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Lately when I read posts here, I see thread after thread with bent pushrods. Having been around pushrod engines all my life, it is startling to see so many threads with the same problems. Can anyone explain to me why so many of these engines suffer bent pushrods? Are the valve springs too weak so the valves float and the rods fall out? Are the clearances so close that valves are hitting pistons? Are the pushrods too soft? Some people have had it happen multiple times, even with new heads? Are modern fuels causing valves to stick?

I would say the majority are related to the overheating head effect which causes the valve guides to migrate toward the rocker arms to the point they interfere with valve opening.

how? If the guide is too far to the valve it could hold it open. If it is too far to the crankcase side how does it interfere? VW heads are air cooled and they don't have guide migration problems. Type 4 heads in buses run sometimes close to 450F. I am trying to understand the process that is causing all these bent pushrods.

The valve guides on Briggs due to overheating always migrate toward the valve spring keeper to the point it will prevent the valve from fully opening during its cycle. Overtime the pushrod will go from straight to flexing on each open attempt to finally bending and falling out. The issue which may be a difference in manufacturing. I don't know how other guides are installed on different equipment, but on Briggs they are press fit after head molding not cast into the head, and when the head overheads due to air obstruction the aluminum expands causing loss of adhesion on the metal guide and then it starts moving. Heating and cooling cycles may cause a caterpillar effect. This is present on the Intek series and the Vanguard single cylinder series of engines. Not as common on the Vanguard V Twin engines but will see on occasion a bent push rod. On those it seems to be more of a temporary stuck valve and almost the same issues with the Kawasaki engines. You can replace the bend push rod on those engines and most of the time you will not find the initial cause of failure.

Also on Briggs you have to watch the aluminum pushrod where it contacts the push rod guide because it will wear it thin which leads to push rod collapse. Normally seen in cases of improper maintenance where the oil isn't changed on a regular basis.

On the Kohler Command engines, you have to watch for over expanded hydraulic lifters if you remove the heads. If not collapsed before starting will bend the push rods on first start up.

And it happens of Kawasaki engine too from overheating. I have replace several Kawasaki heads with valve guide migration. One engine of a Walker walk behind commerial mower I had to replace both cylinder heads because the hydro oil cooler was clogged so much that over heated the engine. A very stupid design where they are using the engine to pull cool air through the hydro oil cooler. It over heated so much that I had also to replace the engine's shroud and cooling fan because they melted. Engine shroud is very expensive item @ $103.47 along two cylinder heads.

interesting. Thank you for the insight. In the years I worked in automotive machining and racing we saw maybe 10,000 heads, mostly water cooled but many air cooled. I never once saw a guide migrate that I can recall, although I have seen them break, and wear out. I suspect from the description given above, carbon build up on the valves may be the culprit that allows the valves to drive the guides towards the keepers. Just guessing. It just seems so common a problem in these threads that the manufacturers should find a fix like putting a snap ring or collar on the valve side to lock it from migration towards the keepers if that is the direction they all go.

sgkent said:

interesting. Thank you for the insight. In the years I worked in automotive machining and racing we saw maybe 10,000 heads, mostly water cooled but many air cooled. I never once saw a guide migrate that I can recall, although I have seen them break, and wear out. I suspect from the description given above, carbon build up on the valves may be the culprit that allows the valves to drive the guides towards the keepers. Just guessing. It just seems so common a problem in these threads that the manufacturers should find a fix like putting a snap ring or collar on the valve side to lock it from migration towards the keepers if that is the direction they all go.

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The only similarity between auto engines & mower engines is you put petrol in both .
Mower engines are the cheapest nastiest engines that it is possible to make.
Every cheap & nasty cost cutting measure that can be taken and the thing still run has been done.
All of the quality engines have gone so now we are left with nothing but crap .
Some are just higher on the Bristol chart than others
Valve stem migration was a common thing in the motorcycle engines of the 40's & 50's then they learned how to keep them in place
Many auto engines cast the guides into the heads with a flange on them so they can never move ( or be replaced ) as now days worn guided can get sleeved but with the advancement in guide materials worn out valve guides only happen in race engines where you are pushing everything past it's reliability limit or with an abused engine running on filthy oil
I have not been to a mower engine factory but I could almost guarantee that the guides are pressed in then not honed before assembly to cut costs.
This was tried by the Japanese motorcycle factories in the 60's and eventually they got it right but there were a lot of valve train failures in the 60's & 70's till they got it right .
When you are pushing in a finished to size guide the amount of interfearance in the fit is reduced to the minimum that is required to hold the guide in place and modern pressure die casting allows the guide hole to be formed in the casting such that a tiny , or no finish machining is required .
Heat treating is cheaper than machining .

All of this is fine as after all they are just mowers, throw away items designed to go for 5 to 10 years if looked after properly .
Modern computer modeling of heat exchange has allowed cylinders to be cast with the minimum possible number of the shortest possible fins which again saves metal CHEAP, CHEAPER , CHEAPEST & still needs to be Cheaper next year

yeah I get the manufacturing thing. It becomes part of the self destruction of the west. We give more and more to the 1% and deal with inflation at the same time from the middle class feeding those who don't feel like taking off their bunny slippers today. The accountant tells engineering that instead of making a mower to last 15 years make it last 13, 11, 10, 9, 8 ... . The sign says new and improved, and the price is only $450 more than last year. But last year $4700 \ 10 years was $470 a year cost, and now $5150 \ 8 is $643 year so Fred starts looking at used ones cause he only mows 5 months a year so with oil, gas and a new mower he can't afford it anymore. Meantime he is told buy electric and save a political weasel. The end result is a bad guide and a bent pushrod. Makes sense now.

Goes like this and Ryobi is the master of it
1) design a product that isa delight to use
2) make it impossible to repair
3) owner will buy a new one because the old one was so comfortable & worked so well .

Then from B & S's point of view
They could make a bigger profit at sale time on a more expensive mower, but then for the next 10 to 20 years, the dealers will be making the profits on repair
Or make junk that lasts a short time and is not repairable so you get another sale 5 years latter.
The customer pays about the same amount but in case 2 B & S make all of the profit
And that is the real reason for the horrid state of the planet right now because no one counts embedded pollution in makeing new products every 5 years as compared to the 5% more that would be emitted making something that lasts 3,4,5 or 6 times longer .

It just plain greed that is involved now. Plus all the pencil pushers are thinking about is the here and now, not the future.

With Briggs they sold to an investment company and now prices for anything is going through the roof. A lot of parts that is available has nearly doubled in price and others are simply NLA. I ran across one their engine that was less than a year old and all the parts were listed as NLA. In one example a fuel bowl that I need for conversion jobs now cost as much as a complete carburetor yet I can buy a clone carburetor pull the fuel bowl and throw the rest in the recycling bin dirt cheap.

On top of this my Briggs distributor is demanding me to buy $2000 in parts that I don't need or they will dump me. If I needed them I would buy them if available but a lot of the NLA Briggs parts I am having to buy junky aftermarket parts to do the repairs which i must remake to fit the intended purpose. It like those dang camshafts that are currently on back order until after Christmas.

In general getting parts this year has been a nightmare. I got equipment sitting on the repair yard now for over three months awaiting parts. Personally I just about to quit repairs and retire.

And top of this every dang company is being a pain this year. Even the grocery stores are showing their behinds. Now they are trying to force us into buying the over price name brand items of their choice. All they are doing is causing me to change what I am buying for everyday use. I have eliminated nearly all of the impulse items which they seems to be always over stocked on. I am basically down to potatoes, Ramen noodles, beans and rice primarily now. I am not going to pay $15/lb for steaks, it can just lay there and rot for my part. Besides the doctors keep griping about my cholesterol levels and put me on a no salt diet.

I usually go through 20+ lbs of sugar per year and now I just brought my 4 lbs in the last 18 months. I no longer buying soda pops or candy. I am buying very little in fresh produce now too.

@StarTech I know the feeling about Briggs. I don't think I have worked on $2000 worth of Briggs engines this year, Let alone ordering $2000 in parts. I haven't worked on a single Honda this year. Kohler has been hit and miss, but Kawasaki has been about 90% of my repairs this year. Most of my repair work has been on the mower side of the equation.

ILENGINE said:

@StarTech I know the feeling about Briggs. I don't think I have worked on $2000 worth of Briggs engines this year, Let alone ordering $2000 in parts. I haven't worked on a single Honda this year. Kohler has been hit and miss, but Kawasaki has been about 90% of my repairs this year. Most of my repair work has been on the mower side of the equation.

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It been about the same here. It is strange that so Kawasaki engine are coming in this year. But who knows next year might quite different.

StarTech said:

It been about the same here. It is strange that so Kawasaki engine are coming in this year. But who knows next year might quite different.

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I think what is going on is the type of equipment we are seeing in the shop has changed. Not so much low end equipment like past years. More mid to high grade mowers, that just happen to be fitted with Kawasaki engines. My market has went from the typical box store $1200-2500 riders to $6000-15000 Z turn mowers.