick some yarn in a coil or two then crank down on valve stem to mimick lobe lift and rocker ratio and a bit more then see if yarn pulls right out or not and go from there. Spring swell with heat and bounce waves through them so fudge factor needed. A real anus might check it at almost 400'F too.

Matt Spencer said:

Vone Vay is to throw it on . Remember that cam timing is checked at 20 thou. clearance

Click to expand...

Depends on the camshaft. Are you and Hobot in some kind of unofficial competition as to who can make themselves the least understood. Would hate to try and get directions off either of you two. :lol: :lol: :lol:

I'm still wondering if anyone knows the shim thickness that would put the stock springs in the ballpark before I put the head on to test for coil bind. It would be cool to have it all together and find that it is good without having to take it off again to readjust the shim thickness. Some seem to feel that just plunking the head on is no big deal but I found it quite hard to get off in the frame and don't look forward to a lot of putting it on and off. Any tips on getting the head off easily in the frame would be cool.

I also still need advise on the two different types of guide seals that I have. They are in pictures previously posted.

I guess I'll call Old Britts to ask how far to machine the valves to move the collets up for the Kibblewhite springs. JS has detailed drawings for doing it on valves for their beehive springs but it does not follow that it would be the right amount for the stock style Kibblewhite springs.

I got .060 oversize exhaust valves which should put the seat right in the middle of the valve as it should be but I have been warned that there is the possibility of valve to valve interference on a Combat motor. Has any one experienced this? It is only .030 closer and it would be hard to determine how close the valves come to each other due to cam timing. I'm taking the warning seriously while at the same time having a hard time believing that the valves come that close to each other in a running engine.

Here is the stock size exhaust valve seating out by the edge.

This is WHY you spin it over with the Valves set 40 past Zero. IF theyre going to HIT , they will there . :twisted: :twisted:
This is why you use youre fingertips judiciously . NO Force . If they catch, PANIC .

Your bound to do some simple 'MATH' as these Americans call it . Simple addition & Subtraction .
The 40 there is giving you the Max. full lift movement , set Full lift plus 40 , for installed length .
A bit of plasticene in the valve cut outs in the pistons will give record of clearance there . Or putty .

biggest likelyhood is opening to early - inlets advanceing whilst pistons still raising , simultaeneously .
Thus Valve to piston clearance is seperate consideration from valve to valve - inlet to exhaust ' TANGLING '
where the inlets are thrown into the exhausts before theyve shut .

THIS later is normally exhibited when overreving with weak springs .

Eyeballing the proximity of the valves in the chamber , say 2.5 mm ( 3/32 in. ) is likely adequate between outer diameters .

VISUALISEING the actions of the valves , at overlap - is a key to understanding the action .

Its only necesarry to nip the bolts down with a old head gasket in for the Big Test .
Ya wanna play the Blues ya gotta pay the Dues . Go toit . It willnaye do itself .

One small step for man , beats watching television . 8)

Once you undestand the Objective - you should actually ENJOY doing it . Or at least , Having DONE It . !

motorson said:

Here is the stock size exhaust valve seating out by the edge.

Click to expand...

That's pretty close to where it should be seating.
I'd normally have the seat 0.010" from the outer edge of the valve head,and certainly no more than 0.020".You could possibly grind 0.030" off the head diameter of your new valves to get that.That would leave you with more valve to valve clearance and valve to piston clearance,without deepening your valve seats for nothing (the seats are then more repairable next time).
There is a time during valve overlap,where both valves will be open about 0.150",and that's not including any valve float on the exhaust valve.Hopefully there will be less than 0.030" valve float.