Hi All mechanical experts! Is there an easy potential cure for an apparently sticking hydraulic tappet? (In a conventional water-cooled engine that is, not VW.) eg a bottle of a magic-additive-something to add, that might free it up? The tappet noise occurs at low rpm, when cold, for about a minute or so, but seems to disappear after that. We have so far tried engine-flush; changed to somewhat thinner oil (from 25W-60 down to 20W-50, then 20W-40), and added some molybond -- all with no change at all to the noise. If oil viscosity contributes to such noise, does going thicker or thinner improve it - or make no difference? It's an old engine in apparently good condition that may have been sitting around unused for a while.

thinner oil improves cold hydo tappet/lifter noise as it allows them to fill and actuate more easily and quicker when cold.

But if they are noisy all the time with correct grade of oil at low rpm can be oil pressure problems but more often they have faulty check valves or physical wear/damage resulting in inoperable hydraulics, usually from over revving too much like i did which killed all my hydro roller lifters

Mine come somewhat good after warm up, but clatter about just off idle and come good again at higher rpm. thinner oil helps but at cost of engine wear ran 10w instead of 25W as stated by the owners manual. I run 15W which worked out good. little engine wear and not too noisy, good in summer and winter. Winter is a killer in Macedon ranges and thick oil. might as well not have any oil at all and use honey as its thinner.

Anyway, replace them is the best bet if oil doesn't fix it immediately.

I went to the pick and go wreckers and got a full set of real clean OHC rocker sets for our Suby engine. The whole set cost me $24 and it made the engine go from clattrey until warm, and changed it to not even a tick after firing up.

old lifters can stick due to build up of tarnishes and this is enhanced if the engine hasn't run in a pretty long time. If the lifter isn't worn then after soaking in brake cleaner for a few days and re-priming then brings em good but you won't know unless you remove them and give it a whirl so if a set from the wreckers is that cheap then probably the easiest option

My son had sticking lifters in his Skyline (it appears to be a common problem) and what he did was to remove them and soak them in thinners then as psimitar says reprime them and refit. As I say it worked on a Skyline.

Thank Guys. Was hoping for an easy cure in a bottle, rather than dismantling the valve gear & removing the tappet/s, but realise that may have to be done if nothing else works! (Will try thinner oil first.)