Combat Engine Specs

Thanks again to all who sent along suggestions regarding my "Real Tight" 750 engine. I have begun using some of these techniques and am hopeful for a looser outcome.

As I wait for parts to break free, a question I have remains regarding engine specs. I have been told on this site the engine number (212408) indicates a post Combat specification, and I assume this to be a '73 750 model. As I contemplate the rebuild, I am curious what made the Combat engine a hot rod. Higher compression pistons for sure. What else? Hotter cam, etc? is the process relatively easy considering te engine needs to be split anyway?

One last related question. I noted the gearbox number stamped on top of the bix has a number that matches the engine number, except it includes an "S" at the end. Is this just to identify it as the gearbox? Just wondering..

Thanks again for the help! This site rocks!

Cheesehead Commando said:

I am curious what made the Combat engine a hot rod. Higher compression pistons for sure. What else? Hotter cam, etc? is the process relatively easy considering te engine needs to be split anyway?

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The increased compression ratio was achieved by machining approx .040" from the cylinder head gasket face and not from higher compression pistons, although the Combat pistons were stronger. The Combat engines also had the "2S" camshaft and bigger 32mm carbs (32mm carbs were eventually fitted to all Commandos) different valves and cam follower locating plates (both of which appeared to have been adopted on all 750 models) and no heat insulating washers under the inlet valve spring seat, which also allowed more valve lift.

Cheesehead Commando said:

I noted the gearbox number stamped on top of the bix has a number that matches the engine number, except it includes an "S" at the end. Is this just to identify it as the gearbox?

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The extra letter denoted a production change, but those changes were only known to the factory. Crankcases are sometimes stamped with a code letter on the rear mounting boss for the same reason.

As with most engines you have to spend a lot to gain a little. On the big scale of things your stock bike vs a combat version is still no power house compared to modern superbikes. I'd just plan to rebuild it without spending any un-necessary money. Once you can get it stripped down and assess the damage you can upgrade those things that need replacing along with other complimentary mods.

"On the big scale of things your stock bike vs a combat version is still no power house compared to modern superbikes."

Well, that's true. You MIGHT get 65 real HP out of a combat bike ( which I'd love to own) but a current Ducati 848 produces 120 HP on the dyno and it's the lowest HP of the Ducati sport bikes. That's about 40 HP more than the current Norton "Sportbike" which has 100cc more displacement, costs 5k USD more and weighs more...what's up with that?

Maybe 65 HP at the crankpin on a perfect bike on a good day but probably would struggle to get 50 real HP at rear wheel. Just going by what I've read over the years. It still blasts off darn well, dances the front wheel in 2nd gear and makes a lot of lovely noise so I'm not complaining at all.

DynoDave at a rally some years ago with a dyno contest for all the hot rods, got the best in upper 49 rwhp. He had set up similar to accidental combo of Ms Peel Combat, small head ports and mildly hot cam close to 2S specs. Commando lose about 9% in drive train so implies close ~54 crank hp.

If just the head trimmed to get 10 CR, how do ya explain that Combat pistons all stick up .050" beyond barrel bores. Do any non Combats stick out too?

hobot said:

If just the head trimmed to get 10 CR, how do ya explain that Combat pistons all stick up .050" beyond barrel bores. Do any non Combats stick out too?

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I don't know, but here's a quote from Service Release N2/4 "Combat and standard Commando pistons" April 1972:

"In order to meet early 1972 production requirements, an interim piston has been produced to accommodate the additional valve lift of the Combat engine. As the new interim Combat pistons differ only from the previous Commando type in the depth of the valve cutaways, the new pistons are therefore currently specified for both standard Commando and Combat versions and supersede the previous pistons....." (etc.)

Ok thanks for the quote, Combat just needed more cut out for 2S cam lift and valves being closer to pistons with head trimming. So I still don't know it other non Combats have pistons that extend through gasket at TDC??? Jim Schimdt had asked couple week ago about Combat piston height above barrels, can't imagine that question unless he's only familiar with non Combats opened up which don't stick up. Confused yet I am.

I will not do performance mods I did to Ms Peel on my factory Combat as its not rear linked so couldn't handle the sweeper loads and drive train not up to it nor do I ever want to go so far over 100 w/o engine beef ups.

Clymers and Haynes as well as Dunstall Notes and maybe even Jim Schmidts Race Notes have the details to make a Combat out the other sows ears.

hobot

So how do you make a "combat" 850 that will rev and not explode? Jim's pistons/rods?

Nope #1 on list is lighter steel flywheel, the lighter stronger pistons/rods just allow less bob wt mass to get 50's BF or higher, so nice features but not a key issue to make power. A Combat don't rev safely much higher than an 850 as same long stroke crank whip becomes limiting factor before piston acceleration limits. Cam and CR and head flow are the other features to bring 850 up to speed. A Combat that's right don't make more torque than an 850 but it keeps building power while the 850's just sorta stay at same power as rpm rise or even drop off as rpm rise to top.

I'm sold on small ports myself now as by time the big ports begin to be an advantage the rpms are doing damage. W/o many $1000's for rpm aids, building for torque is about only way to get more reasonably out a Commando. Its about a toss up to spend for engine aids or lost mass.

Do 850 piston pass through the head gasket?

MexicoMike said:

"On the big scale of things your stock bike vs a combat version is still no power house compared to modern superbikes."

Well, that's true. You MIGHT get 65 real HP out of a combat bike ( which I'd love to own) but a current Ducati 848 produces 120 HP on the dyno and it's the lowest HP of the Ducati sport bikes. That's about 40 HP more than the current Norton "Sportbike" which has 100cc more displacement, costs 5k USD more and weighs more...what's up with that?

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Hi Mike

How about you get a motorcycle that doesn't cost an arm and a leg every 6,000 mile for servicing and is much more comfortable. At $800 per service that $5K dollars difference is dead even at 37,500 miles. After that the Norton is a bargain.

If you think you will live forever, get a 250 scooter as best sense in 2 wheels.
If you know you only got so long to kick up heels its hard to beat a well spent on fully fettered Cdo for life time of joy and involvement. Spend like me and can kick butt in elite sports crowds yet cruise forever comfortable. You call of course.

"How about you get a motorcycle that doesn't cost an arm and a leg every 6,000 mile for servicing and is much more comfortable. At $800 per service that $5K dollars difference is dead even at 37,500 miles. After that the Norton is a bargain."

I agree that I probably would no longer be able to handle the 848s riding position - at least when riding slower than around 90 which is where sportbikes start to get reasonably comfortable. But I was still tracking a 996 Duck in late '06 before I sold it to move here (Mexico). I did the service work on my Ducati myself so that's not really an issue to me. But I'm not serious about an 848 (at least I don't think I am).

What puts me out about the new Norton is they chose to call it a "sport bike" and price it 5kUSD above sportbikes such as the 848 and larger Ducks as well as the equivalent KawaHonYamSuZ company. I'd LIKE to see Norton do well but there current "model" makes no business sense to me. But I'm not a business expert so who knows. Lots of people said the Ipod was pointless when it appeared...(No, I wasn't one of them)

Mike wrote:
What puts me out about the new Norton is they chose to call it a "sport bike" and price it 5kUSD above sportbikes such as the 848 and larger Ducks as well as the equivalent KawaHonYamSuZ company. I'd LIKE to see Norton do well but there current "model" makes no business sense to me. But I'm not a business expert so who knows. Lots of people said the Ipod was pointless when it appeared...(No, I wasn't one of them)

Hi Mike,

So if Norton calls the 961 a "cruiser" and it's several thousands less than a Harley...it's OK right? When a company only makes one model, they really don't need a marketing label to differential their line up.

Choosing a Ducati and choosing a 961 Norton to me are two different motivators regardless of the "genre" ...a term I hate.

By the way....at last night's NCNOC meeting I heard we will have two new 961 Commandos on display at the San Mateo Motorcycle show Nov 19-21. I'll be at the vintage booth in Fiesta Hall on Sunday from 10:00am.

I don't think they're trying to market to the same crowd. I think they are marketing to the elite, people with cash who don't mind spending a little more because there's less of them around. Don't think they ever want to be a mass market bike like the new triumphs, but maybe I'm wrong.

Back to combat engines.... :twisted:

Hi.
I read here, if i mean well, that the combat head do not has the heat insulating washers under the inlet valve spring seat.
And under the exhaust?
Why there are not the insulators?.
Is better to mount?.
Another question, please: i have two combat head, one is skimmed, the other not and is like the detuned one.
Thank you.
Piero

The insulating washers were dropped as with the greater lift the valve springs got coil bound at full lift, you can put them back in place but only if you find another way of stopping the coils going coil bound eg by usig a std cam, or thinning the valve spring seat or getting different springs that will not go coil bound or some other clever alternative. Whatever you do check the springs for the gap at full lift before running the bike.