My friend Brad's D8K has developed a problem with valve lifter guide yokes coming loose. The first was #6 cylinder a few months ago. Then #3 came loose and he repaired it and it came loose again after only 30 hrs. He told me the D8Ks 342 engine uses the same lifter retainer system as the D7Es 339 engine. 
I never had a lifter retainer problem with either of my D7Es regardless of the hours used.
The picture is of my D7E parts book.
I hope Old Magnet or someone else has the answer to why this is happening and how to fix it.
Thanks,
Craig
 

 

One possibility is the stelite (Spelling?) valve seats have come loose in the head.

Wombat

Thanks for the replies, I'm waiting for Brad to answer your questions. I believe he knew the history of the D8K when he bought it from the original owners. I think it is a fairly low hour machine. He has had the tractor for a few years and has done no work to the valve train. 
I will get back with info when I can.
Craig

Are the bolt locks in place? might add some loc-tite as well.

i would use a high temp loctite. remember to use the loctite cleaner after a thorough degreasing as any oil will interfere in it working

Oh, snap! What a difference a day makes. Brad finally answered my calls. He told me his D8K is actually a HIGH hour tractor! The only time I saw the tractor up close was at his shop when he bought it. The D8K had a very nice appearance, the paint was nice but not new. The blade was removed at that time and I never saw it up close. I ASSUMED the tractor was a low hour machine. I did know that he hadn't done any previous valve train work. Brad said he wasn't sure how many total hours were on the tractor but there was only 1,100 hours since a engine overhaul when he bought it. Brad asked me to post this question because he had heard two instances of yoke loosening problems on D8s over the years but didn't know why they happened or how they were fixed.
Brad told me he had started repairs and had noticed that the lifter faces were so worn that the faces were concave rather than convex! Apparently the recent engine overhaul didn't include the camshaft. Brad is going to replace the cam and lifters.
I think he's found the answer to his problem.
Thank you to all who tried to solve the problem. I apologize for my misinformation also.
Craig

if its only a extra 100 bucks its well worth the investment in new fastners.
its part of why many machines are expensive to fix is so much gear now uses torque to yield fastners particularly japanese engines and vehicles. it allows them to get higher clamp forces and thus smaller fastners which is less weight, so more fuel efficient and less material so its cheaper to make.

Thanks trainzkid,
Your advice is good for many modern engines, especially when it comes to cylinder head bolts and valve train components,
However, the D8Ks 343 engine uses the same retainer/bolts as the D7C,D7D and D7Es 339 engine. That proven(since the 1950s) design doesn't require replacement unless damage has occurred.
Brad has replaced all the bolts/retainers that loosened with the ones he had from a scrapped D7E engine.
Craig