Coolant overflow tank cap
 

Replacing this inexpensive threaded cap again. Previous replacements seem to come apart after only moderate use. Is this common for our XK8s?

for the inexpensive ones - yes - have gone through about 5 that the threaded part came off, but the rest remained in the overflow tank and was a pain to remove. Paid the extra for OEW from my indie and been good to go since

Another suggestion, do not overtighten it. It is sealed by the O Ring on the inside of the part of neck. Had mine come loose and looked closely. The inner cap neck seals to the tank interior cavity by the O Ring before the caps bottoms out. Just install it firmly and back off a tad,(small amount)

^^ This....

Thanks for the install tip!

Moderate use? What are you using it for? Had mine XKR for 5 years now, havent touch this cap yet.....

I ignored this because I thought it was referencing the OVERFLOW TANK (AKA atmospheric catchment tank) in the wheel well.(the part that is open to the atmosphere)
The part you need is the the COOLANT RESERVOIR TANK CAP.

Sorry but words mean things.
ETA
pictures help.

It’s very very easy to cross thread the cap while putting it on. That’s what contributes to their failure rate. If you feel it’s going on to hard, back it off a little and try again. It goes on with very little effort when right. If the bottom of cap is not nearly flush with the tank when tightened , then you’ve cross threaded it.

Z

these are two most important posts you will read today.

Z

I had a valley hose and then a radiator go. Probably not the cause but I ordered a new cap. Gates brand. I know not to tighten down much as the o-rings make the seal. First time removed top broke off. Got a replacement and same thing happened. Either Gates are crap or bad batch.

Apparently the Gates and other A/M are not quality caps. My Stant A/M cap also broke after 3-4 months of careful usage. Several years ago, I had to replace the Coolant Res. tank near the Brake M/C. and cap that came with it still sealing well. BE CAREFUL

In places like RockAuto.com you can buy the whole reservoir for what just the cap costs on most places , and it comes with a cap . My cap has lasted a few years and I’m always fooling around with it ( for no good reason ).

Z

Expansion tank cap
 

I've replaced two expansion tanks–one due to the tank leaking and one due to a faulty level sensor. I used A-Premium brand on both cars and they came with a new cap. I thought that
they didn't feel like they were sealing tight, so I turned them on harder. That made them difficult to remove. I thought that they might need a bit of petroleum jelly on the plastic threads as other plastic threaded fittings do, but after exploring further I found that it was the outside bottom edge of the cap that was rubbing on the top of the tank and sticking, so I stopped turning them after I could feel the O-rings contact the tank and have had no problems since.
Best,
Rick

Thanks for the correction!

By the way, someone resurrected a similar thread I started on this topic when this happened before. Sorry for the confusion. And yes, this is the pressure cap for the “coolant reservoir” near the left center rear of the engine compartment, not the “atmospheric expansion tank”.

Great advice! Once the cap engages the tank threads properly, it tightens down easily!