Engine rebuild question

[zenithblue71T]

Hi All, I have a 1971 911T with stock 2.2, zeniths and 901 tranny. I've had the car 12 years and have slowly restored it over time. The carbs and engine have never been rebuilt or opened up. I'm about to remove my carbs and ship the dirty zeniths to Paul in Chico. My car smokes on start up, as it has for 12 years. The valve stem seals are gone, I'm sure. The smoke typically then goes away. I found a reputable shop to perform a top-end engine rebuild for roughly $4,500. However, my gear head friend says that if I rebuild the top-end and not the whole engine, I'll blow up the bottom half. He's not a Porsche guy.

I'm trying to find someone's experience with only a top end rebuilds and what effect that can have on a 40 year old bottom end of a magnesium 2.2T.

[gled49]

When I use to do just top end work, I didn't disturb the pistons and cylinders. I felt if you did, you should do the bottom end as well, or sometimes oil leaks would drive the bottom end work. You pay your money and take your chances.

[304065]

All good questions.

I don't know that you will blow up the bottom end, but it probably makes sense to split the case while you're at it.

The early magnesium cases did not use a bearing insert for the intermediate shaft, which means they are worn in this area. It makes sense to split the case once you have it torn down that far.

The trouble is that Magnesium cases are very flexible, and more than one rebuilder reports unbolting the case, which then distorts out of shape, so much so that when new bearings are installed and the through-bolts torqued the crankshaft won't turn. To properly do a magnesium case, you should have it resized, line-bored, oil bypass mod and any of the other machining updates. The cost of all these services runs into the thousands of dollars performed by a reputable shop such as Ollies.

It would be heartbreaking for you to spend $4,500 on a "top end" only to experience problems later on. The majority of that expense is labor, not parts or machine work, hence if you do have a crank/intermediate shaft problem you would have to repeat the exercise.

Also you should buy Wayne Dempsey's book about 911 engine rebuilds if you don't already have it, this is essential to understanding the process and will ensure that you are armed with information in your discussions/negotiation with your mechanic. A good top end includes not only a valve job, guides, seals, a complete gasket set, any necessary cam/rocker reconditioning, but also replacement chains/sprockets if necessary, idler arm update, chain tensioner update, piston rings, cylinder work, rod rebuild, rod bearings and rod bolts. You can access ALL of that stuff as part of a top end. The only thing you can't get at is the main bearings and intermediate shaft, which is why it's so tempting to go all the way.

What other symptoms does your engine have besides smoking? What is compression and leakdown in each cylinder?

Good luck!

[raspy2point2]

I believe the 70 -71 cases got the bearing insert for the intermediate shaft. The 1969 cars did not have the bearing as the previous aluminum case did not need them and the factory just went without on the first Mag cases. Chris