MK3 commando head

worntorn said:

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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I have always thought the commando to be one of the best bodge ups in motorcycle manufacturing history
The bike is better than the sum of its parts

worntorn said:

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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I think I read somewhere there are 150 differences on the MK3?

Tornado said:

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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I'm sure that is correct, it took time for the problem to show up. In 1974/75 Norton managers might have also thought the 32 mm port RH4 was the higher performance head.

The 73 850s with the RH4 in my book of tests had the fastest quarter mile times of all the Commandos tested in that book, so they might have gone by that information. The RH4 doesn't seem to be lacking in peak power. It's the midrange that is said to be better with the rh10.



The weird thing is that the cracking doesn't happen to all rh4 heads. Mine has been problem free as have the other local bikes that I know of.







Glen

baz said:

I think I read somewhere there are 150 differences on the MK3?

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That sounds about right. I started a thread on all of the differences a few years ago. I learned a lot from the info posted!
The common theme emerged that Norton was trying to improve the Commando with the MK3. Actually they were trying to improve the bike right from 67 on but some changes weren't successful.

Not long after I bought my MK3 I met the President of the BC Vintage Motorcycle group. He spotted my bike in a store parking lot and was having a good look at it when I came out of the store. I didn't know much at all about the various models at that time but I liked the idea of estart and getting a model from the end of a production run vs the start.
He informed that the nice thing about the MK3 was that it already had the changes that he and his friends were busy adding to their earlier bikes. He mentioned that MK3 vernier isos and the stronger MK3 swingarm were two things he planned to fit to his earlier bike.

Glen

Madnorton said:

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.

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In which case there can't have been all that much difference in the machining cost if both head types were machined from the same casting.
Also although the '73 and '75 heads were RH4 the part numbers are different although the Mk3 parts book does say "with guides" but with a lower part number than '73.

Madnorton said:

The Drg for RH10 was last amended 9/74 and the RH4 11/74.

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And what exactly was that RH4 11/74 amendment?

Brian Slark said that the MKIII's were the best of the Commandos. Obviously top end performance wasn't the deciding factor, rather the overall bike. I certainly love mine!

Tornado said:

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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I think the thinner intake head would have recorded a few cracks from brutal intake guide install ,after factory and later ones machine shops , after sale . Correct me please ?

Torontonian said:

I think the thinner intake head would have recorded a few cracks from brutal intake guide install ,after factory and later ones machine shops , after sale . Correct me please ?

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The RH4 is prone to cracking with over tight valve guide fit and agressive install or removal. The head is not much thinner only 1mm, but the 5/8" valve guides open into a thinner range on the head, this is the bigger issue than the port diameter, though that is not help.

Later in the week I'll see what the amendment were, they may have had more than one machine shop machining the heads and then dropped to just one. I have the process cards from the 70's as well as the MK3 build book so I will look and see if they add any info.