Ruined Pushrods

[Lliamiskool]

Hi all, I was just gifted a broken 2003 MTD yardman with a briggs and stratton 20hp v twin 407577 engine. Supposedly, it lost compression and wouldn't restart. It had very low oil. I pulled the valve covers and one side was good, but the side with the dipstick was all messed up. The aluminum push rod was bent and just sitting in there. I could just barely see the other rod deep inside so I pulled the head and was able to yank out the other pushrod. It must've been caught on the connecting rod or something because it was all chewed up and I had to spin engine back and forth to free the rod. I've only ever worked on 2 strokes before, but I've been doing some research. The whole engine was covered in dirt and oil, so I'm thinking maybe it overheated and that's what caused this. The valve guides probably rode up a bit too. I plan on pushing those back down, and using a punch around it to keep it secure, replace the rods, and hopefully I'm back in business? I'm hoping someone with a bit more experience on here can potentially prevent me from making a stupid mistake. Thanks!

 

[Elkins45]

It’s not an uncommon occurrence with these engines. Overheating is usually blamed for it but loose clearances can do it as well. Good luck keeping the valve guides staked down because that doesn’t always work.

Some folks replace the aluminum intake rod with another steel exhaust one when they have to repair them because they’re the same length.

 

[Lliamiskool]

It’s not an uncommon occurrence with these engines. Overheating is usually blamed for it but loose clearances can do it as well. Good luck keeping the valve guides staked down because that doesn’t always work.

Some folks replace the aluminum intake rod with another steel exhaust one when they have to repair them because they’re the same length.

Not a bad idea. It's a free mower so its worth a try!!

 

[Gyro Gearloose]

I haven't worked on a twin but amoung the causes for bent pushrods in a Briggs OHV single cylinder can be valve guide worked its way up and hit the rocker arm, or the compression release mechanism or the governor gear had a rapid unscheduled disassembly and the shrapnel got caught between the cam and lifter causing over lifting of the valve.

As a general observation, these engines have little lifters in the block between the pushrod and cam so it should be impossible for the pushrod to get into the crankcase and contact rotating equipment unless a lifter fell out of place, and the only way I can see that happening is if the guide in the block broke off. This mower sounds like a "free puppy" deal.

 

[Lliamiskool]

I haven't worked on a twin but amoung the causes for bent pushrods in a Briggs OHV single cylinder can be valve guide worked its way up and hit the rocker arm, or the compression release mechanism or the governor gear had a rapid unscheduled disassembly and the shrapnel got caught between the cam and lifter causing over lifting of the valve.

As a general observation, these engines have little lifters in the block between the pushrod and cam so it should be impossible for the pushrod to get in the crankcase and contact rotating equipment. The only way I can see that happening is for the block to break and let the lifter fall out of place.



View attachment 263534

hmmm okay. What exactly am I looking for in your picture? I'm hoping I can get away with not completely tearing into this thing!

 

[blkwtr]

Your engine is not a 2 stroke. It is a 4 stroke. A 2 stroke does not have push rods or valves with valve stems.
A dirty engine will cause an engine to overheat. Since the valve guides on a Briggs and Stratton engine are pressed into the head, overheating will cause them to move back out of the head. When the cam moves the push rod up against the rocker arm to push the valve valve stems down, it hits the raised valve guide and bends the push rod. If the engine has no other damage it's relatively easy to replace the head and valve guides but a new head will be around $100.00.

 

[Gyro Gearloose]

The picture is the upper half of a Briggs twin cylinder crankcae. The small bearing at the top of the crankcase is where the camshaft runs and the 4 small holes in the two blocks of metal between the cylinders are where the valve lifters reside. The lifters are about two inches tall so the pushrods do not extend into the crankcase and there should be no way for them to get in there as long as the lifters are still sitting on top of the cam. The fact that you have a pushrod that got down into the moving parts and got chewed up strongly suggests to me that something broke inside the crankcase.

 

[Lliamiskool]

The picture is the upper half of a Briggs twin cylinder crankcae. The small bearing at the top of the crankcase is where the camshaft runs and the 4 small holes in the two blocks of metal between the cylinders are where the valve lifters reside. The lifters are about two inches tall so the pushrods do not extend into the crankcase and there should be no way for them to get in there as long as the lifters are still sitting on top of the cam. The fact that you have a pushrod that got down into the moving parts and got chewed up strongly suggests to me that something broke inside the crankcase.

thank you for the explanation. Online, I have seen multiple stories of lost push rods ending up in the bottom of the engine. People would use magnets to fish it out of the dipstick hole. In those cases, they got the old one out, and fixed the problem in the head and they were good to go with nothing broken inside the crank case. Maybe my situation is similar to that?

 

[Elkins45]

I’m pretty sure there’s an opening in the “valve box” where oil gets splashed into the lifters, and it’s not uncommon for the pushrod to fall down into the crankcase. I’ve also seen videos of people trying to fish pieces of the steel ones out thru the oil filler with a magnet.

 

[Gyro Gearloose]

In the picture of the block there is another hole below the lifters which I would presume is the way oil gets/drains from the valve springs and rockers. Ya'll have more and better information than I do on the possibility a dislodged pushrod can fall through there so we will go with your diagnosis. I learned something today.

 

[Elkins45]

In the picture of the block there is another hole below the lifters which I would presume is the way oil gets/drains from the valve springs and rockers. Ya'll have more and better information than I do on the possibility a dislodged pushrod can fall through there so we will go with your diagnosis. I learned something today.

In these engines the fuel pump is powered by a small vacuum hose that is connected to one of the valve covers. There has to be some sort of interconnection to the cylinder or crankcase in order to draw the vacuum.

 

[mk378]

Before buying any parts, you could borrow the pushrods from the other side to see if the valves can still move properly.

I've had a Kohler V-twin of similar design bend and drop the same pushrod twice. Both times it would simply run on one cylinder, the rod did not find its way into the crankcase.

 

[Lliamiskool]

Some great ideas everyone thank you. When it comes to pushing the exhaust valve guide down to where it is supposed to be, does anyone know how much it should stick out? looks like the other cylinder (that had working push rods) was going to fail soon as the guide had moved as well. I attached pictures of both cylinders and two new things I believe to be intake valve guides? I wonder if it is worth replacing those as they came in my cheap gasket kit I ordered. I measured both intake valve guides to stick out 2.34 mm.

 

[z71bill]

When I was 17 (1975) I bought a 1969 Chevelle SS396.

After hot rodding around for a few weeks I started hearing some rattle inside the valve covers -

I pulled the covers off and could see the rockers were rattling -

Not knowing anything - I grabbed a socket and started tightening them down - a little at a time until they were all quiet -

I over did it on some and the engine would bog down.

I then realized I had bent several pushrods - and also found out some of the noise was coming from bad lifters

So I ended up replacing all the lifters and pushrods and also learned how to torque them down properly.

My biggest mistake was I sold the car in 1980 - by then it needed another engine overhaul, tranny was getting hard to shift, clutch needed replacing (again), brakes (again), rear tires (again and again and again and again), shocks, ball joints, exhaust and many other things -

But the body was still straight and 99% rust free, interior was good.

I should have put it in storage - and then when I had the $$ restore it.

Only vehicle I ever sold that I wish I had kept

 

[Fordiesel69]

Hi all, I was just gifted a broken 2003 MTD yardman with a briggs and stratton 20hp v twin 407577 engine. Supposedly, it lost compression and wouldn't restart. It had very low oil. I pulled the valve covers and one side was good, but the side with the dipstick was all messed up. The aluminum push rod was bent and just sitting in there. I could just barely see the other rod deep inside so I pulled the head and was able to yank out the other pushrod. It must've been caught on the connecting rod or something because it was all chewed up and I had to spin engine back and forth to free the rod. I've only ever worked on 2 strokes before, but I've been doing some research. The whole engine was covered in dirt and oil, so I'm thinking maybe it overheated and that's what caused this. The valve guides probably rode up a bit too. I plan on pushing those back down, and using a punch around it to keep it secure, replace the rods, and hopefully I'm back in business? I'm hoping someone with a bit more experience on here can potentially prevent me from making a stupid mistake. Thanks!

View attachment 263514

View attachment 263515

Whatever valve guide moved on you, on the aluminum head, drill diagonally all the way thru the aluminum, until you hit the guide, then knock it out completely and look for the drill bit witness mark. Grind a slot there but not all the way thru. Tap the alumium head before installing the guide. On the slot made put a nice liberal amount of jb weld and press the guide back in. Use an allen set screw and tighten it into the guide. This will prevent it from moving again. Ugly repair but it works.

I have found mine do not hold just by prick punching them. They seem to work loose again.

Also run the gov up to 4000 for better cooling, and less lugging under load. Use a lower ground speed any time the engine has to work hard to avoid high EGTs.