replacing valve guides

[turbo2.0]

I just ordered replacement exhaust valve guides from Pelican, as mine were way gone. Are there any secrets about installing them? I plan on machining the old ones so that the wall thickness is small enough to reduce the interference fit, and then remove them by driving them out. If I heat the heads to 250 degrees, and freeze the guides with liquid nitrogen, the guides should go in fine, right? Hopefully the finished i.d. will be within tolerance when installed, so they don't have to be reamed (Haynes manual). I have access to a complete machine shop, and many years of experience with industrial machinery, but the fact that these are my heads makes me just a little nervous. And what about valve guide seals? They ran them on v-8's but evidently they are not needed on these engines. This is my first try on porsche heads, so any input will be appreciated. Thanks. BTW, I found a 70 914 in the junkyard near my house. It's picked over by now, but I got some good stuff. It still has two rear calipers, one rear disc, complete engine, trans., heat exchangers, suspension, and the pad that goes behind the seat is in pretty good shape. It's in Anaheim,ca.

[Dave at Pelican Parts]

I would suggest reaming the guides to size regardless. The metals can and do distort some when heated and chilled.

Hmmm.... Liquid N2? That stuff is frighteningly cold! I got to play with it a bit when I dated a chemist a number of years ago. I might be a bit worried about something cracking from the extreme temperature differential between the guide and the head. Perhaps simply freezing the guides would work?

Then again, if it has worked for you in the past, then go ahead!! (No pun intended.)

Good luck!

--DD

[JP Noonan]

I agree with DD, one or the other. Heat heats in oven, guides in freezer overnight. OR heads room temp and N2 the guides.

There is a special "drift" made to fit the guides. Basically a shaft that has about 2" turned down to the dia of the guide I.D. the resulting shoulder puts force on the guide without distorting it. Good ole Bob Hoover talked about getting the guide in with one good "whack". Repeated taps let the guide heat up, and risk distorting the guide.

Any way you do it, check the ID with the rocking test, or better yet, use a "split-ball gauge" (O.K. LOL....that's enough....seriously THAT IS what it is called) to check the guide.

If the guide is to be reamed see if the shop has the correct 8mm (???maybe 9mm) ream. The shop I used had to order one because it evidently is an air-cooled VW only part (the shop worked mostly with domestics). Oh, wait-a-minute, maybe the ream wasn't the problem it was the pilot for the seat cutter. I dunno everythings geting fuzzy.

[turbo2.0]

Please disreguard this post. I talked to a local machine shop that is highly regarded as a quality shop, and they told me that they charge $6.00 each to replace worn valve guides. Thats too cheap to pass up. Now what to do with the $7.50 guides that are on their way?

[Ian Cubus]

Why not tell the machine shop you already ordered the guides and have them install them? They would more than likely have to order them and then charge you for them. One other thing about heating heads is the possibility of the valve seats dropping out in the oven due to expansion. Taking the heads to a reputable machine shop in my opinion, which take it for what its worth is probably the best course of action.

[John Rogers]

I am glad you are having a shop replace them, since they also have to be concentric with the seat and if not the valves won't seat well. Also as a note, we used to use liquid nitrogen to make freeze seals on the nuclear power plants I worked on in the Navy and the extreme cold can cause most things to shatter when put in the liquid nitrogen. We used to demo this by freezing a hotdog,pencil and a screwdriver and even the metal shaft of the screwdriver would shatter.