Hey guys I need some help. My head is ready to have the seats replaced with the hardened steel ones but my machine shop won't touch it without any specs. Ie., what distance should the valve stem should protrude on the top of the head and where do you measure from.

If anybody knows of a shop that has worked on the nissan head (N42) that might have the specs on it could you give me there number. I really need your help here guys. Thanks

Ed

Not really sure what the question they have is....

The valve stem length above the surface of the head? What does that have to do with installing the seats? There are so many variables involved in even specifying a certain length that the point would be moot IMO.... The only spec length I can think of would be the compressed spring length after the valve springs are installed, but still, that shouldn't have anything to do with installing the seats.. unless what they are needing is the spec for the valve guide, and how much is is supposed to sit above the head surface....

Maybe we should get Phred into this, as he is our "resident engine guru".....

Hi Ed,

You might try Barry Brown at Riter Restorations in East Rochester

(585)385-4151. Not sure if they do their own, or farm it out, but they may be able to help..

Jeremiah

What my machinist described was: Once you cut out the old seat the new seat needs to be placed at a particular depth into the head. If you place the new seat too deep then the end of the valve stem will stick up higher on the top of the head and the springs wouldn't compress enough. Don't put it deep enough then the valve stem won't stick up high enough requiring additional adjustment on top.

He wants to know if there is a specification for this. Like when you replace the valve guide, so much of the guide needs to protrude above the surface of the head and this is measured from the spring seat. So what I'm asking is how deep does the new valve seat go? And where do you measure all this from? I guess it would be the same as the compressed spring length. Thanks guys.

Jeremiah, I'll give Barry a call tomorrow. R U going to the car show by Syracuse this sat?

Ok, now I can answer the question.. I thinkLOL

I was just curious about why they wanted to know the spec on the valve stem height... since it is never really going to be one specific height due to the tolerances on the valve stem lenght, etc. etc.

The counterbore for the seat should be the same as the thickness of the insert, simple isn't it?

Counterbore depth will be about 6.7mm for the intakes, and 7.4mm for the exhaust. Funny, the book has a misprint when giving the measurement in inches.. it says .0266in for intake and .293 for exhaust... hmm, have to figure out which on has the decimal in the wrong place....:stupid:

Now, do you need the diameter of the counterbore?

Here's what I got. The interference fit for intakes is .0032-.0044 in or .081-.113mm.

For exhaust it's .0025-.0038in

The interference fit is determined by the nominal diameter of the valve seat insert MINUS the specified interference fit measurement.

For instance, if an intake valve seat insert measures 1.792in(45.5mm), subtract the specified nominal interference of .0038in (.097mm). Using this example, 1.792-.0038=1.754in (45.6mm)

FWIW, if the valve seat counterbore is not within spec, valve seat insert can be had in a .020 oversize diameter but the valve seat counterbore would then need to be machined to the proper size for th eoversize insert to be installed (using the same formula above).

Originally posted by 2ManyZs

Counterbore depth will be about 6.7mm for the intakes, and 7.4mm for the exhaust. Funny, the book has a misprint when giving the measurement in inches.. it says .0266in for intake and .293 for exhaust... hmm, have to figure out which on has the decimal in the wrong place....:stupid:

Well, they're both wrong:

6.7mm = 0.26378 inches

7.4mm = 0.291339 inches

(divide mm by 25.4 to get inches)

Guess their proofreader took a break when that page was checked huh? That wasn't the only typo I found on the page.. just hope the measurements are correct....

Well, I don't know what the tolerances are. Other than the obvious decimal place problem the .293 vs .291 may not really matter. However, if you're going to give the precision to 3 decimal places you might as well get it right.

Me giving it to the nearest millionth of an inch is rather pointless as well.

Oh boy, what a wicked mess. First, find a new engine builder/machinist. This should be basic stuff for a qualified shop.

Even without any specs this job can be accomplished. Let me explain, and take notes, there will be a test later.

In my cyber shop I have a bare head, with cam towers. A cam, new valves, and as yet, no new seats. I also have my retainers, locks, and lash pads. First we need to remove the old seats. Tip #1, if the new valves are not larger in dia. than the old ones, theres no need to cut the head for larger dia. seats. If this is the case, then we will remove the old seats, and install new hard seats right in the old seat bores. If we can't find hard seats the right dia., Then the head will be cut to accept the new seats. Tip#2, only cut to the same depth as the old seats. Tip#3, removing old seats in an aluminum head is easy if you are skilled with a TIG (Heli-Arc) welder. Simply start your arc, and flow a puddle without adding filler, around the inside of the old valve seat. This will shrink the old seat And the heat from the process will transfer through to the aluminum, expanding the seat bore. Then the old seat falls or gets popped out with a couple of 90 degree scribe pics. Magic!. Step two. Install the new seats. An earlier post listed the press fit, but the factory specs are not right.(surprise) An alu. head should have .004 / .0045 press on the Intake, and .0045 / .005 on the Exhaust. (tech. note: the Exhaust will see a higher temp than the Int. and will expand the seat bore slightly more, so it gets a tighter press) Tip # 4, when installing the new seats, special alignment tooling for holding, and beating them in are required. Yes their actually beat in, not pressed. Tip #5, an hour befor you install the seats, put them in the freezer. That shrinks them a couple thou. If you really want to make them go in easy, Get the rose bud tip out and heat up the head to about 275/300 degrees, then they'll just about fall in. Ok time for dinner. Part 2 after desert.

Phred

Ok, my belly's full and I can think again.

I forgot to mention that when installing the seats in an alu. head to always use press lube, center point lube, or moly. They should also be installed flush with the combustion chamber. . Now, we have a head with new seats. Next is to cut/grind the proper three angles into one seat. (60,45,&30 degrees) I said one seat, not all of them. Install one valve, using only the inner spring, with retainer and locks, and lash pad. Now install the cam towers, and slip in the cam. Install the rocker and adjust the clr. (int.008 exh.010) Now wipe a thin layer of Prussan Blue marking compound (permatex,at NAPA) on the rocker contact pad. Turn the cam clockwise one revolution. Observe the wipped area, Center the contact patch on the rocker by, (a) Thicker or thinner lash pad. ( Grinding the end of the valve stem. © sinking the valve in its new seat, ( don't do this unless all else fails, we want to keep the head of the valve proud in the chamber) When satisfied with the valve height/contact patch of the rocker, then cut or grind, (depending on the equipment used) all the seats to the same height in the head. Make sure all valves are the same length. Presto! New seats/valves all set up properly. Now class, if you have been listening carefully, and still have questions, jot them down, and I'll get to them tomorrow night, after I have dinner with the Principal's new secretary.

Professer Phred

Thanks guys, you have all been very helpfull! I will take this info to the machine shop. Ya'll are the best! Beers are on me.:classic: