I finally got a chance to pull the head and valves out of my 235, but I'm not entirely sure what I'm looking for. The head looks to be in decent shape. I haven't noticed any cracks at least, and all the oil and water p***ages seem to be clear. I haven't measured the valve guides yet (the reason I pulled the head in the first place) because I don't have anything to measure them with. The #3, 5, and 6 exhaust guides seemed to have a lot more oil caked around the top. The rest of them looked pretty clean. The #3 guide in particular seemed to have a bit more wear around the top. I had to run before I could check the valve fit at the seats. What's the best way to check this? The valves themselves looked kind of old (especially the exhaust valves), but I want to clean them off for a better look. A lot of them seemed to be worn unevenly. A bit thicker on one edge than the other. Is that normal? What's the easiest/fastest/most convenient way to clean all the carbon off? Is there anything else I should be looking for here, or does that just about cover it?
The best way that i have found to clean the valves is to sand blast them BUT PUT TAPE ON THE STEM the chrome part that goes in the Guide try to keep them isolated to the they came out of. Then after cleaning the valves and guide put the valve in the guide and wiggle the vlave if it wiggles much the guide is worn and needs to be replaced. look at the face or the edge of the valve if it looks like you could shave with it they need replaced if you can see an edge 1/32 or so then you can have them ground. were the rubber seals cracked or missing this could have lead to the carbon build up or the guide wear.
A slick tip for protecting the valve stems while sandblasting, is to use a piece of rubber fuel line. Cut off about a 4 or 5 inch piece, slide it over the stem and blast away. If you don't have access to a mic., a quick way to check your valve guides is to wipe them dry, slide them back into the guide, then pull them back out quickly with a finger over the end of the guide. It should "pop" if your clearences are at all close. As far as wear goes, most service manuals should have them. From checking mine, it looks like the clearences vary from year to year. Some other things you'll want to check are, head warpage, valve recession, stem and guide diameter, and installed height. Does the head have hardened exhaust seats? You may want to take it to a machine shop and let them go through it.
If you aren't able to do your own machine work, take the head to a GOOD machine shop. They'll bake it overnight, cleaning all the rust and carbon out. Then they'll magnaflux it, and if they've been in business 50 + years, they'll pressure test it as well. Have them install silicone bronze guide inserts in the old guides, and if you can afford it, "L" or "Z" seats in the exhausts. They'll surface it to clean, or if you want more compression, mill any amount you want within reason ... new springs are better than shimming the originals. I've always used the exhaust valves from 283 or 327 truck engines,as they have a hardened tip as well as the chrome stem, and I was able to get 6 excellent used truck valves for the price of a cold 12 pack. Bear in mind if you don't do something about the rings and at least Plastigage the bearings while you're doing the head, it will very likely lose the ring seal. Oil consumption will be so bad you won't be able to drive it. 302
If you've got wear (to the untrained eye, no less) on the top of a valve guide chances are the head needs a bunch of work. Like was already suggested, take the head to a shop with a good reputation. Guide/guide liner replacement usually runs 5-10 bucks a guide, sometimes the price includes liners, typically full guides are extra these days. There's nothing wrong with a correctly installed bronze liner...however, if you have excessive guide wear you may have to go to either a full guide or at least a heavy wall liner. As for pricing.......A bake and blast is about 20-40 bucks depending on the shop. Its 25-35 to magnuflux the head. Its typically $40 to resurface the deck surface. Three angle valve jobs run 8-10 bucks a port, the fancy five angle ones run 12-15+ per port depending on how much work is done to the bowl below the seat. So a rough estimate would be about $275 for the basic machine work plus whatever you'd have in new valves, fresh springs and valve seals. Make sure they have the cam specs so they can set the installed height and get correct seat pressure. If they have to machine the heads for larger valve springs and/or to adjust for installed height that can run into extra machine work (and money) as can rocker stud replacement and machining for positive type seals. Good luck with it. -Bigchief.
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