Valve Guide Replacement

[Northern]

Alright guys, I have a couple M20 heads, one of which needs new guides.

Most of the M20 related posts I've seen prefer to drill/ream out the old guides before driving them out, vice just driving them out, but everything is very vague.

Does anyone have any numbers on what sizes to drill/ream the guides to before driving them out, and to remove them would it be better to shoot them out or to press them?

[AndrewBird]

I don't see why you would ream them before driving them out. Usually you press them out, press new ones in, then ream the news ones to size.

[Northern]

I don't see why you would ream them before driving them out. Usually you press them out, press new ones in, then ream the news ones to size.

I feel that guides are just bushings, and thinning the wall thickness will reduce the force required to remove them.

Also, does it matter which way I drive it in/out? Meaning can I use one fixture to drive it down and out through the valve pocket, then drive the new one in from the top?

[TobyB]

There's a fitting that goes into your pneumatic chisel that does a pretty good job.

If your driver mushrooms them at all, you need to push them in one direction-
but otherwise, no, it doesn't matter much.
Depending on whom you get them from, you either need to ream the head oversize, or not-
at least one aftermarket version comes .0005 or so over, and claims that you get
a tight- enough fit that way. Others say you need to ream oversize, then drive/press in
the correct oversize guide.
They then need to be reamed internally, and then the valve ground concentric to the guide-

and for that reason, I've never done 'em myself.

Yet.


t

[jeffnhiscars]

If you get the right guide you just have them pressed in then have the seats ground so they are true to the guide. I let my machine shop swap them and it was very reasonable.

[ForcedFirebird]

I have experience with hundreds of guide replacements in m20's over the years, there's a couple ways to go about things. I have my machinist do all of this for me as he has some top notch equipment.

First, you shouldn't knock them out unless you plan on reaming the heads and going with larger guides, as mentioned they will mushroom as they are bronze, and gall the aluminum. Heat the head and with little force, they will come out, but make sure the head is 100% cleaned first, dirt can also gall the guide bosses.

I tend to buy multiple sets of guides and measure them. After being pressed out, you will often need to go to a .0005-.001" larger guide in order NOT to ream them. Heat the head and they will go in fairly easy.

As Jeff said, you will 100% NEED a valve job. The guides will no longer be concentric, and be aware if they move too much the seats may have to be replaced if they are cut too much.

An easier way to do it is to line the guides. They will be drilled over size,then a bronze liner installed. This keeps the valve concentric to the seat. We try not to do this too often. Never had a failure on one, though.

Finally you can knurl and ream them. This is a trick to get a little more life out of a stock head, it's an old trick that should have never been used in the first place, but lots of machine shops still do this since it's very quick to do, keeps the valves concentric and only involves cost of the tooling.

I don't trust many machinists, and have gone through them like tissues until I found the right people - not so much based on them, but the equipment they use. I always disassemble the heads and bring them in bare as many people (even the Bentley) says to knock out the rocker shafts, but this is also VERY wrong, plus this gives me a chance to check the valve stem play and the work completed by the machine shop. You remove the rocker retainer springs and slide the rockers back and forth until they are all off the lobes, spinning the cam each time you move a rocker. The shafts should slide in/out by hand.

[Northern]

^ thanks. I may just try to see how the valve laps without the grind. I'm a lot less gung-ho now that I know the head doesn't fit flat in the oven I wanted to use, but now that I'm thinking about it, I could probably fit it diagonally to preheat.

If there is a large amount of resistance, I'm sure it will mushroom the guides (planning on using my 4X rivet gun with a turned down air chisel bit with pilot to fit the guide over, and if that doesn't work, I'll use a brass snap?)

I'm hoping that installing with the head preheated to 200 degrees and guides frozen in liquid nitrogen will make the install much easier.

[ForcedFirebird]

It's not something I would attempt at home, you will never get the valves lapped back in, even if you do lap them enough to seal, you will ruin the back cut and seat angles by the time they do.

[digger]

lapping should only be done after the seats have been properly cut/grind as a means to check the contact relationship and thats if you lap at all, many dont lap, i wouldn't be relying on lapping to fix any problems

[Northern]

I didn't see John or Jeff's posts before I replied last, whoops.

Neither of these heads are likely worth putting money into.

Head A is missing a section of rocker journal from when the timing belt snapped, bent nearly all the valves and snapped 4 rockers. Otherwise, it was previously in great shape.

Head B came off a mystery parts car and was intended to be the replacement head, but it has heavy oil deposits (rocker shafts would not move unless violently beaten with a hammer) and oiling issues which completely destroyed the cam, almost all the rockers, and gouged the cam journals. It is visibly warped when laid on a precision granite surface, and the valves have well beyond a full mm of radial play.

I was hoping to get the second one decked and see what I can make of it for cheap just using tooling I have, but if changing guides moves the valve and I need a valve job, then I'm out. When the time comes for another M20, maybe I'll have found something locally in alright shape, or I'll cave for a rebuilt one.