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

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?

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

elefantrider said:

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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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.
Cheers,
Thomas

pierodn said:

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

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

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

elefantrider said:

pierodn said:

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

Click to expand...

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

Click to expand...

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

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?

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?
Cheers,
Thomas

elefantrider said:

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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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.


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

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.

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?

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

elefantrider said:

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

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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.

Slick

BillT said:

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

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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
thanks

elefantrider said:

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

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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....

elefantrider ,
Just out of curiosity... LOOKING AT YOUR PICTURE. Regarding oil leaks. Did you use the original head bolts on your rebuild? These would tend to be stretched. Recommendation to use new bolts. Did you just want to keep the patina?
Cheers,
Thomas

Ron L said:

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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There would seem to be a bunch of transitions in production.
pre commando, early thru 65: spigot
The early NHT small bolt pattern 500,650, mold # 23166 on the bottom, not part#, is 3/8 bolts and with spigots.
The early NHT med bolt pattern (750) mold # 23166 on the bottom, not part #, is for spigotted barrels with very little clearance to the combustion chamber...5/16 bolts.
pre commando head 66-68 no spigot
The later NHT head small bolt pattern 500, 650SS,PN 25358. mold # 25319 on the bottom, , NO SPIGOT, 3/8 bolts This same application was remade with 06-0380 commando core for mercury.

The later NHT head med bolt pattern, (750), mold # 25319 on the bottom, seems it kept the 5/16 bolts for some applications and obviously (NOW) used 3/8 since the spigot was gone and now had more distance to the combustion chamber.
I have one of these heads and am using it with commando barrels for my 65 atlas/combat/hot rod engine. I previously thought it was a home brew 3/8 conversion but now I know better.

A lot of transitional components possible with the introduction of the 06-0380 cores small and med bolt pattern heads 5/16 and 3/8 bolt pattern for featherbed/mercury or the atlas/hybrids.

If you know the norton part # for these "new to me" heads please PM or click on my website and contact me.. I am missing many norton IPL's
Thanks guys for this new data.

CanukNortonNut said:

elefantrider ,
Just out of curiosity... LOOKING AT YOUR PICTURE. Regarding oil leaks. Did you use the original head bolts on your rebuild? These would tend to be stretched. Recommendation to use new bolts. Did you just want to keep the patina?
Cheers,
Thomas

Click to expand...

Yes Thomas, on that bike (124888), we did use the 5/16 original bolts to keep the look consistent with the rest of the bike (pictured right).
On this bike (121687), we are using 3/8 bolts because the original barrel was damaged and the replacement already had 3/8 holes.

BR,
Chris