Combat engine query

Hi All,
I'm trying to get some details on a 72 Commando I'm restoring mainly on whether it has a combat engine or not.

As purchased, the bike was in pieces and came with a spare frame to replace the original which was bent. The new frame is from early 73 as shown by the stamping on the coil bracket and is for an 850, the frame has been jig straightened and powdercoated. Additionally the swinging arm has been installed, forks fitted, re-laced front wheel installed, engine cradle with new vernier adjusters and engine/gearbox installed. I have also fitted new exhausts and silencers as well as rebuilding the gearbox.

The bike already has a Pazon surefire and 34mm Mikini installed all I needed to do was re-install the twin coils and add an ignition circuit. The bike fired up virtually first kick and appears to run well. Next steps are to install wiring for the headlight & indicators etc. I'm designing my own harness which uses a 6 way fusebox mounted somewhere near the battery and will probably use a solid state regulator/rectifier as well as relays for the headlamp.

What I'm now trying to understand is whether the engine is a combat or not. I have checked the cylinder head and there is a "C" stamp under the head steady, additionally the spacing between the head fins and barrel is small, probably showing the head has been skimmed. I checked the inlet valve lift at the tappet end and found it was 9.2mm (0.3622 inch), I guess this is less than the 2S combat cam and probably shows I have a standard comando cam. The breather is at the rear of the engine between the cradle, there is no sump plate, only a magnetic drain plug.

The engine number begins with 2204** which I believe shows the engine cases are from 1973, I havent taken the engine apart as it appears in good condition, I'm assuming Superblends have been fitted at some point.

My question is whether this is a genuine combat engine, or more likley a blown up combat engine with new cases reusing the original combat head and a std commando cam.

All info welcome.

gunnag said:

What I'm now trying to understand is whether the engine is a combat or not. I have checked the cylinder head and there is a "C" stamp under the head steady, additionally the spacing between the head fins and barrel is small, probably showing the head has been skimmed. I checked the inlet valve lift at the tappet end and found it was 9.2mm (0.3622 inch), I guess this is less than the 2S combat cam and probably shows I have a standard comando cam. The breather is at the rear of the engine between the cradle, there is no sump plate, only a magnetic drain plug.

The engine number begins with 2204** which I believe shows the engine cases are from 1973, I havent taken the engine apart as it appears in good condition, I'm assuming Superblends have been fitted at some point.

My question is whether this is a genuine combat engine, or more likley a blown up combat engine with new cases reusing the original combat head and a std commando cam.

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Combats supposedly ended at 211110, so 2204** is unlikely to have been an original Combat.
The low rear breather and lack of sump strainer was normal for '72 and '73 750 crankcases, not just Combats.

that might be a combat cam... do not forget the lift is given by the rocker geometry as well the lobes

Thanks Guys, will next check the cam timing by adding a timing disc in the crank drive side end and measuring when the inlet/exhaust valves open/close. Additonally I wlll do a compression check on each cylinder and see what readings I get. I'm assuming readings over 170 psi would show approx 10:1 compression?

gunnag said:

Thanks Guys, will next check the cam timing by adding a timing disc in the crank drive side end and measuring when the inlet/exhaust valves open/close. Additonally I wlll do a compression check on each cylinder and see what readings I get. I'm assuming readings over 170 psi would show approx 10:1 compression?

Click to expand...

When you said: "I checked the inlet valve lift at the tappet end and found it was 9.2mm (0.3622 inch)," did you actually mean you checked at the valve adjuster end of the rocker, so you measured the valve lift and not cam lift which would be measured at the 'tappet' (cam follower).
https://andover-norton.co.uk/en/shop-de ... 72-h12-75-

0.3622" would be about the correct amount of inlet valve lift for a standard camshaft, but it would not be enough for a Combat inlet cam if it was cam lift (0.390") or valve lift (0.441").

A serial number 2204XX is in the range assigned to (1973) MkV 750. These were dispatched with the standard spec cam, but either an RH5 (low compression) or a an RH6 (high compression) cylinder head. If the head has a 'C' stamping it should not have been original to the 2204XX serial number. Perhaps it was replaced at some point.

Does the original frame show 2204XX on the headstock tag? Does it have the large square taillight and deep black clock holders like the later 850's?

gunnag said:

I'm assuming readings over 170 psi would show approx 10:1 compression?

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depends on the cam. a hot cam can have lousy compression at kicking speeds.

i have a cammed triumph with 11.75 to 1, and all i can get out of it is 140 psi.

gunnag said:

Thanks Guys, will next check the cam timing by adding a timing disc in the crank drive side end and measuring when the inlet/exhaust valves open/close. Additonally I wlll do a compression check on each cylinder and see what readings I get. I'm assuming readings over 170 psi would show approx 10:1 compression?

Click to expand...

Best way to measure static CR is: get the pistons to TDC, select the combustion chamber that has both valves closed, measure how much oil it takes to fill the combustion chamber, then calculate as follows: (swept volume + combustion chamber volume) / combustion chamber volume = CR.

As speedrattle says, hot cams lose some effective CR and so hot cams and high CR are kinda a package. If you have a Combat head with a standard cam, I would worry that the effective CR would be too high.

When you said: "I checked the inlet valve lift at the tappet end and found it was 9.2mm (0.3622 inch)," did you actually mean you checked at the valve adjuster end of the rocker, so you measured the valve lift and not cam lift which would be measured at the 'tappet' (cam follower).
https://andover-norton.co.uk/en/shop-de ... 72-h12-75-

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Yes well noted, I measured how far the tappet was pushed down taking and into consideration the 113% rocker ratio, this seems to point to a standard cam.

Does the original frame show 2204XX on the headstock tag? Does it have the large square taillight and deep black clock holders like the later 850's?

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I have transferred the original headstock tag from old frame to new so its the same. As bought, there were no clock holders or taillight so sadly this doesnt help.

I might try filling the chamber with oil and calculating the CR, I will also do a compression check.

The inlet valve lift listed in the manual is .375 for a standard cam. So unless you feel there is some error in your measurement I would think it points to a custom cam or a worn one. Do you have reason to believe the person assembling the engine knew what they were doing?

Russ

I have owned both '72 combat and '73 hi compression 750. The 220xxx is a '73 model year and would have originally had a head with stamp either RH5 or RH6 near the timing side valve cover. Someone has replaced the head on yours through all the years. Your 220xxx engine would have had a standard camshaft - a little harder to replace than a head. The combat cam profile gives a power surge above 6,000 rpm as compared to a standard cam profile which is what made the combat self destructive in the first place. Don't worry too much about how the factory build yours. Build it the way you want it.

I'm interested in the comment about the Combat cam giving a power surge above 6000 RPM. I recently bought a 2S cam with a view to using it in my 850 motor. It appears to have too much lift. But when I race I usually keep the motor spinning between 5,500 RPM and 7,000 RPM by using the close ratio box judiciously. I have had trouble cutting the oil scroll into the cam, however your comment has made me think it might still be worth using it. What killed the Combat motors - was it revs or the lack of Superblend bearings ?

acotrel said:

What killed the Combat motors - was it revs or the lack of Superblend bearings ?

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Or oil starvation?

acotrel said:

What killed the Combat motors - was it revs or the lack of Superblend bearings ?

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Both.

Oil was probably only a problem when racing.

I got a few bits of an 850 motor from a young guy in the west of Victoria. He used to use it to race other guys on modern bikes on our Great Ocean Road. He regularly revved to 8000 RPM and it split the crankcases through the drive side main bearing. To me that gives an indication of the design limitations of that engine. I always try to keep my 850 revving below 7000 RPM and I still expect to self-destruct sometime, but so far it has been very good. I'd built it in about 1978 and never raced it for about 20 years, simply because I did not believe in it. I don't take it near big race circuits.

I've never had an oil pressure gauge on my 850 motor. What pressure is normal in an 850 motor ? My experience has only been with Triumphs and I think 80 PSI is normal for those.

acotrel said:

I'm interested in the comment about the Combat cam giving a power surge above 6000 RPM. I recently bought a 2S cam with a view to using it in my 850 motor. It appears to have too much lift. But when I race I usually keep the motor spinning between 5,500 RPM and 7,000 RPM by using the close ratio box judiciously. I have had trouble cutting the oil scroll into the cam, however your comment has made me think it might still be worth using it. What killed the Combat motors - was it revs or the lack of Superblend bearings ?

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You need the compression for the combat cam to work, so on an 850 that will need to be worked out. I don't care for milling the head as Norton did. There were Hepolite Power Max Pistons (slight dome) in days past for 750. I have no idea if they were made for 850 or if you can find them.

If you really want to explore putting a combat cam into an 850 motor, you might want to start a thread specific to it. There is probably someone here who has done it. There are a lot of people here with more technical expertise than I. Below is a link to a previous discussion on this:

combat-cam-850-t13585.html

JS Motorsports has high compression pistons available:

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

As to why the combat motors blew up, it is sort of a chicken and the egg thing. But it wasn't the fault of the cam unless you blame it for the fact that people ran them at high revs until they blew up.

Russ

If the power surge occurs above 6000 RPM with the Combat cam, the usable rev range is probably too high for the rest of the motor. However a 2 into 1 exhaust might move it downwards ?

acotrel said:

If the power surge occurs above 6000 RPM with the Combat cam, the usable rev range is probably too high for the rest of the motor. However a 2 into 1 exhaust might move it downwards ?

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My understanding is that the 2S is strongest 5,000-7,500. A CR box would indeed work well with this.

The 2-1 Maney system definitely helps boost mid range. Whether or not yours does would have to be established through testing.

An 850 doesn't need domed pistons unless you want to go above 10.5:1. HC (ie raised crown) Flat pistons will give a nice squish effect and 10.5:1 if set up correctly.

The 2S cam and raised CR will definitely give more power if set up right. But whether the stock crank or crank cases will sustain repeated use of this power and revs seems to be the question.