Combat differences

quawk

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Since recently purchasing a 1973 750 Hi-Rider, and it being my first Brit bike, I've been researching and asking questions here on the forum, trying to learn about the Norton brand/history. Can someone confirm or not that a couple of the differences of the Combat engine models from the "regular" engine was that they had higher compression and a different (hotter?) cam. Did they maybe also have larger carbs? Some of my research indicates that these engines had a tendency to self destruct, and for a while gave Norton a bad reputation. How long did they make the Combat and was it dropped because of the durability issues? I would guess it could be looked at as being either way, whether they were more or less desirable. There is a bike currently listed on Ebay as being a '72 Combat. It has both front and rear disc brakes. I assume these were both aftermarket add ons or grafted on from later model bikes, as my understanding is the discs weren't offered from Norton until later, '74 or '75 maybe.
 
quawk said:
Can someone confirm or not that a couple of the differences of the Combat engine models from the "regular" engine was that they had higher compression and a different (hotter?) cam. Did they maybe also have larger carbs?
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Yes.

quawk said:
Some of my research indicates that these engines had a tendency to self destruct, and for a while gave Norton a bad reputation. How long did they make the Combat and was it dropped because of the durability issues?
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Yes. The Combat series ended at 211110.

quawk said:
There is a bike currently listed on Ebay as being a '72 Combat. It has both front and rear disc brakes.
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A Combat would normally have had a front disc brake but not rear.

quawk said:
I assume these were both aftermarket add ons or grafted on from later model bikes, as my understanding is the discs weren't offered from Norton until later, '74 or '75 maybe.
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No. 1972 (so Combat) for the front disc. 850 Mk3 (1975) for the rear disc.
 
quawk said:
Since recently purchasing a 1973 750 Hi-Rider, and it being my first Brit bike, I've been researching and asking questions here on the forum, trying to learn about the Norton brand/history. Can someone confirm or not that a couple of the differences of the Combat engine models from the "regular" engine was that they had higher compression and a different (hotter?) cam. Did they maybe also have larger carbs? Some of my research indicates that these engines had a tendency to self destruct, and for a while gave Norton a bad reputation. How long did they make the Combat and was it dropped because of the durability issues? I would guess it could be looked at as being either way, whether they were more or less desirable. There is a bike currently listed on Ebay as being a '72 Combat. It has both front and rear disc brakes. I assume these were both aftermarket add ons or grafted on from later model bikes, as my understanding is the discs weren't offered from Norton until later, '74 or '75 maybe.
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The head has a "C" on it - sometimes under the head steady. It has no RH number on it. It has 32mm carbs, not 30mm. The will say R932 and L932 on their sides. It has a different cam. The head has been skimmed to increase the compression ratio. It has a front disk and no rear disk. If Black, it has the "D" lining (not sure about non-Combat).

If original, could self-destruct for two reasons. 1) Main bearings - unlikely to be original. 2) Original pistons, there a good chance they will eventually lose their tops if ridden hard. Lots of Combats I've rebuilt still had original pistons.

When the main bearings and pistons are good, there's no real issue.
 
The 'easy spot' is the 32mm manifolds have one upper & lower spaced fin cast in, rather than the 30mm groups of 2/3. From a distance they look more plain.
The big issue was if held at WOT for long periods, the oil would migrate to the back of the crankcase and out of the breather, since the scavenge port was located forwards, and would draw air back into the tank instead of oil, resulting in starved big-ends.
Some folks have done high mileages with no issues, presumably due to a more civilised riding style.
Apart from the mods Greg has already mentioned, relocating the scavenge to the back of the case and plugging the forward one fixes the root cause, but fitting Superblend mains & non-spotted pistons are also highly recommended.
Shortening the pushrods to restore the correct valvetrain geometry is something the factory never got around to, so another bit of work.

The Combat is my favourite Commando, and a lot quicker than some folks realise.

This is the first one I did...

Combat build - first steps

I'm finally making some inroads into a Combat Interstate I acquired in rather lamentable condition last year. Maybe a contender for 'most improved? We'll see.... It came with an 850 engine installed, but is a '72, so I've tracked down the correct stuff to put it back to how it came out of...
www.accessnorton.com www.accessnorton.com
 
I just read the linked post to your first Combat build. So from info in your post, am I to understand that the Combat engine was only built in the 750cc displacement?
 
quawk said:
I just read the linked post to your first Combat build. So from info in your post, am I to understand that the Combat engine was only built in the 750cc displacement?
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Yes, the 850s came around in 1973 and the Combat started and ended in 1972. However, a skimmed RH4 head and 2S cam, and you've basically built an 850 "Combat". Personally. I wouldn't do it - there are better ways to raise the compression and the extra displacement of the 850 was due to getting rid of the Combat as I understand it.
 
quawk said:
I just read the linked post to your first Combat build. So from info in your post, am I to understand that the Combat engine was only built in the 750cc displacement?
Click to expand...
Yes, only 750. The bike in question had a replacement 850 unit fitted at some time in the past, but left the factory as a 750, so I rebuilt it to 750 Combat spec.
 
B+Bogus said:
The 'easy spot' is the 32mm manifolds have one upper & lower spaced fin cast in, rather than the 30mm groups of 2/3. From a distance they look more plain.
The big issue was if held at WOT for long periods, the oil would migrate to the back of the crankcase and out of the breather, since the scavenge port was located forwards, and would draw air back into the tank instead of oil, resulting in starved big-ends.
Some folks have done high mileages with no issues, presumably due to a more civilised riding style.
Apart from the mods Greg has already mentioned, relocating the scavenge to the back of the case and plugging the forward one fixes the root cause, but fitting Superblend mains & non-spotted pistons are also highly recommended.
Shortening the pushrods to restore the correct valvetrain geometry is something the factory never got around to, so another bit of work.

The Combat is my favourite Commando, and a lot quicker than some folks realise.

This is the first one I did...

Combat build - first steps

I'm finally making some inroads into a Combat Interstate I acquired in rather lamentable condition last year. Maybe a contender for 'most improved? We'll see.... It came with an 850 engine installed, but is a '72, so I've tracked down the correct stuff to put it back to how it came out of...
www.accessnorton.com www.accessnorton.com
Click to expand...
So, do all 72/ 73 750's utilize the same engine cases, regardless of whether Combat or not?
I suspect so.
That would make the scavenge and piston issue the same for all.
 
Deckard said:
So, do all 72/ 73 750's utilize the same engine cases, regardless of whether Combat or not?
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Yes, that's why the cases should not be called Combat cases but 72/73 cases. Only exception is some 235XXX cases which used the 850 cases and meant for the short stroke 750 but some got used for normal 750's.
 
Deckard said:
So, do all 72/ 73 750's utilize the same engine cases, regardless of whether Combat or not?
I suspect so.
That would make the scavenge and piston issue the same for all.
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Yeah but probably more associated with Combats as they tended to spend more time at WOT !
 
Makes me feel OK about my '71. I milled and ported the head for 32mm and replaced the pistons. Later regretted porting the head to 32mm, but we didn't know better then about better flow. Commnoz later flowed the head, so even better now.

OP Quawk would do well to leave head stock with 30mm ports if that's what he has.
If 32mm carbs are desired, use '74 RH10 manifolds to mount 32mm carbs to 30mm head. Well proven to flow better.
 
marshg246 said:
The head has a "C" on it - sometimes under the head steady. It has no RH number on it. It has 32mm carbs, not 30mm. The will say R932 and L932 on their sides. It has a different cam. The head has been skimmed to increase the compression ratio. It has a front disk and no rear disk. If Black, it has the "D" lining (not sure about non-Combat).

If original, could self-destruct for two reasons. 1) Main bearings - unlikely to be original. 2) Original pistons, there a good chance they will eventually lose their tops if ridden hard. Lots of Combats I've rebuilt still had original pistons.

When the main bearings and pistons are good, there's no real issue.
Click to expand...
When you say "1) Main bearings-unlikely to be original.", do you mean that most owners relized they were a potential problem area and had them replaced? Also, what do you mean by "if Black, it has the "D" lining?
 
Deckard said:
Makes me feel OK about my '71. I milled and ported the head for 32mm and replaced the pistons. Later regretted porting the head to 32mm, but we didn't know better then about better flow. Commnoz later flowed the head, so even better now.

OP Quawk would do well to leave head stock with 30mm ports if that's what he has.
If 32mm carbs are desired, use '74 RH10 manifolds to mount 32mm carbs to 30mm head. Well proven to flow better.
Click to expand...
I do intend to leave the entire engine and carbs alone/stock (non-Combat). Was just curious about the Combat and its' differences, since some history I'd read indicated how it's durability issues had given Norton overall such a bad reputation for some time, maybe undeservedly.
 
quawk said:
do you mean that most owners relized they were a potential problem area and had them replaced?
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The original main bearings were not 'if they failed' but 'when they failed', usually in a few 1000 miles. Either they would be changed by the dealer before sold or the first customer had the failure and they were replaced under warranty. There are some 72's still around that were driven gently and with not many miles that could have the original main bearings. Only way to find out is to split the cases and check the markings on the main bearings.

The main bearing failure is not catastrophic, your engine will rumble from the bottom end telling you you need new main bearings. Piston tops coming off will cause issues such as bent valves.
 
quawk said:
When you say "1) Main bearings-unlikely to be original.", do you mean that most owners relized they were a potential problem area and had them replaced? Also, what do you mean by "if Black, it has the "D" lining?
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Dealer repaired or owner repaired. Look at the 1972 portion of the second table here to learn much more about the iterations of bearings: https://gregmarsh.com/MC/Norton/CommandoID.aspx The first two are normally gone. I've seen several that still have the third and they are probably OK, but I always change them. Also (begin war starter) the 1st service release specified C2 clearance. The others say nothing about it. Many here insist that C2 is required. C2 from then is not the same as today and very difficult to find. AN sells the correct bearings. (end war starter).

When painted black, the "D" stripe was applied at least on Combat Roadsters. Also, Combats had black cylinders and the rest were silver as best I can tell. I'm not clear on the Combat Interstates - pretty sure the black ones had lines (not "D" shaped), but the brochure shows blue Interstates with silver lines and black cylinders.
 
quawk said:
When you say "1) Main bearings-unlikely to be original.", do you mean that most owners relized they were a potential problem area and had them replaced? Also, what do you mean by "if Black, it has the "D" lining?
Click to expand...
The Black tanks had 'D' pinstripe around 'Norton'.
 
marshg246 said:
Dealer repaired or owner repaired. Look at the 1972 portion of the second table here to learn much more about the iterations of bearings: https://gregmarsh.com/MC/Norton/CommandoID.aspx The first two are normally gone. I've seen several that still have the third and they are probably OK, but I always change them. Also (begin war starter) the 1st service release specified C2 clearance. The others say nothing about it. Many here insist that C2 is required. C2 from then is not the same as today and very difficult to find. AN sells the correct bearings. (end war starter).

When painted black, the "D" stripe was applied at least on Combat Roadsters. Also, Combats had black cylinders and the rest were silver as best I can tell. I'm not clear on the Combat Interstates - pretty sure the black ones had lines (not "D" shaped), but the brochure shows blue Interstates with silver lines and black cylinders.
Click to expand...
Not trying to drag this thread into eternity, but when you say " the first two are normally gone" do you mean the first two sets of bearings? If the non-Combat engines used the same lower end and cases, were they failure prone also? I guess what my concern is, does this relate at all to my '73, since the Combats ended in '72?
 
I might be a doubting thomas, but I would almost guarantee that when most guys skim a head to get more compression, they ride the bike without changing the jetting or ignition timing. It the motor is jetted slightly rich or the ignition is slightly retarded, increasing the compression will give an increase in power. Perhaps the same increase might have been achieved by optimising the jetting and ignition timing, without skimming the head ?
When you skim the head, do you worry about the squish band losing clearance ?
 
A 73 would have left the factory with upgraded bearings also called Superblends.
 
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