Zmanco Posted July 6, 2008 Share Posted July 6, 2008 I have the head off the car and torn down. To determine if a valve guide needs replacing, is it as simple as inserting a valve and trying to wiggle it while it's off the valve seat? I'd guess any play at all is bad. Or is there a more precise method? Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 6, 2008 Share Posted July 6, 2008 There are limits for what is termed 'the wiggle test' and has to do with simple geometry. If you look at the distance from pivot point to measurement of lateral movement you can compare that to sidewall clearance. Using simple geometry you can determine the maximum clearance radially in the guide to the valve stem. Failing that, you can look up the specifics in the FSM, which I believe has dimensions for that (or it's one of the knock-off books like Haynes or etc...) "Any play at all" is NOT bad. You need oil clearance. Excessive clearance is bad, but what excessive is has to be determined as above. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted July 6, 2008 Author Share Posted July 6, 2008 Thanks Tony, I should have thought about the FSM. Here's the relevant page. page 9.pdf Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 6, 2008 Share Posted July 6, 2008 I WANTED to say "0.008" but I simply could not positively remember the number off the top of my head. And being in Tarragonna Spain right now, my reference books are some few thousand miles away...LOL .008" is a calibrated wiggle. You can feel the difference between 8 and 4, and say 8 to 10. You will get the hang of it and realize when it is 'too much' and it's time to break out the precision measuring equipment for exact numbers. Quote Link to comment Share on other sites More sharing options...
Zmanco Posted July 6, 2008 Author Share Posted July 6, 2008 I'm nearly finished with unshrouding the valves and will be taking the head in tomorrow to have it shaved, the valve job done and some of the valve guides replaced. A few are clearly too loose - I don't need any measurements with a gauge to tell me that I'm hoping the machinist can help me make a decision on a few that feel borderline. Quote Link to comment Share on other sites More sharing options...
Tony D Posted July 7, 2008 Share Posted July 7, 2008 Valve Guides are cheap. Change them all while you're in there. Don't insert them, don't Knurl them. Just replace them and be done with it. And change the guides BEFORE you have the machining on the gasket and top surfaces done! Heat makes the job go easier, so it's best to do the guides before truing the head surfaces. I heat lightly, and use LN2 or Dry Ice and Alcohol to freeze the guides for installation. Less chance of warpage to the head. But you can heat the hell out of the head to open those bores up while you drive em home. Filling the centers with LN2 helps them drop out when removing them as well... I digress... Quote Link to comment Share on other sites More sharing options...
1 fast z Posted July 7, 2008 Share Posted July 7, 2008 Valve Guides are cheap. Change them all while you're in there.Don't insert them, don't Knurl them. Just replace them and be done with it. And change the guides BEFORE you have the machining on the gasket and top surfaces done! Heat makes the job go easier, so it's best to do the guides before truing the head surfaces. I heat lightly, and use LN2 or Dry Ice and Alcohol to freeze the guides for installation. Less chance of warpage to the head. But you can heat the hell out of the head to open those bores up while you drive em home. Filling the centers with LN2 helps them drop out when removing them as well... I digress... The seats must be ground after any new guides are installed, its a MUST. Quote Link to comment Share on other sites More sharing options...
Challenger Posted August 8, 2008 Share Posted August 8, 2008 So how do you get the suckers out? Do they install from the top or the bottom? Quote Link to comment Share on other sites More sharing options...
1 fast z Posted August 8, 2008 Share Posted August 8, 2008 The drive from the block side, UP. You need a speacial driver. Take a peice of steel to the lathe, turn down one end to .312" for about 1.5" then take and turn down another portion behind that .312" step to .470" for clearence on the guide bore in the head. VERY IMPORTANT, make sure there is ZERO radius between the .312" Diameter to the .470", as if there is a radius you WILL crack the guides. Quote Link to comment Share on other sites More sharing options...
Tony D Posted August 8, 2008 Share Posted August 8, 2008 I agree, I had my driver made out of USAF Aircraft Specification Steel, and case hardened back in 1985... The drawing from Nissan was VERY specific like Bryan said: NO RADIUS! Cut STRAIGHT with the bit so it fits the guide snugly. Quote Link to comment Share on other sites More sharing options...
Challenger Posted August 8, 2008 Share Posted August 8, 2008 What if Im going to replace the guides. Is there a way to get them out without damaging the surrounding area. If I had a lathe I could make it easily... Quote Link to comment Share on other sites More sharing options...
1 fast z Posted August 8, 2008 Share Posted August 8, 2008 I would guess any machine shop would not charge more than 25.00 to make the tool. Quote Link to comment Share on other sites More sharing options...
Tony D Posted August 9, 2008 Share Posted August 9, 2008 Yep... And if you take some alcohol, mix in some dry ice, and swirly it around, then put it down the center of the valve guide (after heating the head in an oven to around 150F) the guides drive out much easier. If you try and use a press, unless you have a jig set up to put the guides 90 degrees from the point of force, you stand a good chance of breaking the guide off right at the head/guide juncture in the port... Quote Link to comment Share on other sites More sharing options...
dr_hunt Posted August 9, 2008 Share Posted August 9, 2008 To answer your question, the layman can lightly put oil on the entire valve stem, insert it into the head with the top of the stem even with the top of the guide. Then place your finger on one hand over the hole, use the other hand to pull the valve back out of the head. If it draws a vacuum on your finger and makes a pop sound when the valve comes out, then chances are guide is ok, if not then the guide and or the valve needs to be replaced. IMO if you mic the valve and it has .0015 or more wear then it needs to be replaced. Mind you the FSM is about stock motors and running a motor stock. Performance applications are something quite different IMO. Quote Link to comment Share on other sites More sharing options...
Z2000 Posted August 9, 2008 Share Posted August 9, 2008 Now you have spent ten times more time on this thread trying to guess what to do then it would have taken to run the head by a machine shop and have the head properly measured and checked. They usualy dont charge to check... Quote Link to comment Share on other sites More sharing options...
Zmanco Posted August 9, 2008 Author Share Posted August 9, 2008 FWIW, I just had the exhaust guides replaced on my new-to-me P90 head at $15 each (10 labor, 5 guide). The intakes were ok. Quote Link to comment Share on other sites More sharing options...
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