MK3 commando head

worntorn said:
That reputation of being slow is there for a reason- the stock black cap silencers. These were also on MK2A 850s and those bikes were also very slow on tests. The good news is that with the older type free breathing silencers in place, full performance is restored.
I have read various professionally written reports that claim the MK3 was detuned internally and weighs a huge amount more than other Commandos. This isn't so.
Other than the exhaust, It is tuned the same as an early 850 and those went a bit quicker thru the quarter mile than the 750s that are in my test collection
The reputation about the high weight is a very overblown thing.
The weight amount difference between the lightest Commando and the heaviest is very small as far as performance is concerned. That 30 pounds would make about 1/10 of a second difference in quarter mile time.
I weighed 4 Commandos on my hanging scale.
-A stock 71 750 with Interstate tank filled to 3" down weighed 444 lbs.
-A stock 73 850 with Interstate tank filled same weighed 447 pounds
-An Alton equipped 71 750 with Interstate filled same weighed 474 lbs
-My 850 Mk3 with Interstate tank filled same weighed 477 lbs.

I plan to try the JS tubes in my RH4 head soon. In the meantime, I don't find the bike to be at all lacking in power. The midrange seems very strong and the top end is pretty good as well.

Here is the 850 MK3 ( with open pea shooters and 260 mj) against that 750 with the Alton. The 750 has a Comstock ported head with D ports, high compression and a big cam.
The only time this modded 750 pulls away is when I let the MK3 rev above its power band in 3rd. When I finally put it in top the 750 stopped pulling away.
The over rev was due to a slight mixup. We were supposed to stop at 80 mph but he kept the throttle pinned to about 110 mph!
The 750 rolls on first which is an advantage. The time advantage means that if power is equal then the first bike to roll on should start to pull away immediately and keep pulling away. Up to about 80 mph that doesn't happen. One day we will do a rematch with the MK3 rolling on first and I am fairly certain it will pull away.

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Agreed about the black caps, I did back to back tests on an original black caps equipped standard Mk 3 and straight after with a set of straight through black caps, the difference was like night and day, the original ones killed the bike.
 
Sounds all too familiar, just slot in any other british bike manufacturer. :-(
 
baz said:
I hadn't realised the MK3 only came with the rh4 head
I remember commoz testing the RH10 against the RH4 and finding the top end the same but a boost in the midd range with the RH10
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I am very surprised reading this. I always thought RH10 was standard fitment at the Mk3.

Madnorton said:
Money - they had none when the MK3 was built so they hammered together anything they could find laying about.
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Do you think NV "found" 5000 cylinder heads? That is rather unlikely. Although JIT hadn't been invented back then, the call-off from the foundry would have been in the order of 1000 pcs, I guess.
There must be another explanation.

- Knut
 
mdt-son said:
I am very surprised reading this. I always thought RH10 was standard fitment at the Mk3.


Do you think NV "found" 5000 cylinder heads? That is rather unlikely. Although JIT hadn't been invented back then, the call-off from the foundry would have been in the order of 1000 pcs, I guess.
There must be another explanation.

- Knut
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Typo on the drawing?
 
I believe the factory simply got it wrong. They had no time or money for comprehensive flow bench and Dyno testing and they just believed that 32mm would be better, just like they did with the Combat.

Although a well ported 30mm head is widely accepted as the best for road use these days, 32mm can be made to work extremely well, as can 34mm and even 36mm ports, especially for pure performance (as proven by Maney amongst others). But the factory clearly lacked the time and / or money and / or knowledge when it came to detailed porting work / theory at that time.
 
Fast Eddie said:
I believe the factory simply got it wrong. They had no time or money for comprehensive flow bench and Dyno testing and they just believed that 32mm would be better, just like they did with the Combat.

Although a well ported 30mm head is widely accepted as the best for road use these days, 32mm can be made to work extremely well, as can 34mm and even 36mm ports, especially for pure performance (as proven by Maney amongst others). But the factory clearly lacked the time and / or money and / or knowledge when it came to detailed porting work / theory at that time.
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I think this is the most likely explanation
It'd be interesting to know what head was going to be used on the ill fated Norton 76 with the SU carb
 
baz said:
I think this is the most likely explanation
It'd be interesting to know what head was going to be used on the ill fated Norton 76 with the SU carb
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I believe that was just a lash up Baz rather than a seriously thought through production motorcycle.

So…probably had whatever head they had lying around…
 
Madnorton said:
Money - they had none when the MK3 was built so they hammered together anything they could find laying about.
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Considering how many changes were introduced on the 850 Mk3 and going by the numbers there were around 11000 Mk3s made I find that difficult to believe.
 
They were in administration from from 1973, and receivers were called in 1975, that would suggest they had no money.
 
Madnorton said:
They were in administration from from 1973, and receivers were called in 1975, that would suggest they had no money.
Click to expand...

They must have had money to introduce the electric start, left foot gearchange and rear disc or that they had several thousand RH4 cylinder heads laying around unused so I still don't believe it.
 
Madnorton said:
Money - they had none when the MK3 was built so they hammered together anything they could find laying about.
Click to expand...
When you look at the scores of little and big improvements made with the MK3, things like trans and engine circlips added, vernier isos added, crank strengthened, cases strengthened, rear swing arm stiffened, and rear disc brake added, that statement doesn't work.
All of those changes and many others required thought, design and capital input. They weren't just hammering the bike together.They were correcting earlier issues and adding an estart.
The RH4 is a puzzle. It's very doubtful that they had accidentally made thousands extra in 1973, and if they had wouldn't they have just used them in 1974 when they were also short of money?
The decision must have been made to cast that design as someone in charge felt that it was the better design.
Time has shown that the RH10 is superior, although I've had no issues with my RH4 which is now 1 year shy of 50 years old.
My friends 73 has 135,000 miles on his RH4, so they can last a long time.
We do know that they are more subject to cracking than the RH10 is.

Glen
 
baz said:
Hinckley thaiumpth!
🤣🤣🤣🤣🤣
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Triumph or Thaiumpth has bags of money and they keep making more all the time. They are in really good shape which gives them plenty of money for R&D, so far at least. I guess so far is now about 40 years already!

Glen
 
Also strange, RH10 drg approved 5/73 RH4 drawn 5/72. Strangely the drawing says that they should be machined from the same casting 06.3920. The Drg for RH10 was last amended 9/74 and the RH4 11/74. From the Dennis Poore notes we have that are not public, they had no money. AN had NOS castings of cylinder heads (type unknown) in 1991, we have proof of sale of these being sold - maybe they are still about.
The MK3 looked like a stop gap to satisfy the market with a LH gearchange, as we also have drawings dated 1974 & 1975 of parts that would have made a later model that would replace the MK3. Once in receivership the new drawings ceased and so did drawing updates.
 
The entire Commando range was a stopgap with a 1948 design engine!
The MK3 had about a hundred little and big expensive changes so that doesn't sound like just going with the same old same old for penny pinching purposes.
All of the Commandos ( except the Combat) worked and went remarkably well considering the age of the engine and trans design.

Glen
 
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Something happened at NV between September and November 1974 which prompted a change of direction. Maybe this change of direction has to do with plans for a successor to the Mk3?
Or the introduction of a revitalised electric start 750? If the planned successor was a higher performing 750 model, using the short-stroke engine as core for instance, commonality of parts meant a decision in favor of the RH4 head. Maybe Norman White is the man to ask about what went on in terms of forward planning at NV.

- Knut
 
I recall hearing the rh4 head was more prone to cracking. Though maybe that was not apparent back in the days when the bikes were new and men were men ;-)
 
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