Atlas head and barrel query

My understanding is that it is ok to machine the spigot off. You can buy an alloy recess ring to fill the gap in the head.

See RGM part # 025494; £6.49. www.rgmnorton.co.uk

I can't insert a link, for some reason. Check before you buy that it is for a 750, not 650.

It sounds like he doesn't need the alloy recess rings as he's got the later head without the recesses.

The later head also has the 3/8 holes for the outer 4 head bolts. The tapped holes in the spigotted barrel are are only 5/16 so these will require opening up to 3/8" cycle as well as machining off the spigot.

Correct, Jim. I should have read the post properly.

By coincidence, I now have a similar issue. Having stripped the top end of my 1964 650SS engine in order to fit new rings, I spotted that the liners are starting to crack where they protrude above the barrel. They are +0.40". A local engineering company can machine the spigots off.

My question is, do I need to fit spigot rings? I seem to recall this coming up in the past, but can't find it, and my memory says they are not necessary. It will result in a slight reduction in CR, but probably not enough to notice.

Another point is that, if the head has been skimmed at some point in it's 52 year life; if only to true it up; the rings may then be too thick?

If the rings are not needed, do I use the head gasket for the spigotted barrel? The gasket for the non-spigotted barrel would only be supported on one side otherwise, which I cannot imagine can be a good thing.

Ian

Just a point to watch,i have a '67 Atlas with non-spigotted barrel and head.The outer head bolts are 5/16ths-not 3/8ths.Apparently slightly later ones were 3/8ths,as per Commando. Another one of those small anomalies which keep us interested!!

If you head is for spigoted barrel and its now flat then you need the rings, thickness needs to match the depth of the recess in the head so if its skimmed then the rings need thinning.

If you don't fit the rings, the resulting recesses gradually fill up with carbon. Doesn't seem to do any harm. Well, it didn't when I did it.

That was what I thought, Gordon, but I needed to make decision on it, and 10 minutes ago I ordered the rings and appropriate head gasket! A bit annoying really, as it has added £40 to the job, and I had just bought a top-end gasket set, for the spigotted setup. I just hope the rings fit without issues.....

Having had the spigots machined off, and spigot rings and non-spigotted head gasket at the ready, I have just re-fitted the barrels. However, now the spigots are gone, the piston crowns are 1/10" above the barrel face at tdc. Therefore they clearly went that far up the spigots. This means that if I use the spigot rings and appropriate gasket, the pistons are likely to touch them.

Does this mean that the non-spigotted barrels are slightly taller, measured to the barrel face, than the older spigotted ones? Remember this is on a 650, not an Atlas, as was the subject of this thread originally.

Whatever, to ensure clearance, I will have to re-assemble without the spigot rings, using a head gasket for the spigotted barrels.

Hopefully it will carry on happily as yours has done, Gordon!

Relating back to the original post, I do not know whether the same applies to the Atlas and the 650SS, but in the light of my findings as detailed in my previous post, I certainly could not fit a later head onto the earlier spigotted barrel, with the spigots machined off.

Paul, your friend needs to take some careful measurements before going down that route, although it will be obvious whether it will work or not once the spigots are machined off and the barrels re-fitted, but it will be a bit late by then.

.... or my con-rods too long! lol... I did turn it over with the non-spigotted head gasket in place and the pistons touched. In theory it shoudn't, but in practice aftermarket gaskets seem to fit where they touch, and unless it is absolutely centralized over the bores, which it did not want to do, then it is running too close for comfort. The rings were also too thick, holding the head off the barrels, so would need turning down, a delicate operation considering how thin they are anyway. This has not enhanced my view of aftermarket parts....!

You can get a compression plate that fits under the barrel.

The tech digest shows #124370 as 1st 68 and 125770 as the last atlas..

FWIW my 68 dunstall atlas is a relatively high # atlas 1253xx. It still has 5/16CEI head bolts.

Robert. I see RGM supply compression plates, which are .030" thick. My piston crowns are actually .125" above the barrel face, so they won't really help.

Also, that could lead to issues with pushrod lengths. running out of adjustment etc. Been there.....!

OK Paul, thanks for that. I will use the gasket for the spigotted barrel, which hopefully should be fine, but I will certainly bear that in mind.

Out of interest, is that in solid copper, or composite materials?

Ian

Ian

You could just use a spigotted barrel gasket as Gordon suggested and I'm sure it will be ok, but the preferred engineering solution would be to have the barrels re-sleeved to standard to re-establish the spigots.

On a standard bore, the spigot is only 2mm thick reducing to 1.5mm when bored to the recommended maximum 40 thou oversize which yours is on, therefore no surprise that they are prone to cracking.

I was surprised to read that recess rings were available for the 600/650 head because of the lack of spigot thickness compared to the 750 Atlas, therefore your question "Does this mean that the non spigotted barrels are slightly taller, measured to the barrel face, than the older spigotted ones" is very valid and one I would be interested in knowing the answer.

Jim. A preferred engineering solution it may be in theory, but in practice plenty are run without the spigot, and a re-sleeve and new pistons will end up costing the thick end of £400, when I have good +.040" BHB pistons in there at the moment which have barely covered 1500 miles, so I have no intention of consigning those to the bin just yet.

As you say, the spigot recess rings are very small in cross-section for the 650. I didn't feel too comfortable using them, particularly as they are too tall, so needed thinning anyway.

Work is getting in the way of me putting the head back on and finishing it off ready to fire up, but hopefully I will find time in the next week.

I will report back!

Ian

Having returned the spigot rings and non-spigot gasket to Norvil, I had a conversation with them on Friday. It seems the spigot rings are designed to work with a maximum of 0.020" oversize pistons, as 'that was all that was available at the time'. Unfortunately there is no mention of this when one buys them. This is why I found the pistons were touching the rings when I offered them up at first. I don't know how long +0.040" pistons have been available, but quite a long time, I would have thought.

Their suggested solution is to machine an appropriate amount off the perimeter of the top of the pistons, above the top ring obviously, to provide clearance. This because the pistons protrude 1/8" above the barrel face at tdc. Then, stick the rings in place with some sort of high temperature silicone and machine them in situ so they are flush with the head. I am not an engineer but I suspect this could be a difficult operation, as the thin alloy rings could move and break.

I am inclined not to do this, which means putting it back together with the spigot gasket. Not ideal, but the head has already been skimmed flat, the barrel has been skimmed flat, so the gasket has a fighting chance of doing it's job; hopefully......?

Hi Ian,

If you plan to run without the spigot rings but use a non spigot gasket, be very careful of your gasket choice.

When I first had the head off my 650 (many moons ago...) the only gaskets available at the time were for the spigoted heads and the one I got had no fire ring - the crimped metal edge that goes around the bore. Ignorance is bliss as they say, and never having had a Norton before, I assumed that they were all like that as I had no knowledge of the two head types.

The result was the push of shame as the gasket failed on both cylinders within 4 miles on my first ride, and yes my barrels and head had been freshly skimmed.

Perhaps the modern gaskets are now better, but my fix was to go initially to a solid copper gasket, but when I got fed up with the oil leaks, I got a friend in the trade to use his contacts with Reinz who's apprentices made me a super duper proper gasket that is still fitted today.

Regards,

George.

George. It is the gasket for the spigotted barrel which I intend to use. It is a composite gasket, which has a crimped edge to it.

Using the non-spigotted gasket without the spigot rings would mean the inner edge of the gasket would be unsupported from above.

Hi, Paul, I know this is an old thread, but, I have a Proper head or two in my collection, so, If you are stuck, let me know, I might be of assistance. I have a mill and lathe, I used to be a toolmaker once. Best regards, Paul