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was born to pimp tractors
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Discussion Starter #1
I was working on a friends lawn tractor today that has a vertical shaft 18hp Tecumseh V-Twin engine on it. My friend said it was only firing on one cylinder well after checking the basics I pulled the valve cover off... the rocker had popped off the pushrod but was still attached to the rocker arm stud... I pulled the pushrod out and see that its bent. what could make the pushron bend? could very low oil level make the pushrod bend cause the oil level wasn't even reading on the dipstick?
 

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Master Service Technician
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748 Posts
Normally what causes pushrods to get bent is they can hang in the guide, the guide loosens in the casting and moves or it gets bent when it falls out while running.

If you just replacing the push rod make sure the guide is still in place where it belongs and the pushrod moves freely in the guide.
 

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You can bend a pushrod if the valve sticks in the guide -- due to lack of lubrication or to overheating [caused by lack of lube oil or running at low speed or fins clogged with debris].
Over revving the engine can also bend pushrods. If the rocker arm is galled, that can add extra friction, and possibly lead to a bent pushrod. Basically, any thing that will prevent the pushrod from moving when the cam lobe rotates into place puts the pushrod between a rock and a hard spot. The valve mostly won't bend if it is stuck in the guide, and the rocker arm is significantly stronger because of its shape, so the pushrod is the weakest link in the valve train.
tom
 

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was born to pimp tractors
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172 Posts
Discussion Starter #4
You can bend a pushrod if the valve sticks in the guide -- due to lack of lubrication or to overheating [caused by lack of lube oil or running at low speed or fins clogged with debris].
Over revving the engine can also bend pushrods. If the rocker arm is galled, that can add extra friction, and possibly lead to a bent pushrod. Basically, any thing that will prevent the pushrod from moving when the cam lobe rotates into place puts the pushrod between a rock and a hard spot. The valve mostly won't bend if it is stuck in the guide, and the rocker arm is significantly stronger because of its shape, so the pushrod is the weakest link in the valve train.
tom
it had both these problems... it was very low on oil and that cylinder had gotten so hot (there was a rats nest packed tightly around the cylinder :fing20: ) that what oil was left had gotten baked onto the valvetrain componets
 

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You can push the tip of the valve stem with a block of wood, a hammer handle or a screwdriver handle, and your force should push the valve open. It should then spring back. If you can do this, you might take the head off, and clean the valve stem and guide, or you can just hope, replace the pushrod, put the cover back on, fill the sump with oil and see if it works.
tom
 

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dirtgeezer
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1,441 Posts
Yes, what I have done is while I had the rocker off , make sure the piston is about an inch past TDC and use a amall hammet to tap on the top of the valve stem. If the valve is stuck it will go thud (solid) soak the valve stem down with PB Blaster and see if you can get the valve to move around and around and up and down before you install the new pushrod. With the engine being hot and low on oil you are going to be luckier than a goat if you get it to run ok. I have seen a valve stuck because a piece of a steel bolt got sucked into the engine on top of the piston and hammered the valve. It looked nasty but ran ok.
 
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