Hi I've asked this on the BSA Owners Facebook group the following
Your help/thoughts please as per attached sketch. As I see it. The deeper the seat is cut affects where the valve sits within the crown of the head which in turn affects the length beyond the guide = spring compression. On my bike the exhaust valve protrudes a little beyond the crown ( & the spring are compressed more) where the inlet is cut deeper and sits below the crown. Hope that makes sense'
Hi,
Relevant to all models with valves. I would say that the head of the valve should sit slightly proud of the head/seat when the seats are unworn so your exhaust looks fine. As the seat and the valve are worn and probably lapped in many times the valve will sit deeper in to the head.
The difference in valve spring tension is unlikely to affect the running of a B33 whereas the shrouding of the valve will. When the valve is opening it will shrouded by the recessed seat and gas flow will be reduced and from there to fully open it will not allow the full amount of mixture to enter the combustion chamber. This could be seen by holding a valve in the fully open position and then closing it by the amount of seat recess that you have. It will not seem to be a lot, perhaps a couple of mm max, but on valves that don't open that much in the first place it could be significant.
A recessed exhaust valve is probably slightly less of a problem as the gasses are being forced out by the pressure in the cylinder.
Does this make sense?
Dave.
Quote from: DAVE BRADY on 07 Nov, 2025, 08:09 Hi,
Relevant to all models with valves. I would say that the head of the valve should sit slightly proud of the head/seat when the seats are unworn so your exhaust looks fine. As the seat and the valve are worn and probably lapped in many times the valve will sit deeper in to the head.
The difference in valve spring tension is unlikely to affect the running of a B33 whereas the shrouding of the valve will. When the valve is opening it will shrouded by the recessed seat and gas flow will be reduced and from there to fully open it will not allow the full amount of mixture to enter the combustion chamber. This could be seen by holding a valve in the fully open position and then closing it by the amount of seat recess that you have. It will not seem to be a lot, perhaps a couple of mm max, but on valves that don't open that much in the first place it could be significant.
A recessed exhaust valve is probably slightly less of a problem as the gasses are being forced out by the pressure in the cylinder.
Does this make sense?
Dave.
Thank you. OK I understand. So a recessed inlet is reducing flow and weakening mixture?
Quote from: RichardAB on 07 Nov, 2025, 08:30 Thank you. OK I understand. So a recessed inlet is reducing flow and weakening mixture?
The amount of mixture will be reduced but it won't be any weaker, just less.
Quote from: RichardAB on 07 Nov, 2025, 07:29 Hi I've asked this on the BSA Owners Facebook group the following
Your help/thoughts please as per attached sketch. As I see it. The deeper the seat is cut affects where the valve sits within the crown of the head which in turn affects the length beyond the guide = spring compression. On my bike the exhaust valve protrudes a little beyond the crown ( & the spring are compressed more) where the inlet is cut deeper and sits below the crown. Hope that makes sense'
also make sure to top and throat cut your seats to achieve the correct width valve seat and position on the valve, approx 1/3 down and two thirds up on your valve face. Better flow and a seat width that cuts through the carbon.
Cheers