Yanmar - Bent push rod

[Bay Banger]

On my 1979 1 cyclinder SB8 Yanmar, I pulled off the valve cover after i could not get compression and found a bent rod, what would cause this and if that is bent what else might need looking into further outside of just replacing the push rod.

Thanks for any help, Tony

 

[Bill Roosa]

Getting bent

Welllll, there is really only two things that can bend a push rod, the piston hitting an open valve and forcing it against the cam shaft lobe or some sort of valve train breakage. The push rod or valve usually takes the brunt of the punishment in these situations.
The odd part is you didn't indicate that the engine stopped running and now "sounds funny" when I crank it.
For the valves and piston to come into contact in a working engine you would have to have the engine either jump valve timing or a timing belt/chain break. I don't think Yanmars have timing belts I believe they use gears which would make the latter pretty hard to encounter.
Broken valve trains are pretty easy to spot if you pull the injectors and hand rotate the engine while watching all the valve train bits and pieces. Look for a cracked rocker hold down. The part that is cast into the head and has two bolts over a cap that holds the rocker in place.
Did you recently rebuild the engine?

 

[Bay Banger]

The piston hitting an open valve sounds right. I just Got the boat cheap with out the motor running. I just fixed the fuel lines and moved on to compression and stopped everything when i saw the rod bent, now i'm waiting for the engine manual before i go any further. So the rockers and possibly the valve and the head of the piston could be damaged. I know all engines are differant but typicaly do you have take engine apart to change the valves or is that something that can be done from under the valve cover?

Thanks for the help
Tony

 

[Clark]

You can replace the valves, etc. by pulling just the head. Relatively easy job (compared to pulling the engine). While your at it, why not have the shop do a valve job - not a terribly expensive job.

 

[Benny]

Check for a stuck valve or broken valve spring.

Overheating or corrosion can weaken valve springs causing them to snap. Also from overheating or carbon build up valve stems can become stuck in the valve guides creating havoc. Lack of lubrication can freeze up the rocker arms. I would remove the cylinder head and perform a complete valve job.