Combat or not?

tpeever · Nov 14, 2013

ashman said:
If its done over 10,000 miles the the laysharft bearing should be OK as the ones that blew up didn't make it that far and it was mostly the 850s that had the problems.

Ashley

My 850 made it to 12K on the infamous Portuguese layshaft bearing. I got lucky as no dramatic rear wheel lockup at 75 mph. Mine gave up the ghost at 30 mph. Probably better to pull the gearbox and replace it regardless. Peace of mind is a wonderful thing.............

Bob Z. · Nov 15, 2013

When the kick start lever gives you a sharp slap to the back of your calf, you know it's time!

Torontonian · Nov 15, 2013

Mine went south after about 20,000. Kicklever was doing the love slap combined with an intermittent tick sound. Rear wheel locked up at about 30 mph. Honking car up my derriere. Limped home at 5 mph. It was an English made ball-bearing so not just the cheapo portuguese ones go on vacation. Cage twisted and 2 balls on bottom of box. Take note. :cry:

Johnnymac · Nov 15, 2013

I am in the process of swapping my layshaft bearing out too. Might as well for safety sake. It is goes south, it could be bad.

ashman · Nov 15, 2013

My bearing blew at 12,000ks (about 6,000 miles) outside of my house when I put it in first so was very lucky, my friend 850 blew when we were about 100 miles from home, we go it into 4th gear and he rode it all the way home in 4th, his bleww about the same milege on the clock.

Ashley

L.A.B. · Nov 15, 2013

Ron L said:
dragonfly said:

Ok....that's the clincher. The carbs are 932 32mm and the head is stamped RH6. So it's a standard 750. Gotta look for a layshaft bearing.

Click to expand...

Not quite. The RH6 head has 0.020" milled off for a compression ratio of about 9.5:1 and 32 mm ports. The "standard" 750 has 8:1 compression with 28.5 mm ports. The Combat head had 0.043" milled for a 10:1 compression ratio and a 32 mm port job. This puts the RH6 more or less halfway between "standard" and Combat spec.

Yes, maybe so-until the demise of the Combats-after which, the "32mm" RH5 & RH6 heads were introduced. Commandos fitted with either the "High" (RH6) or "Low" (RH5) compression cylinder heads then seem to have become the "Standard Commando" models for late-'72 and '73 (see Service Release N3/23, below).

G-Force · Nov 17, 2013

Yes, maybe so-until the demise of the Combats-after which, the "32mm" RH5 & RH6 heads were introduced. Commandos fitted with either the "High" (RH6) or "Low" (RH5) compression cylinder heads then seem to have become the "Standard Commando" models for late-'72 and '73

Now I'm confused, I got a 73 MK5 #221393 yet it has a C stamped on the head, So not a combat but the RH6 variant then?

swooshdave · Nov 17, 2013

G-Force said:
Yes, maybe so-until the demise of the Combats-after which, the "32mm" RH5 & RH6 heads were introduced. Commandos fitted with either the "High" (RH6) or "Low" (RH5) compression cylinder heads then seem to have become the "Standard Commando" models for late-'72 and '73

Now I'm confused, I got a 73 MK5 #221393 yet it has a C stamped on the head, So not a combat but the RH6 variant then?

Or someone put a C head on your bike. Not unheard of.

L.A.B. · Nov 17, 2013

G-Force said:
Yes, maybe so-until the demise of the Combats-after which, the "32mm" RH5 & RH6 heads were introduced. Commandos fitted with either the "High" (RH6) or "Low" (RH5) compression cylinder heads then seem to have become the "Standard Commando" models for late-'72 and '73

Now I'm confused, I got a 73 MK5 #221393 yet it has a C stamped on the head, So not a combat but the RH6 variant then?

An RH6 head would normally be stamped "RH6".

The Combat "C" head was supposedly RH3 (although I believe it was never marked as such).

http://atlanticgreen.com/images/cylhead.jpg

What about the camshaft, is it Combat (2S)?

Steve G. · Nov 18, 2013

Checking out my trusty Roy Bacon Twin Restoration guide, it shows the first Combat engine is # 200976,,,and the last Combat as the engine before # 212278 which was the first 'detuned' engine. March 1973 coincided with engine # 220000, with the last 750 being made Oct 1973, at engine # 230935,,,,,,so certainly yours is a 1973 Commando. Your bike will have been equipped with either head RH5 [8.9 comp] or RH6 [9.3comp]. Might be able to spot the part number cast , but can't remember where it is.
BTW, all 1973 750's and all 850's had Superblend main bearings.

L.A.B. · Nov 18, 2013

Steve G. said:
Checking out my trusty Roy Bacon Twin Restoration guide, it shows the first Combat engine is # 200976,,,and the last Combat as the engine before # 212278 which was the first 'detuned' engine.

My copy of Roy Bacon's book shows 211110 as "First detuned" (which also ties-in with the factory Service Release literature).

Steve G. said:
March 1973 coincided with engine # 220000,

We know from certification plate date stamps that the "March 1973" date given for the beginning of the 220000 (750 MkV) series is not correct.

220000 production apparently began sometime around Oct./Nov. 1972.

Combat or not?

tpeever

Bob Z.

Torontonian

Johnnymac

ashman

L.A.B.

Moderator

G-Force

swooshdave

L.A.B.

Moderator

Steve G.

L.A.B.

Moderator

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