915 with 4 out of 5 gears working
 

I purchased a 73 Targa several years ago that was barely running. I rebuilt the engine (using Wayne's book). I am driving the car to test the engine. The transmission is shifting fine. Then 4th gear is no longer there! There is a slight tapping noise like perhaps something is bouncing on a gear. I think I have to pull the engine/tranny and take a look but hoping someone has other thoughts.
This is my first Porsche and I am a bit of a novice.
Regards,

cannot tell, and cannot fix without pulling the engine - transmission. Good luck

That is where I am at. Thought I would throw a hail mary just in case.
Thanks,

I'd check the shifter first. Sounds like you may not be getting enough throw in the mechanism to get the gear into position. Inspect all bushings, disconnect coupler in the access panel and attempt manually engaging the gear.

Assuming it's not the shift coupler.... You can remove the plate on the bottom of the trans an have a look inside. Probably should check before anymore driving.

Look to see if the synchro ring is coming loose. I would do that before dropping. There was a recent thread on this.

While in there check to see if the fork support has not come loose. like this http://forums.pelicanparts.com/porsche-911-technical-forum/856329-915-shifting-problem.html

Are you able to move the stick into the position where fourth gear should be? Or does that position feel like it is blocked?

I had first gear cease to be available on my 915 transmission. I took the transmission apart and the synchro ring was jammed so it couldn't compress. I disassembled, replaced some various parts, and it works great now.

If you can rebuild the engine, you should be more than capable of basic transmission work. Just keep track of all the parts!

Are you able to move the stick into the position where fourth gear should be? Or does that position feel like it is blocked?

Yes I am able to move the stick into the position but there is simply nothing there.

Look to see if the synchro ring is coming loose. I would do that before dropping. There was a recent thread on this.

This looks like a good first step. I will take a look at this tonight
Thanks, I appreciate all the suggestions

And it is just for fourth gear? Second and Third work fine? As does fifth?

I'm going to guess the fork isn't in the right place and won't completely engage the slider onto the synchro teeth. This would require disassembly, even if nothing else is wrong.

And it is just for fourth gear? Second and Third work fine? As does fifth?
Yes, All other gears work fine. Fourth gear did work briefly, perhaps I shifted into fourth 4 or 5 times and then the next time it simply was not there. There was no shifting problems before that and no noises, it simply disappeared.

If you can get 2d and reverse, it is not a shift coupler or bad cup bushing up front. Those gears require pulling the shift coupler forward about the same distance.

If the shift lever, when 3d is engaged, is in the same forward position as it is in 1st and 3d, then it doesn't seem likely that the Y shaped yoke/shift fork inside the transmission has come loose and slid forward on its shaft inside the transmission, away from the 4th gear. But perhaps somehow it slides when pushed toward 4th, but slides back and then catches when moved to 3d?

Removing the shift fork plate (4 bolts, drain oil first) underneath will let you look in and see some of all this. You can push against the slotted forged pieces which move the 1/2 and 3/4 shift rods. Maybe you can see the brass shift yoke/fork, which will be to the rear of the other one serving 1/2, which is toward the front of the transmission/car.

I'm seeing a transmission disassembly in your future, as others do. Doesn't hurt to look first, though, as you'll be draining the tranny oil when you start disassembling it on your engine stand anyway.

I don't quite know how, if all these other tests show things there are right, you could pull all the way back on the shift knob for 4th gear and not have something happen. Nasty grinding sounds due to failed synchronizer pieces or worn engagement teeth. Though I have seen the "dog" engagement teeth, the ones which engage the teeth on the slider, worn down to less than nubs.

This should leave quite a bit of metal shavings in the transmission oil - more I'd think than the magnet in the plug can hold?