Hi All,
As you probably know by now, I don't always play by the rules and one job that I found very tedious ( in a non-hydraulic tappet engine ) was adjusting the clearance on the tappets. The tool that Nigel Woodford developed, to help hold the three nuts, helped a lot, but it was still not easy, especially if your camshaft was a bit worn.
One answer, given to several of us by an experienced Jowett mechanic, was to set the tappets with a cold engine and when the follower was on the bottom of the cam lobe, so that the push rods would just spin. When the engine warmed up, the expansion in the crankcase would nicely give the requisite clearance for the inlets ( colder ) and a bit more for the exhausts ( hotter ). This always seemed to result in a quiet engine and didn't seem to result in any adverse problems.
I wonder what other folks think?
All the best,
David
Adjusting Jav and Jup tappets
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David Morris
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PJGD
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Re: Adjusting Jav and Jup tappets
David,
I am not sure that your strategy is totally sound. The hotter exhaust valve expands more than the inlet, because as you say it is hotter, but that works to decrease the clearance, not to increase it. I think that your strategy is OK for normal motoring, but is a little bit risky for hard driving on a hot day. If the exhaust valve were to grow to the extent that it took up all the expansion provided by the crankcase, then the valve would not fully seal and result in a burnt seat.
I am not sure that your strategy is totally sound. The hotter exhaust valve expands more than the inlet, because as you say it is hotter, but that works to decrease the clearance, not to increase it. I think that your strategy is OK for normal motoring, but is a little bit risky for hard driving on a hot day. If the exhaust valve were to grow to the extent that it took up all the expansion provided by the crankcase, then the valve would not fully seal and result in a burnt seat.
Philip Dingle
aka, PJGD
aka, PJGD
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David Morris
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Re: Adjusting Jav and Jup tappets
Hi Philip,
That's a good point, but I would guess that, with the coefficient of thermal expansion of aluminium being about twice that of steel, the small increase in the length of the exhaust valve stem with heat might well be offset by the growth in length of that part of the aluminium crankcase? Probably a moot point, but we haven't had much trouble using this method.
Others may disagree?
All the best,
David
That's a good point, but I would guess that, with the coefficient of thermal expansion of aluminium being about twice that of steel, the small increase in the length of the exhaust valve stem with heat might well be offset by the growth in length of that part of the aluminium crankcase? Probably a moot point, but we haven't had much trouble using this method.
Others may disagree?
All the best,
David
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Forumadmin
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Re: Adjusting Jav and Jup tappets
You might well get an engine without tappet noise but there is a risk of burning valves and loosing compression.
The main problem of using feeler gauges is that often the faces of the rocker and valve stem are worn so that a true gap is not indicated.
Yes you can do by feel, once your muscle memory has fully learned the gaps required.
Yes I did reduce gaps slightly when racing as that increases valve lift and hence the charge into and out of the cylinder.
The important step is to do the adjustment at the correct part of the cam lobe and to have the valve timing correct.
How much expansion is there in each component involved in the cam through follower, through push rod and rocker, through valve stem and valve seat, head , gasket, liner and back through the block to the camshaft housing when heated through 75C or more where the valve is concerned? Not only are different materials involved but different parts heat to different temperatures.
Perhaps measuring immediately after a very hot run might give us a clue but valves will cool down very quickly.
The main problem of using feeler gauges is that often the faces of the rocker and valve stem are worn so that a true gap is not indicated.
Yes you can do by feel, once your muscle memory has fully learned the gaps required.
Yes I did reduce gaps slightly when racing as that increases valve lift and hence the charge into and out of the cylinder.
The important step is to do the adjustment at the correct part of the cam lobe and to have the valve timing correct.
How much expansion is there in each component involved in the cam through follower, through push rod and rocker, through valve stem and valve seat, head , gasket, liner and back through the block to the camshaft housing when heated through 75C or more where the valve is concerned? Not only are different materials involved but different parts heat to different temperatures.
Perhaps measuring immediately after a very hot run might give us a clue but valves will cool down very quickly.