Atlas pushrod length

Atlas intake pushrods are .080" shorter [correction - they are longer] than Commando pushrods. Why? I only have Atlas heads on hand or I' d figure this out myself. Are the rocker spindles in different locations?

exhaust is the same on Atlas & Commando.

Because Commando valves are .100 longer. Jim

Hi, looks strange for me , i had always thought the atlas pushrods were 2mm longer than Commando ones......will check tomorow, Pierre.

marinatlas said:

Hi, looks strange for me , i had always thought the atlas pushrods were 2mm longer than Commando ones......will check tomorow, Pierre.

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That is correct. The atlas pushrods are longer and the valves are shorter. Jim

According to the '66-68 twins parts book and the '68-70 Commando parts book, 125871-on Atlas inlet and exhaust pushrods, tappets, valve and rocker part numbers are all identical to the early Commando.

Yes I think the early Commando used the Atlas parts but then they found excessive valve tip and guide wear because of the higher lift Commando cam so they lengthened the valves. I read that somewhere. Jim

Just to add to the mix and not intended to hijack the thread.... I remember reading a recommendation to remove .040 or was it .060 from the pushrods in the early combat engines because of the head milling? I need to go back and review part numbers etc but I don't think I ever did it to mine. WTF wouldn't one think the factory would have automatically done this? heh heh

I remember having my bottom spring cups on the intake side thinned down years ago to give more room for the higher lift Combat cam. Maybe this was to have the same effect as the pushrod shortening. Need to look at the geometry now. Also fuzzy memory on the heat insulating washers on the intake side -- a recent thread brought this up -- there or not there? I think I only have them on the exhaust side.The first time I ever took my head off one of the pushrods top cups was very wallowed out but back then all I did was replace the entire pushrod. Since then I have never seen this problem.

Back when all this was spinning around there was no Internet to get opposing viewpoints - just Norton News and long letters. Would be nice to create a table of what is "proper" for the various years. Hopefully by the time the 850s came out all this was all already worked out. Right...?

comnoz said:

Yes I think the early Commando used the Atlas parts but then they found excessive valve tip and guide wear because of the higher lift Commando cam so they lengthened the valves.

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According to Service Release N3/33 Feb. 1973,

"An improved inlet valve incorporating a stellite material hardened tip, similar to the exhaust valve has been fitted as standard on production models from Engine Number 221317.
The new inlet valve Part Number 064034 is fully interchangeable with previous types NM25501 and 063283 and should be used for all replacement purposes."



So the inlet valves apparently changed for 1972 (from NM25501 to 063283) and then changed again in early 1973 (to 064034), which also corresponds with the parts book numbers, however there doesn't appear to be any corresponding change to the inlet pushrod, as the part number (NM25515) remained unchanged all the way through production (eventually allocated modern part number (06-7940) for 1975.

batrider said:

Also fuzzy memory on the heat insulating washers on the intake side -- a recent thread brought this up -- there or not there? I think I only have them on the exhaust side.

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According to the '72 parts book: "NM23392 Valve Spring Heat Insulator....4 (2 only on Combat at exhaust position)"

That corresponds with the "Combat spec." Service Release information.

This from the Norton Tech Digest Vol.2 Pg.3:

Atlas VS Commando Valves & Push Rods
Beginning with eng. 125871 in 68, valves were lengthened by 0.80", and the push rods were shortened by 0.70" to improve valve geometry.

I don't recall exactly when the valve length changed but I was thinking it changed early in 68 and the recommendation was to replace the short valves with the longer ones with the corresponding pushrods. It may have been during the N or P15 run that the change was made instead of Commando.
I know that whenever I get any head for a rebuild with the short valves it goes out with long valves and shortened pushrods. The longer valves are available with the stellite tip which is not available on the short valve as far as I have been able to find. That in itself is a large improvement. Jim

comnoz said:

I don't recall exactly when the valve length changed but I was thinking it changed early in 68 and the recommendation was to replace the short valves with the longer ones with the corresponding pushrods. It may have been during the N or P15 run that the change was made instead of Commando.

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Atlas serial number "125817" I think would date to around Feb '68 (P11A?), which was also around the same period the first Commandos were being produced, so the combination of longer valves and shorter pushrods I'd guess were simply fitted to all 750 engines from around that time.

Also - do the black diamond valves have the same tip problem as stock valves - do they need lash caps as well?

According to the tech digest quoted above the same treatment was done to 650's at the same time so a few of them will have commando push rods and valves also.