I am tomorrow going to refit a head that has just had two stainless steel exhaust valves fitted to replace the older steel ones. Someone mentioned that I would need to set the tappets at a bigger clearance as the SS expands more?
Comments invited from
1. Anyone running SS, what have you done?
2. Any calculations based on coefficient of expansion?
3. Any just gut feel?
Clearance on stainless steel exhaust valves
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Alan Stanley
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Mike Allfrey
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Dear Alan,
I have been using stainless steel exhaust and inlet valves now for a few years. My method is to use the Royal Enfield Bullet trick:
Ensure that the engine is cold.
Rotate the crankshaft until the valve to be adjusted is fully open.
Rotate one more exact turn, the tappet will now be against the heel of the cam.
Adjust push rod length so that rod can be spun freely between finger and thumb.
Do the same for all valves.
I am running hardened valve seats and the clearances have not altered after two checks to monitor the situation, I am also using Bill Lock camshaft and tappets.
Regards,
Mike Allfrey.
I have been using stainless steel exhaust and inlet valves now for a few years. My method is to use the Royal Enfield Bullet trick:
Ensure that the engine is cold.
Rotate the crankshaft until the valve to be adjusted is fully open.
Rotate one more exact turn, the tappet will now be against the heel of the cam.
Adjust push rod length so that rod can be spun freely between finger and thumb.
Do the same for all valves.
I am running hardened valve seats and the clearances have not altered after two checks to monitor the situation, I am also using Bill Lock camshaft and tappets.
Regards,
Mike Allfrey.
E0 SA 42R; Rover 75
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The expansion difference between the two part valves and the stainless would be minimal. The same could be argued about tie bolts and head studs that could also be made of stainless.
The Jowett engine is a complex beast with regards to expansion, since not only are there other materials in the bores and their seals and head gasket; but also differentials of temperature such as hot and cold spots around exhaust valve and front of engine would also affect the strain exhibited by all the components under stress (and hence the gap of the tappets).
I also have aly push rods that would change any such calculations based on a standard build.
It is important to realise that all valves need to be closed at the right time so as not to let gas in or out at the wrong time. Thus valve timing (camshaft setting and cam profile) might greatly affect the decision on what gap to choose. Exhaust valves need to be closed too for the cooling factor which also depends on the sealing area created when they were ground in.
The basic assumption for having valve clearance is that when the engine heats up the gap will close a bit. That might not be true the Jowett engine under all build configurations or uses.
Competition work requires geting the most gas in and out of the cylinder so minimizing the tappet clearance is a priority, otherwise some valve lift is lost. Large gaps put excessive strain on the valve gear as the tappet will hit the valve at speed.
I too use Mike's method; but for safety give the exhaust valves an eighth of a turn more gap to allow for any recession on long rallies. Note that my in head thermometers give early warning of exhaust valve burn. Valve gaps will also change if push rod cups and balls are not symettric, the push rod or its end are bent, the tappet or the cam heal are not flat and parallel (which would alter with end float of cam or valve gear). A quick check is to use Mike's rotating method and make sure there is no binding or slackness when rotating push rod or axially moving the camshaft or tappet arm.
Note for this reason I use brass washers, accurately fitted, to replace the valve gear springs.
The Jowett engine is a complex beast with regards to expansion, since not only are there other materials in the bores and their seals and head gasket; but also differentials of temperature such as hot and cold spots around exhaust valve and front of engine would also affect the strain exhibited by all the components under stress (and hence the gap of the tappets).
I also have aly push rods that would change any such calculations based on a standard build.
It is important to realise that all valves need to be closed at the right time so as not to let gas in or out at the wrong time. Thus valve timing (camshaft setting and cam profile) might greatly affect the decision on what gap to choose. Exhaust valves need to be closed too for the cooling factor which also depends on the sealing area created when they were ground in.
The basic assumption for having valve clearance is that when the engine heats up the gap will close a bit. That might not be true the Jowett engine under all build configurations or uses.
Competition work requires geting the most gas in and out of the cylinder so minimizing the tappet clearance is a priority, otherwise some valve lift is lost. Large gaps put excessive strain on the valve gear as the tappet will hit the valve at speed.
I too use Mike's method; but for safety give the exhaust valves an eighth of a turn more gap to allow for any recession on long rallies. Note that my in head thermometers give early warning of exhaust valve burn. Valve gaps will also change if push rod cups and balls are not symettric, the push rod or its end are bent, the tappet or the cam heal are not flat and parallel (which would alter with end float of cam or valve gear). A quick check is to use Mike's rotating method and make sure there is no binding or slackness when rotating push rod or axially moving the camshaft or tappet arm.
Note for this reason I use brass washers, accurately fitted, to replace the valve gear springs.