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MG TD TF 1500 - Tappet Clearance
|While troubleshooting the noise in my engine, my mechanic friend asked me, "If the factory specs say .012 for the tappet clearance (on my '53 TD), where did the .015 come from?" Inquiring minds want to know.|
|Early engines used the .019 cam, which was replaced by the .012 in later engines.|
Most XPAG engines have been rebuilt multiple times by now, so T-series cars have a 97% chance of having an .012 cam. If you have a Crane cam, set it to .018 and .020 - follow their cam card.
.015 was never a setting used on any T-series car; a number of the .015 valve covers have been seen on T-series cars, but their origin is unclear. It matters little what the valve cover says, whether .019, .012 or .015. Assume it is an .012 cam unless otherwise known (see Dave DuBois' treatise on cam identification).
You will not hurt an .012 or .019 cam by setting it to .015, which is the best compromise setting. If the engine is a bit noisy, good - better loose than tight, which will burn valves.
I once experimented incrementally between .012 and .018, and settled on .014-15 as the best compromise between noise and power.
|I just remembered that the factory hotter cam, the AEG122, used .015 as a setting, so I assume that's the source of those pesky valve covers. I don't remember seeing them listed in the parts book, however...|
I'll pass this on to my mechanic buddy. Are you saying that setting the tappets on the late TD engine to .012 as per factory spec risks burning a valve? Sounds like you are saying that at .012 you get less noise but less power. Is that correct? Want to be sure I understand fully.
|More on adjusting/setting valve lash...|
|For example, on the BMC A series engines, when unleaded fuel was introduced in the mid to late 1980's, BMC actually changed the valve clearance spec. from 0.12 to 0.15. The reason for this being, because unleaded fuel burns hotter but slower than leaded fuel. Increasing the valve clearance results in an increase in the duration of contact between the valve and the cylinder head thus resulting in better heat dissipation from the valve.|
This seems nowadays to be the done thing for A series engines but is only really required on the outlet valves.
|Larry - sorry if I was unclear. Setting rockers looser than factory spec may be noisy; setting them tighter than the spec can burn a valve. It is very unlikely that engines still have .019 cams, so setting any factory or standard replacement cam at .015 is a satisfactory compromise. Crane cams and hot regrinds are another matter; follow the cam card.|
When I set an .012 cam at .015 I felt no change in power.
|Valve lash is dictated by the acceleration ramp of the camshaft design.|
|Absolutely. Just in case it adds more - the usual explanation for tappet clearance is that it is required to allow for expansion of the valve train and in particular of the valves themselves. This is true, but when valve clearances are set hot, as on an XPAG engine, some of the expansion has already been taken up. The remainder is taken into account in the shape of the cam ramp, so that at the correct tappet clearance, the valve opening and closing will be as the cam designer intended, and no power is lost.|
|Dumb question, are roller lifters and cam any quieter than the current flat tappets?|
This thread was discussed between 08/11/2016 and 09/11/2016
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