A tale of two heads and one barrel. To spigot or not to spigot?

gpzkat

gpzkat

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My '66 N15CS came with two sets of barrels and heads:

One pair is spigoted that I think may be the originals. Barrels are OK overall but need re-bore due to rust, .020 may do it as it's not bad. It's cylinder head has one exhaust port with badly stripped threads though. It is PN 23166

The other pair is a shattered barrel (junk) and nice head, no damage to it, very clean, it is PN 25319.

I'd like to avoid thread repair and also stripping the spigoted head, as it's painted black. The later head is really clean. Other than the spigot they look very similar.

Q1: If I have the spigots shaved off the usable barrels (that I get overbored), will it be a match made in heaven with the later head? If not I can do plan B, and keep the spigots, have threads on 23166 fixed, but it would seem more expensive.

Q2: Who would be able to do either of these tasks, shave/overbore barrels or overbore/fix exhaust threads in Southern California?
 
According to my Roy Bacon Norton Restoration book the spigots were deleted for the 1966 model year from s/n 114870 so as your N15 is a '66 it did not have spigots, also be aware that from engine # 125871 the valves were lengthened and the pushrods shortened by I believe .100" so do not replace your valves with Commando Items!
 
possm said:
According to my Roy Bacon Norton Restoration book the spigots were deleted for the 1966 model year from s/n 114870 so as your N15 is a '66 it did not have spigots, also be aware that from engine # 125871 the valves were lengthened and the pushrods shortened by I believe .100" so do not replace your valves with Commando Items!
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Thanks for that, I'll measure my pushrods and 2 sets of valves and see. I read that too somewhere, my '66 is 119xxx, I was thinking the blown up barrel and head were mated to the 2nd lower end I have, which met a grisly demise. The numbers on the case are '70 Commando. So neither seem to be original.
 
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If I am understanding you right you are thinking one of your heads may be Commando? an easy way to tell is all pre Commando's had the oil feed to the rockers from the top where as Commando's had the rockers fed from the side of the head above the spark plug location.
 
Thanks again. I was guessing, but researched a bit and the spigoted one is early Atlas, non-spigoted is late Atlas apparently. Valves on both are the same accross the face, smaller than Commando at about 33.5mm / 1.32" intake and 28.5mm / 1.12" exhaust. They don't have the oil feed from the side location. I've cleaned up the later one and it's in very good condition. I think the plan will be to source a matching barrel for the late Atlas head and see if I can sell my spigoted one which is original size and very good overall but needing overbore. I think the bolt size to the head is 5/16 early vs 3/8 late, so mating up the spigoted with mod isn't straightforward.
 
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I have a 650SS and an Atlas both late '66 /early '67 with 119XXX s/n, the 650SS has 3/8 studs where as the Atlas has 5/16, if your good late head has 3/8 holes for the 4 studs is it practical to machine off the spigots and enlarge the 4 threaded holes to 3/8, I don't think there is a difference in the Barrell casting.
 
I just measured and the head and barrel are both 5/16. So no enlarging is needed. It is looking practical to mate these two up but I'm worried still that there's a catch!

Update: I was wondering about the height of the barrel and read another thread that has helpful info and measurements.

I shouldn't have chucked that shattered non spigoted barrel in the recycling bin or I could have measured myself, but the numbers are in the thread and from bottom to top gasket surface appear to be the same for early/late Atlas barrel. Still confused a bit after reading about the piston position at TDC, whether it comes to the top of the spigot or not, and if true, does the non spigoted head accommodate this. There is a bit of a recess. Getting closer.
 
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gpzkat said:
Thanks again. I was guessing, but researched a bit and the spigoted one is early Atlas, non-spigoted is late Atlas apparently. Valves on both are the same accross the face, smaller than Commando at about 33.5mm / 1.32" intake and 28.5mm / 1.12" exhaust. They don't have the oil feed from the side location. I've cleaned up the later one and it's in very good condition. I think the plan will be to source a matching barrel for the late Atlas head and see if I can sell my spigoted one which is original size and very good overall but needing overbore. I think the bolt size to the head is 5/16 early vs 3/8 late, so mating up the spigoted with mod isn't straightforward.
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Those valve sizes are for a Dommie, not an Atlas, so you probably have at best a 650 head. Are the bolt holes for the inlet manifolds horizontal, or arranged vertically across the ports?

If you have a later non spigotted (750) barrel or remove the spigot, and have an Atlas head that was originally for a spigotted barrel, there inserts that go into the head to replace the spigot
 
Seeley920 said:
Those valve sizes are for a Dommie, not an Atlas, so you probably have at best a 650 head. Are the bolt holes for the inlet manifolds horizontal, or arranged vertically across the ports?

If you have a later non spigotted (750) barrel or remove the spigot, and have an Atlas head that was originally for a spigotted barrel, there inserts that go into the head to replace the spigot
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The holes are oriented horizontally and measure 74mm across the chamber. I have a 2:1 intake and Mikuni 34mm VM carb.

A tale of two heads and one barrel. To spigot or not to spigot?


I had a non spigoted head but it was a bit rough so I tossed it :oops:

A tale of two heads and one barrel. To spigot or not to spigot?
 
Ok, so it's probably an "SS" head, from a 500- 650. What is the measurement of the counterbore for the spigot in the head?
 
Ok, so it's probably an "SS" head, from a 500- 650. What is the measurement of the counterbore for the spigot in the head?

Not correct, the Atlas N15 did not have the same size valves as a commando! look it up, we have just been down this road with the '65 N15 that I am helping with- The engine builder mistakenly ordered Commando valves which were a different length and inlets were a different diameter as well.
 
Counterbore is 79.5 or so for spigoted. The counterbore for the non spigot is the same depth but of course smaller. So the pistons will not be impeded in vertical stroke. BTW I have the original lower compression convex pistons, but can't re-use due to the need to bore the barrel. I doubt there's much demand for them but would make an easier kick start I imagine.
 
Counterbore is 79.5?
Not sure what you mean by counterbore? a standard 750 is 73x89 ie 73mm bore by 89mm stroke even a 850 is only 77x89. Re your pistons concave [7.5 compression ratio] are very hard to find, in a solid mounted m/cycle vibration can be a killer, I have 7.5 pistons in my Atlas and it is actually smoother than my 650SS maybe it had been balanced by a previous owner unsure! my friend with the N15 was unable to get concave pistons so had to go for 8.9 Commando pistons and have it dynamically balanced but as yet we have not had that bike running.
 
"but would make an easier kick start I imagine"

There is not a lot of difference in effort to start my Atlas as compared to start my '69 Fastback in my view, though starting an 850 is a different story [for me anyway] so am building a Mk3 with an electric start.
 
possm said:
Counterbore is 79.5?
Not sure what you mean by counterbore? a standard 750 is 73x89 ie 73mm bore by 89mm stroke even a 850 is only 77x89. Re your pistons concave [7.5 compression ratio] are very hard to find, in a solid mounted m/cycle vibration can be a killer, I have 7.5 pistons in my Atlas and it is actually smoother than my 650SS maybe it had been balanced by a previous owner unsure! my friend with the N15 was unable to get concave pistons so had to go for 8.9 Commando pistons and have it dynamically balanced but as yet we have not had that bike running.
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The recess in the head to accept the spigot is what I took as the meaning of counterbore. Yes the bore of the cylinder is for 73mm pistons.
 
gpzkat said:
The recess in the head to accept the spigot is what I took as the meaning of counterbore. Yes the bore of the cylinder is for 73mm pistons.
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Yes, the counterbore in the head. 650/Dommies were around 74mm, which is why I asked. So, the head is definitely for a 750 (although the casting number is the same as 650/Dommie ones)
 
Seeley920 said:
Yes, the counterbore in the head. 650/Dommies were around 74mm, which is why I asked. So, the head is definitely for a 750 (although the casting number is the same as 650/Dommie ones)
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I read that "All 650's and 750's Dominator have top oil feed, and share same casting 25319# (with small exception 06-0380)" on a Norton Heavy Twins Heads page. Seems a bit odd to use the same number but what do I know.

I sort of like the idea of using the original lower compression pistons, which means sleeves or possibly plating - either of which sounds expensive. Thanks to Covid I'm on a pretty tight budget so that will probably be the deciding factor.
 
"The recess in the head to accept the spigot is what I took as the meaning of counterbore"

I understand! I believe it is not recommended to re-sleeve a 750 barrel as it weakens the " flange area where it mounts to the crankcases, possibly what happened to the barrels in the picture you posted.
 
Re; spigot barrel. Don't shave spigot off- the reason the spigot prevents the liners dropping down onto the crank , don't ask me how I know!!!
 
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