So I'm in the process of rebuilding my 305 engine and I've ran into a few issues. I bought an XE268 camshaft from COMP Cams (http://www.compcams.com/Company/CC/c...x?csid=86&sb=2) with .477 intake and .480 exhaust. I have 416 heads and I will be using positive valve guide seals on the intake and umbrella valve guide seals on the ehaust. I checked the retainer to valve guide seal clearance and on the intake it has 0.080" clearance but on the exhaust it has only about 0.020" of clearance. I read that anything below 0.060" is bad. My first thought was to grind down the valve guide boss (I think thats what its called) on the exhaust to get more clearance. However after doing some research I found that it wasn't necessary to use valve guide seals on the exhaust valve because there is no vacuum like the intake. So if I take the valve guide seals off the exhaust valves I'll have plenty of clearance. I would rather do this then have to grind the heads, but I would hate to leave the seals out and have it leak oil. Any input on this?

Another problem I have is with installing the valve springs. I'll be using COMP Cams 981 springs (http://www.compperformancegroupstore...ct_Code=981-16) and it says the install height should be 1.700". So I checked the height from the top of the head where the spring sits to the bottom of the retainer and came up with 1.900". So I would have to put in 0.200" worth of shims to get the desired height. This seems like alot, would it be a problem? Or did I do something wrong?

Thanks for any help!

 

I replaced all valve seals about 5 years and 40k miles ago with (16) intake "positive-type" seals. All has been good since.
Newer GM engines use positive seals on both intake and exhaust and don't have any exhaust guide wear issues.

You shim springs to get the required seat pressure for YOUR cam. Comp has to reference an installed height to quote a seat pressure, and it's up to the builder to adjust that pressure by shimming as necessary. Looking at those springs, they look on the soft side for that cam (105 lbs at 1.700"), so I'd guess you DO want to shim them to that height. However, I don't have any performance-application experience with flat tappet cam setups, so somebody else who DOES please chime in here and correct me if necessary.

 

Good to hear someone actually breaking out the measuring tools, rather than just banging it all together and wondering why stuff breaks.

I had similar concerns with my XE262, 981's and stock 462 heads. I measured the guide boss to retainer clearance on ALL valves, and got measurements all over the shop. Didn't want to pull heads for machine work either; I ended up just using what would fit for each valve.

Found the positive seals used 170 thou clearance, umbrella's 120. In my case, it worked out with the intake using either positive or umbrella seals, and exhaust using umbrella or nothing. I put the o-rings on too just for giggles.

I shimmed for 1.700" too, as Comp recommends. Remember, the stock spring pockets are cut deeper for the exhausts to accomodate the rotators. If you are ditching these, you will need extra shims to compensate. In my case this was typically 120 (two 60's), so your 200 worth does seem excessive, don't know why.

I get occasional puffs on startup, but doesn't last long. Then again, my valve guides are probably shot

 

Thanks for the input guys!

I'm probably going to run without the seals on the exhaust valves and just pray that I don't have problems. I don't mind burning a bit of oil on startup, just as long as I don't have to be constantly adding oil.

As far as the shims go, I did ditch the rotator and there was this other thing I tossed too. It was like a cover for the top of the springs, not sure what its called though. But this doesn't seem like to big of a problem though so I'm just going to try it with three 60s and a 15. This is just for the exhaust valves though, the intakes require only about a 60 shim and they'll be good.

 

You are correct in that you don't really need valve seals on the exhaust valves. If you want to have something, the OEM style o-ring type seal under the retainer is more than enough.

On my race car I don't run valve seals on the intake or exhaust valves. With triple springs, seals just get in the way.

 

That's the oil splash shield, another pretty futile factory attempt to keep oil off the valve stems, along with the o-rings. You can ditch them no problems; I kept mine on and used them as part of the shimming.

 

I've run heads a few times without exhaust seals and I didn't have any issues. Intakes you definitely need.

When you ditch the exhaust side rotator/retainers for standard non-rotator retainers (same style as the intake) you will need a .060" shim in the exhaust spring pocket to make up the difference in exhaust spring installed height. That will get you in the ballpark.

 

Theres no vacuum on the exhaust side like the intake side so leaving off the exhaust valve seals wont cause any oil burning in operation unless something else is seriously wrong. It MAAAAAY lead to a little puff at startup, but thats not really a big deal. Doesn't really do anything except lubricate the valve stems and keep the cylinders from rusting.

 

Thanks for all the help guys. I will be using the stock type o-rings that go under the retainer just because I have them. Now hopefully I can put these heads together and feel safe that there won't be any problems!