650 Manxman head rebuild

I have a few nice Manxman heads that have never had the seats ground. I put a nice original valve in one of them in the intake position and it measured 1.662 from the head to the tip of the valve. Since the head used the same valve springs for exhaust too, I would guess that the dimension would be about the same for the exhaust also. Later on AMC switched to a longer valve and shorter pushrod, so you have to make sure you have the original shorter valve to match my measurements.

My old Manxman valve which looks to be really in good shape is 3.936" overall in length. When I put one of the later longer valves in an exhaust position It was standing 1.788" over the spring seat, an eighth inch higher, and all the later valves I measured were around a tenth longer than the old Manxman valve so the measurements sort of back each other up.

If your intake seats are sunk, you can put a 750 valve in which has a bit larger diameter and it should reach out and grab fresh un-sunk seat. With a street cam you might also get away with putting larger exhausts in too that would reach fresh seat, but you will have to check all your clearances.

I would do everything you can to avoid having new seats put into a head, it is a tough job to do well and true, and I would not trust anyone but the best in the world to do it right.

I did a valve job on a 650 Manxman once that had never before been touched. The valve seats cleaned up with maybe a second or less of grinding each. I then lightly ground a good used set of valves I had on hand, put my best and longest used springs in and threw an extra heat insulating washer under each spring to get them compressed a bit more and to make up for the little bit the valves may have sunk. The bike ran great and would wind right up over 6K rpm and more pulling hard all the way.

The best valve jobs are the ones that take the least grinding. If you have to grind a seat of valve more than a very little bit to get a new face on it then something is out of alignment or bent and needs someone smart to figure out what is screwed up. I have seen morons do valve jobs and grind seats down so far they must have taken a nap while leaning on the seat grinder. Then the seat is sunk, the flow goes down and the spring needs to be shimmed so much that the valve-train geometry is ruined.

The factory did a good job of getting the guide concentric to the seat. Once monkeys start changing guides without heating the head well, and grinding the seats with poor equipment or lack of skill and aptitude for the job, then your head can be in big trouble.....

Previously wrote:

Hi,

Dale or anyone in the know!, can you tell me the max length of valve stem from guide top to end of stem, so that I can decide if new seats are needed,

cheers.. Bob Presley

Hello Rob Hears the info you need Norton Manxman 650, part Number's Inlet Valve, Part Number 17221 head size 1.406" stem size 310"/309" exhaust valve part number T2204,head size 1.312" stem size 310"/309" and did you know that the Norton Manxman 650, Had A breath pipe from the Inlet rocker cover . Part Number,s. 23478 banjo assembly for rocker breather ,part number 23427 sleeve for rear cap , part number T2084 washer for the cap ,part number T1084 washer for breather banjo, part number 23426 stud for rear cap, part number 13937 dowl for rear cap ,part number E3218 domed nut for rear cap , So there' you are all part you need for the inlet rocker breather ?if you need any more part number let me know ?