Valve guide insertion

[Psucamper]

I am inserting new valve guides into some T4 2 liter heads. Core drilled the exhaust guides with the help of an angle plate and heated the heads to open up the bores. Punched out the guide remainders and inspected the bores. Little flakey but not too badly scored (long vertical stripes). Made some measurements of the guide bore and the actual diameter of a new valve. Valve stem was in spec. My measurements show an average of about 2 mils difference that I hope is the "crush" number. Seems reasonable for crush. Anyone have a better number? Can't find a factory value. Surely don't want a guide to slip out under operating conditions.

Also concerned about the slop/rock of a valve head as specified by Bentley. Bentley says you are OK (new) with a slop/rock value on an exhaust of 14 mils. That's a pretty big number considering the guide to stem is only from about 1.7 to 2,5 mils. I know the throw length test is long but that's still a lot of rock for new parts. Thoughts? Going to try to get some "dry ice" and will heat the heads to ease the insertion (wish I was back in the lab to steal some LN2). Bob L.

[Amskeptic]

My measurements show an average of about 2 mils difference that I hope is the "crush" number. Seems reasonable for crush. Anyone have a better number? Can't find a factory value. Surely don't want a guide to slip out under operating conditions.

Head rebuilders seem to like to keep those numbers proprietary.
Chill the guides, heat the heads, use a fine oil lubricant.

Also concerned about the slop/rock of a valve head as specified by Bentley. Bentley says you are OK (new) with a slop/rock value on an exhaust of 14 mils. That's a pretty big number considering the guide to stem is only from about 1.7 to 2,5 mils. I know the throw length test is long but that's still a lot of rock for new parts. Thoughts?

Did you see the wear limit? Type 4 exhaust valves .047". And yes, you are far from the actual guide when testing at the valve face with the stem tip flush with the guide. Your .014" has to be halved and divided by Pii or sumpin to give you actual clearance. Now add 1000* F to the valve face on a hill climb and give it time to transfer up the stem. That'll tighten things up a bit.

Thou Shalt Not Concern Thyself About Factory Specifications
(be more concerned about machinists who do not follow the directions)
Colin

[Psucamper]

Colin...appreciate the prompt reply. However, if I take the Bentley number of 14 mil slop/rock under the specified conditions and I fixture a dial indicator and do the "rock" and get somewhere close to that number, what is the corresponding slop (must stop that)..er..clearance between valve stem and guide ID?? Talked to an experienced VW AC mechanic and he tries to never exceed 2 mils on the stem to guide clearance. Could that kind of clearance possibly relate to the 14 mil rock test? Tried a little geometry to get some relationships but it didn't work out.

On the heating/cooling of the aluminum cylinder head,the bore (OD)/hole in the head grows in diameter just like the linear thermal expansion coefficient. So we can calculate how much the bore grows with applied heat. Pretty slick. Bob L.

[Amskeptic]

However,

if I take Bentley 14 mil slop/rock under the specified conditions and I fixture a dial indicator and do the "rock" and get somewhere close to that number, what is the corresponding slop (must stop that)..er..clearance between valve stem and guide ID??

Talked to an experienced VW AC mechanic and he tries to never exceed 2 mils on the stem to guide clearance.

Ask that "experienced AC VW mechanic" if he was the guy who set my valve/guide clearance too tight in 1986, causing a valve to stick open and catch the air filter on fire. What about your first varnish event, if it is too tight? Gasoline "cokes" as it evaporates in severe heat, whammo, sticking valves. VW allows plenty of room for those suckers, it is much hotter than most water-cooled machinists are familiar with. Leave it alone, Follow the directions!

Could that kind of clearance possibly relate to the 14 mil rock test? Tried a little geometry to get some relationships but it didn't work out.

I cannot answer that. If the factory rock range starts at .014", at "x" distance from "who knows?" fulcrum, can you then do the math? Why? I am so happy with .016" or so, I just plow ahead so I can get back out on the road for another 100,000 thou . . .

I have used Ford's quaint method for figuring piston-to-cylinder clearance where in the factory manual for the 1962 Lincoln Continental (!), you are supposed to use a spring scale and a .002 feeler blade between the piston and cylinder wall, how much pull is it to rip that blade out?! There are senses, touch is one of them, I use them to help me "know" how the parts are going to behave with each other. For example, you cannot know piston/cylinder clearance on a cold piston and cylinder. Pistons are cam-ground. What are you going to do? They are oval when cold to prevent slap, they warm up to round. What are you going to do? Me? I am going to follow the factory recommendations, the factory field short-cuts every time.

On the heating/cooling of the aluminum cylinder head, the bore (OD)/hole in the head grows in diameter just like the linear thermal expansion coefficient. So we can calculate how much the bore grows with applied heat. Pretty slick. Bob L.

Applied heat?
Full immersion oven heat, slow ramp-up heat, slow cool-down heat, when off the engine.
Just making sure, Bob.

We will have to discuss the arcana of aluminum global mass expansion vs skinny little fins expansion co-efficients in the presence of bore holes, cooling holes, port holes, open stud holes vs dead holes, intake charge temperature vs exhaust exit temperatures, these heads have a hard day's work on the best day . . . make sure your under cylinder deflectors fit perfectly at assembly time.
Colin

[Psucamper]

Colin. Indeed, I look forward to a discussion on expansion coefficients when you visit. But,prior to that, I hope to have reliable numbers on valve rock (per Bentley) if my indicator fixture kludge really works. If so, perhaps I can backtrack and find what the real guide clearance should be under Bentleys numbers. That can be an important guide a good rebuild. Bob L.

[Amskeptic]

Colin. Indeed, I look forward to a discussion on expansion coefficients when you visit. But,prior to that, I hope to have reliable numbers on valve rock (per Bentley) if my indicator fixture kludge really works. If so, perhaps I can backtrack and find what the real guide clearance should be under Bentleys numbers. That can be an important guide a good rebuild. Bob L.

Call a good VW machinist having a slow day. About 10 AM before they get irritated at the slow business and not too hungry yet.

I enjoyed chatting with

Performance Workshop
9000 Arlington Avenue Suite 111
Riverside CA 92503
(951) 689-7475

He might get into the physics if he catches your pure joy of learning.
Colin