[Stanley]

FWIW, I recall seeing posts on this and other fora describing the fragility of the accessory Hitchcock seals due to the tightness on the guide. This persuaded me to assemble my new valve train sans seals. The Hitchcock parts diagrams showed no seals for my bike.

[Tarnand]

Is it possible that the seal was hit by the spring keeper causing it to split? Clearances may be tight, since the factory had not installed seals. When did the seal fall apart? Hitchcocks ominous warning about cam size speaks to the clearance problem.

That is true.  However, I am using standard (factory) cams and only the exhaust seal broke apart.  The inlet seal, for now, seems to be fine.  Hitchcock usually sells good quality parts but I have no way of telling quality of this one.  After all for some reasons it might have been defective.   As for the factory ... we know that all too well that our bikes could have been built much better.

[Tarnand]

FWIW, I recall seeing posts on this and other fora describing the fragility of the accessory Hitchcock seals due to the tightness on the guide. This persuaded me to assemble my new valve train sans seals. The Hitchcock parts diagrams showed no seals for my bike.

It appears that it is precisely what I will end up doing especially that next year I am planning on replacing the piston.  The one I want is #90205 but Hitchcock ran out of stock in NOVEMBER and God only knows when it will be avaialbe again. 
By posting this subject I was also hoping to receive some input regarding whether installing these valve seals does make any difference.

[Paul W]

It might be of course that you are trying to fix a problem that doesn’t actually exist.

I was a little surprised to find a lack of valve guide seals on my own bike (a 350) when I stripped down the cylinder head. It had only covered 3,000 miles in the first 13 years of its life with four previous owners but had a soft exhaust valve seat and kept losing compression. However, I was also pleased to see how well the valve stems fitted in the guides and decided to rebuild it as it came, i.e. without guide seals. It doesn’t burn much oil, actually less now I’ve put another 10,000 miles of hard riding on it and removed the breather catch can. I don’t think it was fully run in before I bought it.

[ace.cafe]

It appears that it is precisely what I will end up doing especially that next year I am planning on replacing the piston.  The one I want is #90205 but Hitchcock ran out of stock in NOVEMBER and God only knows when it will be avaialbe again. 
By posting this subject I was also hoping to receive some input regarding whether installing these valve seals does make any difference.

The stock oiling system with stock pumps has a small amount of delivery to the head.
The stock rings are fairly poor, and don't create massive vacuum levels during induction.
This combination is unlikely to cause much oil to go into the cylinder, as long as the guides are in decent working order.
Seals would help as wear becomes greater.
In a modded engine with more oil delivery and better ring sealing, the need for seals would increase.

[Tarnand]

The stock oiling system with stock pumps has a small amount of delivery to the head.
The stock rings are fairly poor, and don't create massive vacuum levels during induction.
This combination is unlikely to cause much oil to go into the cylinder, as long as the guides are in decent working order.
Seals would help as wear becomes greater.
In a modded engine with more oil delivery and better ring sealing, the need for seals would increase.

I have not get to replacing the stock pumps yet.  However, due to lack of a new piston I am planning on using the old one for the rest of this season. I replaced rings to new Total Seal set though.  The factory guides were not long ago, maybe 2000 miles, replaced with Hitchcock's # 25524/3  ("An English made phosphor bronze valve guides from our own workshop.")  so I consider them as nearly new.  I think I should be all right.