question on 930 rebuild?

vichang4 · 11-05-2003, 07:45 AM

I have an 87 911 turbo with 50,000 miles. I am preparing to rebuild the engine to solve some oil leaks (base of cylinders) and a low compression problem. The compression check showed 120 in all cylinders except #5 which had only 90. A little oil in the #5 cylinder and the compression went right up to 120. I think I hurt the #5 rings when the wastegate diaphram failed and the boost pressure went up to 1.3 bars. The engine is burning a lot of oil, and there is a lot of smoke at startup.
My question is, with only 50,000 miles, how extensive should this rebuild be? Rings only? Head work? Change Head studs? I pulled the valve covers and there were no pulled studs. More? Less? The case is not leaking. Thanks for any feedback.
Vic

john walker's workshop · 11-05-2003, 08:05 AM

you probably broke some compression rings from overboosting. turbos are hard on valve guides, so you should disassemble the heads and at least check the slop. the stem seals could be replaced at the same time. turbos have 24 dilivar studs, which are powder coated, and will probably be ok to reuse. the 9mm rod bolts should be replaced with ARP bolts for blowup insurance, and rod bearings while the rods are out because they're cheap, and probably are not perfect anymore. you probably don't need a complete teardown at that mileage, just a few repairs, and at worst, a complete valve grind, depending on what the guides look like.

beepbeep · 11-05-2003, 01:33 PM

Hmm...how did you manage to boost 1.3 bar without hitting overboost protection switch?

When i blew my wastegate it just kept hitting overboost switch with "dash in you face" syndrom at 1.1 bar...

vichang4 · 11-05-2003, 03:38 PM

The over-boost switch came on @ 1.3 Bars.

Wayne 962 · Wayne 962's Garage

Right - probably only a top end rebuild needed here - have the heads redone - valve guides were notoriously bad back then. Then check, spec and re-ring your P&Cs if they look okay. Chances are, at least one may need to be replaced...

-Wayne

JC930 · 11-14-2003, 07:22 AM

Ditto Wayne. That's exactly what I did on my 77' with 40k miles. I put in all new valves, P/C's and different cams. Reused Divalars. I just screwed up and reused nylon lock nuts on valve covers but that's my only leak.
jc930

MoreGAS · 11-17-2003, 10:43 PM

Vic,
Overboost is a killer. That detonation can pound at a rod bearing quickly. While doing the other things you note;rebuild the heads w all new guides and resurface(I have seen 930 exhaust guides shot in 38,000 miles), re-ring if the pistons check out Ok per the factory spec book using a new ring, check cylinders for ovality and taper. You can remove the rods from the crank through the spigot bore holes without splitting the case, starting with #5, and if any rod bearing isn't in xlnt cond , pull it all the way apart . You can rebuild-resize rods completely with ARP bolts, and change out to the 993 threaded type headstuds or all steel studs even without splitting the case if you are not willing to go that far(assuming bearings look perfect).

Good Luck,
Kevin Roush
GAS Motorsport
Performance Porsche
Upland Ca
909 985 2909

David · David's Garage

I'm planning to do the top end on my '79 930 soon. I thought about replacing the connecting rod bolts with ARP while the cylinders were off. Does anyone see a problem with this? Can I even get to them with the case together?

MoreGAS · 11-21-2003, 10:27 AM

David,
Did you not read my post you replied to? -yes, no problem. You must also rebuild the rods big and small end ,not just change the bolts. Then you are good to go.

Kevin
GAS Motorsport
Performance Porsche
909 985 2909

David · David's Garage

Duh, I guess I didn't read it all the way through, thanks MoreGAS.

vichang4 · 01-16-2004, 02:52 PM

I've got the engine torn down, pistons removed, and case not split. The bearings feel very good but I wanted to change out the rod bearings and install ARP rod bolts. Just looking through the spigot holes I can see how to unscrew the rod bolts, but putting it all back together seems impossible. The rod caps can bearly be seen through the opposite side of the case. Is everything done through the spigot opening? Do you install the rod cap and new bearing, and then spin it around on the crank? How do you hold the rod cap in place when installing the rod?

By the way, I had broken top rings on pistons 1, 2, 3, and 5!