I bought my first 914 about 8 months ago. The engine is a stock 1.7L and has 3800 miles on a 3 year old engine rebuild. About 200 miles ago it started to make a knocking noise. My mechanic said it was a wrist pin or bad bearing but the knock only happen between 1000 rpm to 1800rpm. At these rpms the knoking is not consistent. It will knock for a couple of seconds then the rpm goes up to 2000rpm ond higher and it stops knocking but my engine idle is at 1700-2300 because my auxilary air regulator does not close. When i plug up the regulator so it idels normal the knocking is there. Could this knocking noise be somthing other than bad bearing? How hard is it on a scale 1 to 10 is it to swap bearings?

does your mechanic have a lot of aircooled experience ? (i have to ask this - is he the one that assembled the engine ?)

it could be piston slap. (wrist pin failures in these engines are rare and typically an indication of something being done horribly wrong in assembly...)

if it's "just" a rod bearing you *might* luck out and be able to open "just" the 'top end' and replace rod bearings, but no one will recommend that unless you're stuck in the middle of nowhere. the right thing to do is tear the engine down, measure everything, and fix what's wrong. bad rod bearings is often the sign of someone trying to cut corners (a lot) by doing only a 'top end' rebuild ("valve job") and not going the additional step of replacing the rod bearings. all too common a story, mostly heard in the six-cylinder world, but not unknown on the T-IV side too.

on the 1-10 scale, replacing "just" the rod bearings is an 8,5; doing it right with a full rebuild is a 9.

but first, before freaking out completely, i'd MAKE SURE it's not just valve noise. get them all adjusted and make sure it's not just that. i'd want to get a real oil pressure gage to find out what your oil pressure is like, and cut open the oil filter and look in the strainer screen to check for bearing bits and metal particles.

what kind of oil are you running, and are you sure you've always had enough ?

it could also be a dropped valve seat, but these are almost always accompanied by other symptoms, and tend to strike the 2,0 engines more than the 1,7's ...

Check the valve lash and or the tappet condition.
Spray some PB Blaster down into the AAR valve to free it up so the car doesn't idle so high.

Check for other vacuum leaks while you are at it.

Geoff

Had the same knock when i picked up my 74 1,8. Turned out to be valve adj. were way out , some too tight some too loose , and some just right! Sound like a familiar story? AL