Questions
- are hydraulic tappets truly maintenance-free and if so why are they not a more frequent appearance in engine designs?
- shim type tappets are a pain to work on and yet most motors feature them, what am I missing here?
- screw and locknut adjusting tappets can be accurately set and easy to work on, so why aren't these more widely used?
....nudge, nudge, Brian please share your expertise!
Hydraulic lash adjusters are maintenance-free until they fail ... which is not unheard of ("collapsed lifter"). They are common, if not universal, in production engines with pushrod valvetrain designs, because the distance from the camshaft to the valve is so far and goes through so many things subject to differing thermal expansion that it's otherwise hard to pick a consistent single manually-set clearance that will always work regardless of the depths of winter cold starts or hot summer or anything in between. YES you could get some high-performance pushrod V8 engines with solid lifters over the years ... they're noisy.
Hydraulic lifters of the type used in pushrod V8 engines and (I think) Harley engines are pushed up and down by the cam lobe, sitting against the cam lobe with the pushrod on top of them ... adding reciprocating weight. That's why the high-performance versions of some older V8 engines substituted solid lifters ... they're lighter.
There's another design used in overhead-cam engines with rocker arms, in which the hydraulic lash adjuster stays put and acts as the fixed pivot point of the rocker arm. Eliminates the lash adjuster as a source of reciprocating valvetrain weight ... but it's another thing to take up space in what is often an already-congested cylinder head filled with valves, ports, springs, etc.
The direct-acting shim-under-bucket design is lightweight, very low reciprocating valvetrain weight, compact, and with the distance between the camshaft and the valve as short as possible, less subject to clearance variation with engine temperature and operating conditions. Plenty of production car engines use something like this. Toyota builds plenty of engines with bucket-type valvetrains.
There are a couple of variations on the shim+bucket design. One type has no separate clearance adjustment shim - it's built into the bucket itself, and if it needs adjustment, you change the whole bucket. Another type - was common in older bike engines - uses a shim OVER bucket. The shim is much larger in diameter and is on top of the bucket, and is the part that actually rides against the camshaft. The good thing is that with these, it is at least theoretically possible to replace the shim without removing camshafts. The bad thing is that when over-revved, they were prone to spitting out that shim, followed by very bad things happening to the valvetrain. I think these have gone obsolete.
Screw-type adjusters are all well and good but it imposes constraints on the layout of the valvetrain - you have to be able to put tools on the adjusters and you have to be able to get the feeler gauges in there somewhere. Another nuisance is that the contact between the screw adjuster (which swings in an arc, and could be in any orientation due to adjustment) and the top of the valve (which moves in a straight line) is a point contact, which is never favourable in terms of wear and contact stress.
There are always tradeoffs, and what's best in one application isn't necessarily best for all.
there are two-strokes that use valves, disc valves!
