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Old 08-29-2011, 12:35 PM   #1
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Briggs engine eats push rods.

I have a Briggs Intek engine here in the shop on a Gravely ZT unit that has started eating the exhaust push rod on the one cylinder (it falls out and slides into the crankcase through the oil gallery). The initial occurrence was caused by a mouse nest over that cylinder, causing that cylinder to overheat. I cleaned everything up, replaced the push rods, reset all the clearances, and pressed the exhaust valve guide back into position (it had released and moved when it overheated). The unit was running fine, and the customer put about 10 hours on it since it was fixed. Last week Thursday, he brought it back, saying that cylinder had cut out again. I pulled the valve cover to find that the exhaust push rod was missing again, and it had slid into the crankcase through the oil gallery just like last time. The valve guide is in position yet, and the intake push rod is straight (it had bent last time). I have to pull this thing apart again, but I was just wondering what the heck could cause this trouble? The engine has about 273 hours on it and it was around 262 before the first push rod went. What should I be looking for to keep this from happening again? I really, really, really hate rework.

Engine info:

Model: 44P777
Type: 0111-E1
Code: 060519YG



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Old 08-29-2011, 01:04 PM   #2
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Sounds like something is still out of wack... i had a 12HP OHV teccy motor on a 80's dynamark years ago, when i got the tractor, PO said it wasnt running.

So i yanked the valve cover off to find the rockers hanging loose , flopping around, but the push rods were still sitting there- i talked to a local tractor guy and got the proper clearance numbers for the rockers, tightened em down- thing fired right up.

After a few mins tho, the rockers clattered a lil louder, so shut it down, pulled the cover to find they loosened again. I bot a teccy manual to see what could be the issue, manual showed 2 lock nuts for the rockers, while my tractor had only one set.

I had to buy a special threaded nut for both rockers, cut em down on the grinding wheel ( to fit under the cover) and reset everything - worked great till the day i traded the tractor for a motor.



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Old 08-29-2011, 01:45 PM   #3
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Check the cam. I've got a buck that says the exhaust lobe is worn.

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Old 08-29-2011, 04:08 PM   #4
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Quote:
Originally Posted by ErnieS View Post
Check the cam. I've got a buck that says the exhaust lobe is worn.
Will do. I pulled the shroud today and found another big ball of grass over that cylinder. I don't know if it is a mouse or if the flywheel fan is just blowing any grass that gets under the shroud over to that side. I pulled the head on that side and attempted to pull the push rod back out of the engine, but it is wrapped around the cam shaft again, so I will have to pull the engine and pull it apart.

If the cam is worn, wouldn't that affect both cylinders? Just the one cylinder is having issues.
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Old 08-29-2011, 04:39 PM   #5
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Quote:
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Will do. I pulled the shroud today and found another big ball of grass over that cylinder. I don't know if it is a mouse or if the flywheel fan is just blowing any grass that gets under the shroud over to that side. I pulled the head on that side and attempted to pull the push rod back out of the engine, but it is wrapped around the cam shaft again, so I will have to pull the engine and pull it apart.

If the cam is worn, wouldn't that affect both cylinders? Just the one cylinder is having issues.
If the engine shares the same cam lobe for both cyl, it should affect both cyl if it is the cam lobe at fault.

Think I'm thinking along the same line as Ernie. Something isn't right about nom distance between rocker and cam. Here our thought are diverging. Was thinking of possibility of weak valve spring and if engine running at high speed, the spring isn't able to keep adequate pressure on the valve components and distance between rocker and cam is increasing thus letting the push rod to fall free.

Would be nice if you could post a pic of the valves and other components in the head that is giving you problems.
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Old 08-29-2011, 05:48 PM   #6
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Quote:
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Will do. I pulled the shroud today and found another big ball of grass over that cylinder. I don't know if it is a mouse or if the flywheel fan is just blowing any grass that gets under the shroud over to that side. I pulled the head on that side and attempted to pull the push rod back out of the engine, but it is wrapped around the cam shaft again, so I will have to pull the engine and pull it apart.

If the cam is worn, wouldn't that affect both cylinders? Just the one cylinder is having issues.
I doubt that one lobe serves 2 cylinders. An Intek twin is a 90degree Vee, isn't it?
More than likely, the cam has 4 lobes and one wasn't heat treated properly. I'm working on a Briggs single where that happened.
It has about 200 hours on it according to the PO and on tear down, everything looks as expected excepting one cam lobe which has probably only a 1/16" of lift left.
Hitting both lobes with a file, the worn lobe seems considerably softer than the other.
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Old 08-29-2011, 07:10 PM   #7
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pushrods

CB,check the pushrods to make sure they're in the right spot,as one is aluminum,and one is steel.Also,check the rocker arms,and rockers,for wear.Use a micrometer on the cam lobes,as I've had a couple that were worn(they DON'T share the same lobes),more than normal.Also,all4 springs are the same #,so check to see if any feel weak. If the cylinder that is throwing the pushrod,is the one with the compression release,check it to make sure it is not sticking. You might also check the tappets,to see if one is shorter,as I've had one engine that had one that was about .oo5"short,and it did the same thing. THe only other thing I can suggest checking,is the cylinderhead ,since this was overheated,and may have some warpage/stretching/deformation. Hope this helps(by the way..use NEW head bolts).

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Old 08-29-2011, 08:18 PM   #8
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I had an Briggs engine two years ago that had an exhaust lobe grind down to nothing due to poor heat treating. I don't remember seeing damage to the lobe the last time I had it apart, but I will certainly check that out when I get it apart. I did pull the head today, but I didn't get a chance to check it for true. I do know that I set the clearances on the valves right on the nuts when I put it back together last time, and I always spin it over a couple times and recheck. The adjusters on the arms are still tight and don't appear to have moved. It must have overheated again due to that grass being on the head again. The grass seems to be all on the head, not around the cylinder cooling fins. I am still at a loss to figure out where the grass is coming from. There aren't any chew marks on anything, nor is there the smell of mouse urine on anything (or the corrosion that goes with it). Every time I have had a mouse infestation of an engine, there were other signs besides the nest. Mice wouldn't use just one material for a nest either. They tend to shred things in the area and use different materials for different purposes. This is just all grass. Could the fan on the flywheel be sucking it in and depositing it over that cylinder? There is absolutely no grass anywhere else on the engine but on the head on that side. Its the left cylinder as you face the engine looking into the V. The valve covers are on either side of you and you are looking at the carburetor. The right cylinder was totally clean when I pulled the shroud off.

I have off work tomorrow (at least at the store!), so I'll have to do more investigating when I get back Wednesday morning. I'll be building new pens for my calves tomorrow so I can finally get them sorted and moved where they belong. Maybe I can rent out a few of them to the owner of this lawnmower to save him money on the repairs...

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Old 08-29-2011, 10:23 PM   #9
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Not so sure this is just a thermal issue. If we would use 400° rise then with the linear thermal expansion for Al being .0000123"/"/°F which translates to ~.005"/" over the aforementioned 400° rise. For steel, the number are .0000073"/"/°F which translates to ~.003"/" over the temp rise. Now if the push rod happened to be the Al one the expansion between the cyl and head would be canceled out by the same expansion of the Al push rod. With the steel push rod the difference in expansion is ~ .002"/" over the aforementioned temp rise. We're not talking a vary large number for dim changes. I don't have any idea how much overall expansion would be needed for the push rod to drop out but would think it is much larger than the numbers are indicating. If the cam to rocker arm distance is 6" (just guestimating) at most using these figures we're talking ~ .012" increased clearence if talking about the steel push rod.

I think it is a little more complex than than these simple number are indicating but the push rod to rocker arm clearence isn't changing very much.

Maybe my brain isn't running on all cyl tonight but I don't think I'm off by some X factor. I'm open to being corrected if I'm way off.

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Old 08-29-2011, 11:20 PM   #10
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Quote:
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Not so sure this is just a thermal issue. If we would use 400° rise then with the linear thermal expansion for Al being .0000123"/"/°F which translates to ~.005"/" over the aforementioned 400° rise. For steel, the number are .0000073"/"/°F which translates to ~.003"/" over the temp rise. Now if the push rod happened to be the Al one the expansion between the cyl and head would be canceled out by the same expansion of the Al push rod. With the steel push rod the difference in expansion is ~ .002"/" over the aforementioned temp rise. We're not talking a vary large number for dim changes. I don't have any idea how much overall expansion would be needed for the push rod to drop out but would think it is much larger than the numbers are indicating. If the cam to rocker arm distance is 6" (just guestimating) at most using these figures we're talking ~ .012" increased clearence if talking about the steel push rod.

I think it is a little more complex than than these simple number are indicating but the push rod to rocker arm clearence isn't changing very much.

Maybe my brain isn't running on all cyl tonight but I don't think I'm off by some X factor. I'm open to being corrected if I'm way off.
Mickey, You better hope your engine never sees a 400 degree temperature rise or you'll have more to worry about than push rods. You're likely to see 200 to 250 degrees over ambient. 20 minutes at 325 and your oil is nearly worthless and if conventional oil reaches 420F, well, you've got REALLY big problems
Yup a really hot engine would probably have some valve train noise, but I think the cylinder head would melt way before a push rod fell out. Al melts about 1,100 F and with your numbers the valve lash would be spec plus .035"

So let's go with 250 rise and call the steel rod .007 shorter than the stack (block and head) The ends of the push rod are spherical and sit in a spherical detent in the tappet and rocker. Let's say the push rod is .300 in diameter. The spherical detent would then be .150" and in order for something to fall out, it would have to have that much room. If we assume the valve lash adjustment didn't move, there has to be a worn push rod, a worn cam lobe, worn tappet, worn rocker arm, a weak or broken valve spring or some combination of 2 or more accounting for .150" less lash spec plus .007.
Granted I have little experience with Briggs engines, but I've been riding and wrenching v-twin push rod motorcycles for 43 years and have seen all of the above, but never an engine that has gotten so hot a push rod fell out of place without the above factors.
The one Briggs engine I have worked on in probably 20 years has a worn, I mean REALLY worn out lobe, apparently due to improper heat treatment.
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Old 08-29-2011, 11:21 PM   #11
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I don't think you're off,Mickey.That's why I suggested he check the other things,also.The engine would have to get a lot hotter ,and it would usually show on both sides.I'm thinking something mechanical,or not the right part(that's why I suggested the tappet length),etc. Since these engines rotate clockwise(viewed from frt),the grass he mentioned may just be settling from turbulance,and I don't think it hurts the engine,if it isn't excessive. So,basically it's got ,as ErnieS suggested,one,or more items working against normal operation,on that cylinder.All CB can do,is measure EVERYTHING,for proper dims.

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Old 08-30-2011, 11:14 AM   #12
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Since there has been mention of air blockage on the cyl/head in question we don't know how hot things got. I just used the 400° number to show even at these high temps, dim change due to thermal expansion was quite small and not likely THE cause of the problem.

If you can live with uneven firing (think Harley) then one set of lobes is possible.

Does this engine have adj valve clearance? If so I don't see how dimensional variation from nom come into play IF the valve clearance can be set to spec. What ever the problem is, engine won't run for hrs then all of a sudden throw a push rod. This has to be some kind of dynamic issue.

Rereading the initial post I'm starting to wonder if the cause is related to the aforementioned loose valve guide. Possibility of valve getting stuck in partially open position, even momentarily, thus not exerting any pressure on the rest of the valve train which could be the cause of increased clearance and thus permitting the push rod to float free of the rocker and lifter and free itself from its proper position.

A wonderful tool for problem solving can be found using K-T analysis ( How to use the Kepner Tregoe decision making model ) and their problem solving approach. Define the problem, look for possible cause and then investigate the possible cause and does it answer the root cause of the problem and not introduce any additional issue that are not seen. It's a great tool but one an individual would not likely seek out and pay for the trainning.

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Old 08-30-2011, 04:22 PM   #13
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Harley uses 4 cam lobes. A 90 degree v-twin with only 2 cam lobes would fire at 0 and 180 then coast for 1 and a half revolutions before firing again.

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Old 08-30-2011, 04:25 PM   #14
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An opposed twin could run on 2 lobes and fire once per revolution

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Old 08-30-2011, 07:24 PM   #15
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The cylinders on Briggs twins,are staggered,and though,on the v-twins they may share the same crank journal,each has its own set of cam lobes.The opposed twins have their own crank journals,and cam lobe sets.Unless the owner is running the engine above redline,or has a lean running cylinder,or a cracked cylinder,I would start with the rocker arms,rocker studs,adjusters,camlobes,pushrods,or valve components.

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Old 08-31-2011, 08:25 AM   #16
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Just to clarify, the grass mat I found on the head was about 2" thick and covered the entire top of the head. I'll be pulling the unit apart today, so I will know more by tonight.

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Old 08-31-2011, 11:28 AM   #17
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Haven't owned a Harley but from the sound and what I've read they are not even firing engines. But this side discussion isn't helping solving the problem at hand.

CB keep us informed as to what you find.

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Old 09-01-2011, 07:47 PM   #18
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Okay, finally got time to tear this unit apart today. Took some pics which I will post. The camshaft is in perfect condition, the minor scratches and dings from the factory haven't even worn off the lobes. The head gasket on the #1 cylinder is blown, as there is evidence of it leaking on the gasket mounting surface and the block and head. The exhaust valve guide did move, as I can see a shiny area on it looking through the exhaust port. It couldn't have moved far because there is undisturbed soot on the bottom edge of the guide. The intake valve is leaking in one area, and the valve is covered in carbon on the stem and bottom of the valve. The exhaust valve is sealing fine, but the stem is partially coated in a very thin, yet very hard layer of carbon. Carbon extends into the area that slides inside the valve guide. The tappets all measure exactly the same dimensions out to the thousandths place. As far as I can tell, the valve springs all feel about the same, however, I do not have a way of accurately measuring the force needed to compress them. All I have is the feel of them as I compress them by hand.

Here are some pics:

Exhaust pushrods. One on the left is from the last time I fixed it. One in the middle is the current bad one, one on the right is the good one from the other cylinder.



Next few are pics of the camshaft.





Head. You can see the blown out area at the top of the gasket mounting surface in this pic. Its stained brown whereas the rest is still shiny.



Block. Blown area is to the right side in this pic.



The unit the engine came off of. Gravely ZT XL 48"



The exhaust valve. Notice the carbon on the valve turns deep, shiny black where it enters the valve guide.



Intake Valve. Note large amount of carbon on the underside of valve and on stem.



Valve chest. Note burned oil on exhaust valve side (left side) and black mark by intake valve guide (no idea what that is).



Looking through the exhaust port, you can see that the valve guide has moved.



Looking at leaking area of intake valve (port closest to camera). Note carbon buildup in port from where valve is leaking.



Inside of engine crankcase. Just for reference.

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Old 09-01-2011, 09:30 PM   #19
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head

OK,I've noticed 2 things,that may cause the problem.First is the camshaft.It has splined cam lobes,which,under higher -than-normal heat,have been known to "walk"on the shaft,and take out a pushrod.They should be tight,on the shaft,and no wobble. The second thing is the black deposit,at the intake valve guide. It could be an indication of a cracked head.The only way to tell,is to have it checked at a machine shop. Both of these conditions could be present ,at the same time,with one(cracked head),causing the other(lobe-walk,due to high engine temp).The blown head gasket is another sign of possible head damage.The carbon on the underside of the valve looks burned/flaky,indicating high temps,also.

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Old 09-01-2011, 10:41 PM   #20
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I assume the black deposit you are referring to is the one in this pic:



I really can't say for sure if it was present when I worked on the unit the last time, however I did notice it right away this time. I'll have to pull the stem seal and take a closer look at that area. My boss was gone this afternoon, so I was busier than heck all afternoon and only had a few minutes to snap the pics. I plan to do a more extensive investigation tomorrow morning. I'll check those lobes for evidence of being loose or moving. I really miss the old cast camshafts. I'm sure they are more difficult/costly to make, but they never slipped out of time or came loose on a guy. I hate the newer plastic ones with a passion. Been having issues with the gears breaking free and turning on the shaft, rendering them useless. Not a ton of them, but a few. And a few is a few too many in my book.

I am wondering if the carbon on the exhaust valve that extends into the guide could be the culprit. If that carbon would become tacky at high temps, it could cause the valve to drag in the guide and loosen the push rod momentarily. Where the carbon is riding in the guide, its polished shiny much like obsidian. I'll probably replace the valve and guide as a precaution. As I said before, I really, really, really hate rework. I want things to be right the first time, and if they come back with the same problem, it makes me feel like I have failed the customer.



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'76 IH 1066 Turbo Diesel, '73 IH 766 Diesel, '72 IH 574 Utility Gas, '56 IH 350 Utility Gas, '47 Farmall H Gas, '08 Bobcat Toolcat 5600 Turbo

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Stihl, Ariens, Tecumseh, LCT, Lauson, Kohler, Subaru and Lawn Boy Certified Technician with over 10 years experience.

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