On the 98.5, I had to rebuild the motor at 215000 miles, because the tone ring on the crank came loose and broke the block. I got a machined block and all rebuild parts from Enterprise, and had my head redone at a reputable shop near me. At the time, I put in a LSM billet cam that a friend was running in another truck ('01). His has flycut pistons and heavier valve springs, but he also truck pulls with it up to 3200 rpm, and his motor was built for bigger mods in the future. A guy that used to work at eep said it should work fine in my motor with stock valve springs and pistons. The motor ran great for 25000 miles (i use it for towing) and then I noticed oil coming out of cyl 6 in the exhaust manifold. I pulled the valve cover, and nearly all the exhaust valve seals were messed up-still in place on the head, but the rubber was cut at the top, and some of the little steel springs on the seals were misplaced or off. Not knowing if the cam was too big or the head has issues on the piston side, I replaced all the valve seals with Cummins seals, and turned the motor over several times by hand before I put the valve cover back on. No problems. I towed for 40 miles, and had oil smoke out the exhaust again, so pulled the valve cover and already 4 or 5 exhaust valve seals are messed up the same way. Never ran the truck over 2400 rpm. I am thinking that the cam is too big, and either the retainers or keepers are hitting the rubber on the valve seals and ruining them. Anybody have experience with this or have any ideas? Just want to check before I go back to a stock cam. Thanks

 

Could the weak springs cause to much stem movement for the seals with that cam. Does it have a longer stroke for the valves? I would have replace the springs with a much heaver spring. My .02

 

What style of seal are you using? The push on seals, or the top hat seals, that have a steel ring that goes under the valve spring?

 

How much bigger is your cam over a stock one.
I'm also wondering if your springs are too soft for the cam.

 

Thanks for the replies-I had stopped checking because I hadn't got any for a while. The truck is fixed. I ended up measuring the travel of the valves, and the amount of room I had before the retainers hit the top of the valve seals, and was very, very close (by measurements it should have worked) so I ended up taking my retainers and filing them down at the bottom a very small amount and no more problems. I didn't want to put in heavier valve springs if I didn't need to because I don't run the motor at high rpm, and this seems to be working fine, and I didn't want the extra stress on the valve train. I measured some retainers out of another 98.5, and they were actually slightly smaller than the ones in my truck-maybe that was all the difference I needed, but I ground these down just a little more on the exhaust side, and all seems good. The seals I had were just the regular style, but the last set of seals that I got for the exhaust valves have the washer attached so that the valve spring holds them down. Mine weren't riding up anyway, just the retainer was hitting them just a little bit.

 

Not sure what the threshold is for lift where you would need heavy springs. Its probably a good idea to do them regardless. If you put an exhaust brake on there later it could be a problem.
The cam might put some more strain on the head gasket, if its stock thickness.
Youre running a ZDDP additive too, right? Might not be so critical without springs, not sure.