MK3 commando head

[baz]

What rh number was used on the MK3 heads ?
My recent purchase has an rh4 was an RH10 also used in the MK3?

 

[Ron L]

As far as I am aware, the RH10 head was only used on the Mk2 850 (1974). RH4 was used on the MK1 (1973) and Mk3 (1975).

 

[Time Warp]

What rh number was used on the MK3 heads ?
My recent purchase has an rh4 was an RH10 also used in the MK3?

Only the MK2 and MK2A got the RH10.
Starting late 1973.

 

[baz]

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
This can't have helped with the MK3s already perceived reputation for being slow and overweight

 

[baz]

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
This can't have helped with the MK3s already perceived reputation for being slow and overweight

RH4 vs RH10 cylinder head performance

I have read many posts regarding these two cylinder heads. I have not found any hard numbers regarding HP and torque. It would be especially interesting if HP and torque values v RPM were available. Any info or references will be appreciated. Thanks, Ed

www.accessnorton.com

 

[Fast Eddie]

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
This can't have helped with the MK3s already perceived reputation for being slow and overweight

That’s why JS offers the port sleeves Baz, may be worth playing with…

 

[baz]

That’s why JS offers the port sleeves Baz, may be worth playing with…

Yep that's what I am thinking about

 

[Mart UK]

I love my lardy, unported RH10, single carb, balanced pipe, 23T sprocket, left shift, fat, old mk3. It flies.

 

[baz]

I love my lardy, unported RH10, single carb, balanced pipe, 23T sprocket, left shift, fat, old mk3. It flies.

Did you fit the RH10 head?

 

[Mart UK]

Doh, RH4.

 

[baz]

Doh, RH4.

Oh ok

 

[milfordite]

Unless you really care about a small gain in the mid range, just ride it! Like Mart UK, my MKIII has enough juice.

 

[baz]

Unless you really care about a small gain in the mid range, just ride it! Like Mart UK, my MKIII has enough juice.

That wasn't the original reason for my question
But a 3 or 4bhp increase in the mid range would interest me
Especially on a low power machine like a commando
Luckily I have a spare RH10 head on the shelf
And inlet sleeves are now available if I decide on that route

 

[worntorn]

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
This can't have helped with the MK3s already perceived reputation for being slow and overweight

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.

 

[Onder]

...of course, this was on a private road...

 

[worntorn]

Thankfully we still have those here!

 

[maaseyracer]

Its too bad the RH4 has those massive valve guides and they did not get larger valves in there. The JS Port Sleeves make a good mid range difference, and I some some relatively minor porting would pair well with the port sleeves.

 

[worntorn]

I believe the rh 10 uses the same valve guides as the rh4. Same valves as well.

Glen

 

[baz]

I believe the rh 10 uses the same valve guides as the rh4. Same valves as well.

Glen

Yep it does as far as I'm aware

 

[MichaelB]

The question I never understood is why they reverted to the RH4 on the MK III.