1967 Atlas Head Bolts

As some of you know, I’m restoring a 1967 Atlas. Recently I noticed a “ mint” head for sale on EBay that caught my attention. It said for use on Atlas’s from 1965 thru 1967. What caught my attention was that all of the bolts and studs required for this head were 3/8”. It was a top oil feed. Now my head and barrel which I thought was original to the bike have four 5/16” head bolts, one short 3/8” head bolt, three 3/8” studs that screw into the head, and two 5/16” studs up front that screw into the barrel. I know that this topic has been discussed exhaustively but am I actually working with a head and barrel from an earlier Atlas run?

No what you have is typical for your year.

I and I think many others interested in performance had the 4 holes near the spark plus drilled for 3/8" bolts and the barrels drilled and tapped for 3/8" bolts. The 3/8" bolts hold better. At least that is what I told myself when I did it. Torque can be increased 5 ft lbs on those 4 3/8" bolts. Well, it can on my modified 1967 Norton P11 head.

Schwany said:

No what you have is typical for your year.

I and I think many others interested in performance had the 4 holes near the spark plus drilled for 3/8" bolts and the barrels drilled and tapped for 3/8" bolts. The 3/8" bolts hold better. At least that is what I told myself when I did it. Torque can be increased 5 ft lbs on those 4 3/8" bolts. Well, it can on my modified 1967 Norton P11 head.

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Thanks. Did you use Time-Serts in the barrel?

My late 1966 Atlas still has 5/16 bolts and is fine.

I think they used 5/16 x4 bolts to leave a bit more meat in the gasket

Commando side oiled heads use 3/8" bolts and bolt right on my P11 barrels.

My P11 head worked alright with the smaller diameter bolts when the engine was 100% stock dished pistons and all. Not so much after I made some engine changes.

jms said:

Thanks. Did you use Time-Serts in the barrel?

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jms

Time-Certs were not used in the barrel deck. Iron is plenty tough enough. The 3 studs in the head have something like Time-Certs cuz the old heads are kind of soft. Bob Raber's machinist did the work for those 4 holes in the barrels. Those holes need to be 100% straight and perpendicular to the deck.

This has always been a mystery to me. I had assumed that all 750 engines came with 3/8 head bolts from 1968-on, since this was the debut of the Commando. However, my 1967 P11 has 5/16 bolts, my 1968 P11A has 3/8 bolts (probably retrofitted), and both of my 1969 (late 1968 build date) Ranger 750 engines have 5/16 bolts despite being a Commando head casting! Both my 1968 Commando engines have 3/8 bolts. Go figure!

Ron L said:

This has always been a mystery to me. I had assumed that all 750 engines came with 3/8 head bolts from 1968-on, since this was the debut of the Commando. However, my 1967 P11 has 5/16 bolts, my 1968 P11A has 3/8 bolts (probably retrofitted), and both of my 1969 (late 1968 build date) Ranger 750 engines have 5/16 bolts despite being a Commando head casting! Both my 1968 Commando engines have 3/8 bolts. Go figure!

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It does get a little crazy doesn’t it!

jms said:

It does get a little crazy doesn’t it!

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I don't think so. Remember, these are different engines wrt. compression ratio. Obviously NV found that the Commando engine needed the 3/8" bolts. Besides, maintaining compatibility with existing parts lists is an argument by itself. Dominators, G15 series and P11 series were running out. Why change something that works and create an additional logistic and documentation challenge? Plus having to answer all the questions and allegations of "poor engineering" that owners of earlier engines would have brought forward. NV did right.

- Knut