mattd860 162 #1 Posted May 31, 2012 I was tuning up the Onan motor on my 1996 520H with 620 hrs on it and noticed that the valve seat had come loose. So I took the head off and removed the valve to see this horror. What in the world am I looking at?? Is this normal damage? It almost looks like the valve seat was previously replaced with an over sized seat and a couple of spacers. The seat that came out measures approximate 1.5 inches OD. The standard procedure is to drill out a larger bore and press an oversized seat in there but I'm not sure that's possible without re-welding the hole and drilling a brand new hole altogether. HELP!!! Share this post Link to post Share on other sites
coldone 35 #3 Posted May 31, 2012 Take a look at onanparts.com. he has some that are very oversized. Share this post Link to post Share on other sites
JackC 616 #4 Posted June 1, 2012 The curse of the ONANs, the weak valve seats. 1996 with low hours? I did read somewhere that there may be a series of serial numbers that were prone to the problem. I wonder if anyone has done a study to see if they are all prone to the problem or if it is a bad batch? The heads are supposed to come off for the de-carbon procedure and the valves adjusted every 200 hours. The rear cylinder on the 520H may not get sufficient cooling. It would be good to find out if there are maintenance procedures that would help avoid the problem or if it is just a manufacturing defect. Onanparts.com is the place to look and the people to talk to. Was it the front cylinder or the rear cylinder that has the cooling issues, probably the rear? Let is know if you find out anything. Share this post Link to post Share on other sites
woodchuckfarmer 333 #5 Posted June 2, 2012 I had both intake valve seats replaced in my 1988 520 Share this post Link to post Share on other sites
mattd860 162 #6 Posted June 2, 2012 It was the rear cylinder. I'm a little short on money right now so I'm going to do a quick fix until I can fix it right. I have a complete spare motor with good valve seats that runs but blows oil past the rings pretty bad. So I'm going to put the good pistons & rings from the motor with the failed intake valve seat into the motor with the good valve seats. I know the motor with the good seats blows smoke because both the rings and the cylinder bore are worn but I figure that installing a set of good pistons & rings that aren't very worn at all will get me by for a little while. I brought the motor with the failed seats to a shop yesterday and the shop owner said it will be an easy fix for him. He will bore out the hole and install a custom seat that he will make himself. Share this post Link to post Share on other sites
