The plug in the front of my chain tensioner popped out.

[Suckerpunch]

I found it. Do I smack it back in? use some silicone to hold it? Do I really need to have it?

[petejw]

yep, you need to have it in to stop oil
leaking, put some rtv around it and pop it back in,
just make sure your chain isnt loose and the bolt
is holding the piston in place and isnt moving slightly in the adjuster.

[Ken Talbot]

just make sure your chain isnt loose and the bolt
is holding the piston in place and isnt moving slightly in the adjuster.

This little nugget is key. There really is not much pressure behind that plug unless the adjuster bolt is loose enough for the plunger to move in and out. If it is that loose, and if the chain is nearing its wear limit, the odds go way up on it slipping a tooth or two and bending valves.

Clean the plug and body of the tensioner unit well, and glue the plug back in with a dab of RTV silicone. BUT - be sure to read over the adjusting procedure a couple of times to make sure you get it right.