Broken Exhaust Valve Guide SBC
 

Hello,

This is my first post, usually I can find my answer from everyone else's information but I am stuck on this one.
I have a 1986 260 Mercruiser (SBC), rebuilt 4 years ago. The stock heads were rebuilt and a Comp Cams XM 256 cam installed with Comp Cams 918 springs and Magnum Roller Tip Rocker arms. I used Magnum pushrods that were 0.050" shorter then stock which I though gave the best geometry.
This week I discovered I had a broken valve guide on #1 exhaust after trouble shooting an erratic miss at idle and a ticking from the rocker arms. After pulling a couple other valves I noticed the other guides look pretty good.
The guides were installed 5 years ago and I have put about 350 hours on the engine since the work was completed.
Any thoughts on the cause? Poorly installed guides? Wrong length pushrods? Just old age?
I find it odd it ran for so long, but I am not sure when it starting actually cracking.
Thanks for your help... maybe I can start to contribute these forums more in the future!

Fixx
 

is the valve stem bent??

i would find a set of vortech heads and a intake to match,,their worth the xtra 40 hp :)

Thanks for the input.
I am using 1.5 ratio rocker arms, the valves look pretty good so I don't think it detonated. The machine shop noted the exhaust valve seats were pretty wide indicating some wear though.
I decided to purchase a new set of assembled World heads and just chalk this up to being worn out. I think I will stick with the self aligning Magnum rockers and do my best to check pushrod length.
Because of the pistons I have I need to stick with 76cc heads to keep the compression ratio down but I went up from 1.94" to 2.02" because they are available sooner. Our season is too short for summer breakdowns!
Thanks again!

more power!!! just make sure that you arent going to have piston to valve issues with the bigger valves!!