OK friends, I need some real guidance here. I have a 1996 XJ, 4x4, A/C. The original 4.0 blew at 190,000 and I realized a long-time dream by stuffing a 4.7 stroker (crate) in. I also had the tranny re-built while everything was out.

After a satisfactory break-in a friend borrowed the Jeep and managed to overheat it and blow the head-gasket. The machine shop found some warping and a couple small cracks. They fixed the cracks, and milled the head and re-assembly went smoothly.

Now however, I can't draw more than about 10 psi of vacum and even that fluctuates. (it should be a steady 16-ish at idle).

Possible cause: The head was milled so much that the push-rods are now "too long".

I have a long and successfull relationship with my mechanic shop. Yes, I do trust them! They are scratching their heads on this. The only suggestion from them is to shim the valve rockers or find / shorten the push rods.

Shimming seems to be the quickest and simplest solution. But, what should I use? Washers? Shim-stock? How thick should it be? Under the worst-case scenario, how much could a machine shop remove from the head and it still be serviceable?

Any and all guidance and suggestions would be much appreciated.

Bill, on the border in Socal  

Here is a link for the shims you need.


http://www.cranecams.com/index.php?show=browseParts&lvl=2&prt=25

Call them and ask for the right part number and how to use them.  

If the push rods are too long that isnt gonna affect the vacume? Is the valve lash set correctly? The only thing that was affected by the shop milling the head would be that your compression ratio changed a little. Even if the pushrod was "too long" that doesnt affect the lift of the cam as long as the valve lash is set correctly. For example on a sbc you can use stock or +/- .05 pushrods and the specs of the cam will be identical when you degree the cam. It just moves the tip of the rocker arm on the top of the spring. sumary: the pushrods are not the cause of the vacum issue.  

There is no valve lash adjustment. That's what the shims are for. If the valves are tight because of excessive lifter preload, then that is the same as overlap from the cam which does affect idle and vacuum. And it's a AMC I6 not a SBC.  

ok, that makes sense then. i didnt know thats how they were adjusted.  

cant you just run a thicker head gasket to make up for the amount they milled?  

Yes, but my quess is that the mechanic did not think that the head had potentially already been milled from the first rebuild. At this point shims make good sense.

Sounds like a lot has been milled. But, hey as long as it's not ping'n away, mo-power!  

OK, now we are getting somewhere..... Good back n forth by the way.

So, shimming should be a good idea... My challenges now are a. how thick? and b. what is a good source for shim-stock. I considered simple washers but they are probably .030 thick and that seems exessive. They also have to be able to fit in the oblong rocker-cut-out underneath the rocker arm.

Suggestions?  

With the time and money, mainly time wasted in trying to find a shim kit that will work, your probably better off hunting fo a head that doesnt need any shimming or head gasket thickness to make it work.  

Did you not see the link to Crane Cams on my first post. Part #99179-1.

Those are the shims you need. Summitracing.com, 4WD.com, and others sell them. They are a common product.  

sorry ftgiles...went right past it, so disregard what I wrote...it comes w/32 shims, cool now when I tear down my 360 thats something I may or may not need.
thanks ftgiles.  

in2baja - Did you see the link in my first post?

See now BT (keithpapa), your bounc'n thingy is mess'n everyone up here. People aint see'n what they're suppose'ta.  

Good morning all,

I sent a reply and thanks to my "pal" ftgiles yesterday, but somehow it didn't get posted.

I had missed your suggested link the first time around but checked it yesterday.

Deep down inside, I just knew that i wasn't the first person in history to experience this problem.

I bought a pack of rocker-stem shims, put them in, and saw a noticeable improvement right away!

I have one rocker that is "tapping" just a little bit, bit I am holding 15-ish psig at idle now.

I buttoned it up for the weekend and will see how it does.

Monday, it goes back to the mech for the Ca Smog check.

I will hope for the best!

ftgiles.....Thanks for the help. My frustration level was huge!

If you hit San Diego I'll buy you a Margarita! Better yet, join us in Nov for the 40th annual Magnificent 7 Baja 1000 experience!

Bill, now DRIVING on the border in Socal.  

Does the noisy valve have the thinnest available shim? Maybe remove that one. A little ticking is better than too tight though.

I might be inclined to make them all noisy and then back off the smallest shim thickness till they get quiet. It takes a mechanic's stethscope to do this. Sounds tedious, but tight valves are BAD (cam lobe wear and burnt valves). If you notice consistency among all the valves it might only take one adjustment.

I don't know how to just measure the preload. Did anyone give you any hints?

I'm also suspecting that your first rebuild decked the block, which would also decrease your clearances.  

LMAO!