Does anyone out there know the stock thickness for a U20 head? I've got one with shims under the cam towers so I'm guessing it's been shaved. Hence the dished pistons I found in the block!
If anyone out there can supply me with the specs for a stock head thickness I would be most appreciative!!! Plus, if anyone knows it, just how thin a U20 head can be shaved and still be acceptable to use....
Okay Brian, I should just stay out of the U20 discussions altogether and leave it to the 2 liter folks. Sorry for the unintended mis-information Dave. I do know I have seen that minimum information else where.
If anyone does have a documented MINIMUM head thickness for the U20, I'd love to see it. The SCCA Street Prepared rules allow milling a head no more than .010 if no minimum is specified. I'd sure like to have that documentation as it currently seems to make most U20 heads illegal.
As far a practial minum is concerned, you usually don't need to worry about it. At some point shimming the cam towers will cause the cam to hit the cam cover. You're talking in excess of .120 here! That would begin to be the pratical limit. You do need to concern yourself with compression as you shave the head more than .030-.040 though as mentioned above. Keep in mind though that as you increase CR by milling material from the bottom of the head, you can decrease CR by doing a valve job. As you grind the valve seats, the seat depth increases which in turn increases the chamber volume. One of my intake valves was seating .050 deeper than the rest and as a result that chamber was 2cc's larger than the others. Worth a bit less than .2 points of CR. Also, you can always do a little grinding on the chambers a la the Bob Sharp manual which will help out with breathing (especially if you've got deeply cut vavle seats as above). Unshrouding the valves and radiusing the sharp edges in the chamber is easy to do and will reduce compression helping out with an excessively cut head. I should point out that cuting the valve seats deeper to decrease compression ratio is NOT a preformance mod as deeper seat depth decreases flow around the valve. Still, it does help if you've got too much compression for your needs.
Wow, that was more info than I thought.
Brian
Just got word back from the machine shop on the heads: 4.480 and 4.460. Yikes! Getting pretty slim there. Both heads need guides and seals. The 4.480 needs seats and some welding too.
Also heard back from my tranny guy (the whole reason I R/R'd the otherwise good running engine to begin with). Input shaft bearing has been spinning in the case, 1st gear synchro is shot, and the forks and rails are pretty sloppy from wear.
Add up the parts and labor on the tranny work, the head work, plus parts for the bottom end (seals, gaskets, oil pump, rings, bearings) and this old U20's got a lot of nails in its coffin. That SR20 DE(T?!) conversion is looking better and better.
At 4.480 and 4.460 I am thinking both heads are goners. If the 5 speed tranny is also dead then you may be on the right track with an engine swap. As someone who has just paid for both a U20 engine rebuild (including replacing the head) and a 5 speed tranny rebuild, I can tell you it will easily approach many thousands of dollars unless you can find a complete replacement set up.
If you want a true blue U20 setup, I would talk to the vendors. They usually have parts laying around and one of them is having a fire sale right now so you might try that.
Thought about an R16 stroker or a L series? Not quite as exotic, but a lot of guys go that way and I hear they get good results.
Anyhow, good luck and keep us all posted on the progress.
If the tranny I have only needed a couple hundred bucks worth of work, I would dump the U20 and bolt up an L20B or an L18SSS in a heart beat (can you tell I used to be a 510 guy or what?). That's still an option should the SR20 swap prove too costly. Although I would probably have to do some digging to find a different tranny. I just don't think mine is worth fixing. Not when the case itself is junk.
I'm off for Hawaii for a week starting tomorrow. That will give me some time to mull this whole thing over and decide which way to go....
I was just wondering, after reading (skimming) this topic, why no mention of using a thicker head gasket? I'm planning on putting one on after I take those shims off my tensioner. I understand that they exist, possibly from Dean or some other source. That would adjust the motor's "goemetry" to quote Russ. Otherwise you encounter all of those other negative dynamics discussed earlier.
Hence the dished pistons I found in the block! 
