Engine Rebuild, Good, Bad, and Ugly

[xpensivewino]

I thought I would share some of my engine rebuild discoveries from yesterdays work. I'm restoring my 69 E, and decided to tear down the numbers matching engine for a complete rebuild. The engine would spin 360 degrees in both directions and then get to a stopping point and go no further. Obviously I knew there was a major issue going into it, but major was an understatement. The number 6 cylinder had a broken intake rocker arm, so I assumed a classic bent valve with piston damage. Once I pulled the heads I found the issue. Amazingly not a single mark on the piston dome, but rather the cylinder was almost completely cracked in half!! The top of the liner was almost ripped away from the rest of the barrel. Huge forces are at work in these engines, and it was one of the biggest failures I have seen. This will go into my "hall of fame" for engine components. The good news is that the engine seems to have had very little miles since whomever built it in the late 80's/ early 90's when it last ran. The great news is that someone on this forum will most likely benefit from this poor little engine, because as I looked at the piston domes I said "that's no E piston". Looks like when the rebuild took place someone bought and installed new 2.0l 'S' Mahle p/c's when they weren't a million dollars a set. The piston in the bad hole is in really good shape for what happened to it, a testament to the toughness of Mahle products, and the other 5 holes are near perfect, with no valve deposits what so ever. The original paint marks from Mahle are still on all the barrels. Someone built this thing and grenaded it very quickly. Since I already bought new E p/c's for the rebuild, I'll have a set of 2L S p/c's as a consolation prize! As is usually the case, the picture tells the story! Ouch!!

[preS]

you might want to check whether the piston is 80 or 81 mm. If it's 81 mm, it can be that due to enlarging the cylinder has split. I have seen that before w/ 81 mm pistons in 80 mm cylinders.

Richard

[xpensivewino]

I wasn't aware of an 81 mm piston? Interesting thought.

[preS]

I wasn't aware of an 81 mm piston? Interesting thought.

This shows as a 906 piston and those were (later on?) available in 81 mm. If it's 80 mm, than please check the the crown (inside) where you will see a casted part number. Tom Racing has posted a list on this board that will tell you what piston it's.

Richard

[xpensivewino]

Funny you say that, because my first comment to my brother when I saw the dome was "that's no E piston, it looks like a 906 piston!" I will check the numbers. I inspected the situation and piston a bit more, and it looks like perhaps a ring broke, and piece was wedged in between the piston and cylinder wall, and it scored, and eventually wore a deep grove in the side of the cylinder wall. Once weakened enough, and with enough heat being generated, the cylinder eventually cracked. The cylinder and piston side wall took a beating.

[xpensivewino]

Ok the pistons are 80P26E, which charts out as a 69 S with 10.3:1 compression. There are some charts that also show a 906 piston as sometimes having the number 80P26E, however these pistons on my engine do not have the 3 holes drilled near the wrist pin, so I would suspect they are 69 S. This begs the next question, were the cams altered? The cams are both E cam part numbers, and appear to be stock, as in not having any machining done to them. I don't believe E cams can be ground to S specs without material being added, so my guess is the cams were left stock. I think adding S P/C's was a popular mod back in the 80's, however pairing them with Zeniths sounds fairly useless. Judging by the amount of blue RTV used everywhere in the engine, and various amateurish mistakes made, this engine was not built by a competent Porsche shop.

[beh911]

Maybe you saw this thread I started already:
Piston info:
http://www.early911sregistry.org/for...67-68S-69S-906

I believe you have the standard 80mm 69S pistons. 9.9 compression, erroneously documented sometimes as 10.3.

They are incredibly tough as you said. Engine mishaps often destroyed rocker arms and bent valves but left the forged pistons intact.