bent pushrod mercruiser 7.4 thunderbolt

[anglosax]

Hi Guys
I have Wellcraft 32 St Tropez with twin merc 7.4 inboards. My Sbd engine developed loud knock esp at low rpm - It went away with sparkplug disconnected - removed rocker and found bent and broken pushrod on No2 cylinder. I have extracted one piece of the rod but the lower end is down in there. Question 1 - can I possibly extract the remaining piece with magnet without further disassembly ?
If not how far past the cylinder head do I have to go to replace the pushrod?
What else should I be replacing at the same time when the head is off?
Is it likely that there is further damage deeper in the engine - The knock sounded more like conrod or piston to me - But Im no expert!!!!

 

[IdahoDoug]

Re: bent pushrod mercruiser 7.4 thunderbolt

With today's strong magnets, I'd bet you could extract it that way even if you had to do something like glue one into a plastic tube so it won't grab onto the block. Once you have that handled, turn to finding why it happened. If the engine's been sitting a lot, suspect a grundged up valve guide. If you pull all the rocker on this side, does the suspect valve sit deeper in the block, or line up with the other valve tips?. If it does not line up it could be a valve guide issue or the valve was bent from piston contact. Dribbling PB Blaster or similar on the valve stem and rapping the valve stem with a brass hammer will help free up a sticky valve guide. How about the valve spring - look good or is a spring broken?

Best case you have a sticky valve and can extract and replace the pushrod and free the valve. Worst case you had piston/valve contact and the head will have to come off for a valve replacement. Of course there is worse yet, but see what you can do or find out so far.

DougM

 

[04fxdwgi]

Re: bent pushrod mercruiser 7.4 thunderbolt

Hi Guys
I have Wellcraft 32 St Tropez with twin merc 7.4 inboards. My Sbd engine developed loud knock esp at low rpm - It went away with sparkplug disconnected - removed rocker and found bent and broken pushrod on No2 cylinder. I have extracted one piece of the rod but the lower end is down in there. Question 1 - can I possibly extract the remaining piece with magnet without further disassembly ?
If not how far past the cylinder head do I have to go to replace the pushrod?
What else should I be replacing at the same time when the head is off?
Is it likely that there is further damage deeper in the engine - The knock sounded more like conrod or piston to me - But Im no expert!!!!

If valves are OK, you shouldn't need to pull the head. Pulling the intake will gain access to the lifter valley and the lifters and any pieces of pushrod there (if it / they didn't fall into one of the slots over the cam and fall deeper into the engine) to extract the pushrod remains, if you can't get it with a magnet.

Usually a knock that goes away when the plug is killed is a sign of a rod bearing / wrist pin going (or completely gone). I think I would be considering a rebuild or long block swap out, if I were you. Too much going on there.