Valve guide reaming problems/honing
 

My local, usually very talented machinist is getting his butt kicked by the guides in my 1986 N/A head. He's on the second set of replacement guides after having problems reaming the first set properly.

He says he needs to buy (or have made) .352 and .353 reamers to "step" the reaming to get to the standard .354. These sizes are not readily obtainable and will probably be an arm and a leg to have made.

I don't understand why he's having this problem. He's worked on lots of import vehicles before, including 944's, 911's, etc. and he's getting frustrated too. He says the guides shrink so much when he installs them that he has to do the "step" procedure, otherwise the .354 reamer is cutting too much material, it grabs and ruins the guide.

At this point, I'm considering going with an exchange head, since mine would still serve as a usable core. Any recommendations on companies that offer rebuilt heads (I know 944 Online does this for $495) or might be able to do the reaming (my preference) are appreciated.

Also, any thoughts on honing guides for clearance and/or final surfacing would be good to hear about.

Thanks - Jim

Call one of the major Porsche head rebuilders. Maybe even Pelican Parts and ask to speak to or get the phone number for whom ever does the work on the 944 heads. They can tell you all about what tooling is needed to do the work.

By the way there are adjustable reamers readily available. Also number drills to rough the guides bore out.

^ +1.

Any time I have installed new valve guides in a 944 head, I have to use a "roughing bit" to enlarge the bore slightly before running the cutting bit in. Usually after 2 passes, the bore can be honed.

-Nick

Good advice and thanks guys, everything else has gone smoothly up to this point. We have great empty highways out here, fun for drivers, but the choices for Porsche repair are non-existant.

Hugh - we lived outside of Pasadena many years ago, but I don't know rebuilders in the area. Any you would recommend?

Nick - thanks, I will hopefully be able to contact you soon about that 9201.

Jim

Jim,

No problem, I should be getting it back towards the middle of next week. Just let me know when you are ready for it!

-Nick

My grandparents and one uncle (and family...know any Denmans?) live in Casper so I've been there many many times. You definitely have some open roads up there....at least in the summer time. You still have to watch out for the wildlife though...seemed to be a lot of that.

Why do the guides need machining out? New guides are not in spec or something? Why does a guide need reamed on a new valve stem?
Getting ready to rework my head and this info has me puzzled.


Dal

Most valve guides need to be reamed to fit. I am thinking that in the olden days, people would reuse old valves and sometimes they would want to polish the stems slightly undersize and ream the guides for a proper fit. Not sure why this is still done, but it is also possible that the guides themselves cannot be guaranteed to maintain the proper ID when they are pressed into the head, so making them undersize and reaming to fit after installation makes sense.

I am not a machinist, I just know enough to be dangerous.

nate

Back on-line
 

Razorback - Sorry, don't know any of your family here. They may have seen or heard me talking about highway issues - I do PR for WYDOT in central and eastern Wyoming. Yes, lots of roadkill!

Can anyone reccommend a head rebuilder in the Western U.S.?

Thanks for the responses.

Jim

toolboy62: you are right about sizing the valve guides.....the factory service manual calls for a .06-.08mm press fit, which is approx. .0024-.0032" press, which is a lot of press on the guide. That is why they also recommend heating up the head and freezing the guides in liquid oxygen before installation. With this much press, the inner diameter of the guides will collapse on installation, which means they need to be reamed afterwards to correct size. The manual recommends using Special Tool 3015, which is probably a step-type reamer.