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Drilling Atlas/P11 head for 3/8 head bolt size?

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Drilling Atlas/P11 head for 3/8 head bolt size?

Postby elefantrider » Mon Apr 17, 2017 12:46 am

Why and when did the factory switch to 3/8 size bolts instead of 5/16 size as used on Atlas/P11?
Is this a common upgrade to do now? The machine shop offered to convert when sent parts there for machine work, but we declined.
Seems to be holding fine.

Now fast forward to another project and we are trying to match up a good used barrel with the bike's original P11 head.
Turns out we have a good used barrel but it has 3/8 holes. It has the exact same stamping number as the original 5/16

Have many of you have upgraded to 3/8 bolts?
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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby pierodn » Mon Apr 17, 2017 11:43 am

I have a 1969 R type 20M3/130... with Commando head but still the 5/16 bolts.
Last edited by pierodn on Mon Apr 17, 2017 12:26 pm, edited 2 times in total.

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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby CanukNortonNut » Mon Apr 17, 2017 11:48 am

elefantrider wrote:Why and when did the factory switch to 3/8 size bolts instead of 5/16 size as used on Atlas/P11?
Is this a common upgrade to do now? The machine shop offered to convert when sent parts there for machine work, but we declined.
Seems to be holding fine.

Now fast forward to another project and we are trying to match up a good used barrel with the bike's original P11 head.
Turns out we have a good used barrel but it has 3/8 holes. It has the exact same stamping number as the original 5/16

Have many of you have upgraded to 3/8 bolts?


If the holes are 3/8th then you probably have a commando barrel. I am not sure when they changed to 3/8. Probably when the tachometer was moved from the end of the timing cover.
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Thomas
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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby elefantrider » Mon Apr 17, 2017 12:26 pm

pierodn wrote:I have a 1969 R type 20M3/130... with Commando head but still the 5/16 bolts.


Do you run washers under the head bolts? Holes in head are not too big?
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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby pierodn » Mon Apr 17, 2017 12:27 pm

But my 1968 Commando has 7/8 bolts and Commando head!
Piero

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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby pierodn » Mon Apr 17, 2017 12:30 pm

elefantrider wrote:
pierodn wrote:I have a 1969 R type 20M3/130... with Commando head but still the 5/16 bolts.


Do you run washers under the head bolts? Holes in head are not too big?

Atlas/P11, i have washers under head bolts.
Holes for 5/16.

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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby elefantrider » Mon Apr 17, 2017 12:44 pm

Here is my '68 P11 with 5/16 bolts. As you can see, oil is on the head, accumulating on the left fins. Looks like the oil is coming from the bolt washer area?
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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby CanukNortonNut » Mon Apr 17, 2017 2:52 pm

Sometimes oil on fins are hard to locate the source. Check the long 1/4 Whitworth nut under front head location. (torque tightening pattern #7) to see if that is loose or stripped :( . Are you using a copper gasket?
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Thomas
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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby BillT » Mon Apr 17, 2017 3:58 pm

elefantrider wrote:Here is my '68 P11 with 5/16 bolts. As you can see, oil is on the head, accumulating on the left fins. Looks like the oil is coming from the bolt washer area?


I doubt the oil is coming from the head bolts. More likely the rocker pivots are weeping around those gaskets and settling around the bolts. My Ranger - built 15 Oct 1968 - has 5/16 head bolts, so I agree they were probably changed with the points relocation (20M3S). Rangers came with Commando head castings.
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The boss between the rockers for Commando oil feed is only found on the Ranger, from what I've seen
'73 Norton 850 Commando - 3030xx
'69 Norton Ranger 750 - P11/1289xx
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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby Ron L » Mon Apr 17, 2017 5:39 pm

We have a 1968 P11A with Atlas style casting head with 5/16 bolts, three Ranger 750's with Commando style casting head, all with 3/8 bolts, and three 1967 P11's with Atlas style castings, two with 3/8 bolts and one with 5/16.
The '68 P11A is a one owner, so has never had the head bolts changed. I suspect the two '67' high pipers with 3/8 have been drilled and probably have Commando cylinders, as they both have Commando pistons in them.

It seems odd that any Commando engine would have 5/16 head drillings.
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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby elefantrider » Tue Apr 18, 2017 4:33 am

It's got a composite flame ring gasket. Wish I knew where the oil is coming from. It is not a lot of oil, but the bike still has less than 100 miles on it since the top end rebuild. I have read on here that those top bolts do not go into oil, so am looking elsewhere.

The 3/8 bolt heads are a little taller than 5/16 bolts - I believe the only way to tell the difference from the outside.

So is the general consensus that 3/8 is a good upgrade on early heads for a better clamping force?
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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby pierodn » Tue Apr 18, 2017 12:03 pm

Oil can come from spindle locking plate or valves cover.
Never seen from head bolts.
Mine lost oil from oil feed banjo bolt/wahers.

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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby texasSlick » Tue Apr 18, 2017 2:15 pm

elefantrider wrote:It's got a composite flame ring gasket. Wish I knew where the oil is coming from.


I have heard of a product, that when added to engine oil, appears as bright red at a leak site. "Seeing Red" strikes me as the name of this product, but do not take that as Gospel. Check with your local auto parts supplier. Once you are certain the source of the leak, or weep, you can deal with it.

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The Second Law (of thermodynamics) rules.
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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby dynodave » Sun Apr 30, 2017 2:45 pm

BillT wrote:snip
The boss between the rockers for Commando oil feed is only found on the Ranger, from what I've seen


The 06-0380 casting/core (68-70) most common for 20M3 commando heads was also the basis for 68 650 Mercury also...
But first time I heard of your application besides commando/mercury
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Re: Drilling Atlas/P11 head for 3/8 head bolt size?

Postby dynodave » Sun Apr 30, 2017 4:25 pm

elefantrider wrote: So is the general consensus that 3/8 is a good upgrade on early heads for a better clamping force?

I would not absolutely agree. If you read Jim Comstock's comments on necking down the commando 3/8 head bolts you might realize why.
I found his logic compelling and in complete harmony with the stock necked down bolts used on Lotus Elans that I have owned since the early 80's. They have aluminum heads like out nortons and the reasoning is the same...thermal expansion differences between steel bolts V.S. aluminum heads.
I am a big proponent of thicker hardened washers to spread the force of the bolt heads instead of softer small diameter washers and which upon thermal cycles tends to dent the head more . Then the head becomes self loosening....
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