Late model V8 and V6 engines that run factory light weight valvetrain components are often subject to valve float in the upper RPM range, limiting the engines performance and full potential.
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Clutch Cover & Pressure Plate Bolt Kits. Automatic/Manual Transmission Pan Bolt Kits. Even Harry Firth used the adjustable (186) rocker gear, as factory standard, on the 202 XU-1, just think how many times their motors were stripped down & rebuilt. This is the main reason why most guys used to use the older style 149/161/179/186 heads with the factory adjustable rockers, when rebuilding 173s & 202s. Fact is, most of the tappets will either too far down or too loose (or a combination of both).Įven if you do a simple head service on an otherwise stock original motor, by the time the valves are ground & the head is faced, & then get the head back on the block, with a new (different) gasket, the tappet clearances will be totally different to original. Most guys just screw on the fixed rocker gear, which in some cases has its own wear issues, & they think it will be OK. If you are building up a new engine, using a different cam, lifters, head gasket, deck height & of course, the head has been fully serviced & faced, then the total lash is going to be different to how it was when it left the factory. An hydraulic lifter can only do so much, if it's working too close to one end of its internal travel, you will have dramas. This is why you hear so many engines with noisy tappets, most of these cars have their valve lash set wrongly. 
If the gear is adjustable then it's easy. If you have fixed rocker gear, you have to grind down the posts down or shim them up. But when you are building up a motor out of bits & pieces, you need to check the valve lash & adjust it if necessary. That's fine if everything is new & to exact factory specs. Everything is pre-calculated so that the clearances will work just right, in a new assembly. Fixed rocker gear was only used to save time on the production line. This is a common misconception, that the fixed type rocker gear was used because the hydraulic lifters will take up any lash.
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