I want to remove the RHS valve cover to check the secondary tensioners on my XJ8. There is currently no oil leaking from either cover, and I wonder if it would be OK to remove the cover and reinstall it with the old seals and gaskets. I would like to avoid the expense of replacing these just to check the tensioners.

 

You might get away with it, for inspection purposes. If the tensioners have been replaced, there is a good chance new gaskets were used, and if they haven't, you will do so when replacing.

Be aware that there have been instances of only one side being replaced.

 

I was very careful when realigning my gaskets and grommets: no leaks, didn't need new gasket.

 

I have seen people mention this too so my plan was to check the LHS only if the RHS tensioner was the upgraded aluminum type. If RHS was still the original plastic type, then no need to remove the LHS cover as I will be taking it to the shop anyway.

 

I mostly follow my grand-pa's advise to slightly grease this kind of gaskets and seals with a slight touch of grease on both sides.
Probably something only done long time ago, but it prevents them from binding, and it helps to let them set.
Never had leakage on any of my re-used seals / gaskets / covers.
Of course you need to be careful on which seals to use this trick, and when only new is recommended.

E.

 

I have had the covers off/on twice without any leaks. A little light grease on the gaskets and no over torque on the bolts will usually allow reuse.

The most common problem with gasket leaks is over tightening the bolts.

 

If you need to know (as I would) then you'll be removing to inspect.

If on inspection all is good then a new gasket would be cheap insurance but if the tensioners need doing you'll need to replace them anyway.

A small smear of black silicone and all should be good,but careful not to overtighten as then are prone to crack on the corners. They are 14 years old remember. Good luck.