It's that toothed belt on the engine that the crankshaft uses to drive the camshaft(s), and while many of us never actually see it, most know it's pretty vital to the proper operation of the engine. We speak of course about the timing belt, and when it comes to replacing or even inspecting it, folks generally defer to anexpert. The only prolem is deferring to said expert can often exceed $250, a sizable sum if you're still in school or just working part time.
Because we have nothing but love for ALL of our readers, not just the rich one, we thought we'd show you how it's done. Or at least how it;s done on fifth and sixth gen single cam non VTEC D-series motors. Our M.E. extraordinare let us use his '99 Civic DX hatchback as the test subject, seeing that it already had 102,000 miles on it and the service manual recommends the belt be changed at 105k, and the entire process took us six hours, so it's definitely a weekend job. We also recommen that ou repace the tensioner and water pump while you're at it. And finally, despite what you may have seen or heard, you don't have to pull he engine. Perhaps more convenient, but not neccessary.
Now there are two schools of thought on creating tension in the belt. Some say that with a slight amount of tension in the belt between the cam and crank sprockets and with the tensioner bolt loose, you can slowly rotate the crank counterclockwise for a distance of three teeth on the cam gear and this should put tension on the belt, at which point the tensioner bolt can be tightened. We did it differently though, basically just leveraging a screwdriver on the block and pushing the pulley up until there was a fair amount of tension in the belt. Then we tightened the tensioner bolt. Seemed to work for us, but you decide what works best for you.
Just a couple of remaining considerations: We turned the crank a couple of revolutions and rechecked the timing marks for proper alignment. We also checked to see if the crank was binding or appearing to hit anything; this could be an indication that the valves are hitting the pistons. If this is the case, your timing is all screwed up. You should remove the belt and repeat the install, making sure all the marks are lined up correctly.
With the belt on we reinstalled everything in the reverse order of removal. We then ran the engine to check for correct operation.