Kubota DH1100 engine bent push rod

[joel.smith1969]

having issues with my little Kubota lately, then quit running completely. Pulled valve cover and discovered a bent pushrod, on 2nd valve. Engine just went through a full rebuild recently. Could this have been a defective pushrod? should I just replace it and try it again or does this warrant pulling the head?

Thanks.

 

[jlrsn]

I don't know the first thing about a Kubota DH1100 or what it looks like but if the engine was recently overhauled a valve guide might have been reamed improperly and too tight or maybe the valve spring went flat allowing the valve to hit the piston. I'd check the valve and top of the piston for damage using a borescope, then look at the valve springs, do a compression test, and if no damage is found I'd replace the push rod and turn the engine through by hand watching for problems. A common cause of bent push rods in cars, motorcycles, and trucks is when a wrong gear is selected while downshifting causing the engine to over rev and the valve springs to float allowing the valve piston contact. Usually there is always a reason. I've rarely found defective push rods.
Good luck,
Jim

 

[rpraymond]

Ditto what jlrsn said. I've rebuilt a lot of engines and unless the pushrod had a bend in it when it was reinstalled, it likely wasn't the cause. There is always a reason. Good luck and look carefully at everything.

 

[firefighter jim]

Being that it was just rebuilt, maybe the valve lash was improperly set ie. too tight or valve clearances not set with piston at TDC.

 

[jlrsn]

Being that it was just rebuilt, maybe the valve lash was improperly set ie. too tight or valve clearances not set with piston at TDC.

If the valves were set too tight you'd have burnt valves, if set too loosely you'd have noisy valves but not a bent push rod. A valve adjusted not at TDC would not cause a bent push rod but mis-adjusted valves. The reason I suggested a Differential Compression Check is to confirm no Compression Leakage caused by a bent valve stem from a piston strike or valve leakage due to too tight a valve adjustment.

The reason that I brought up the possibility of not enough valve guide clearance is that thirty plus years ago I experienced a seized valve on a maintenance check flight with a recently overhauled HIO360A1A Helicopter Engine. I was over the SF Bay and I barely made it to the SF Bay Shoreline. Luckily there was no piston slap but a bent push rod. I removed all the valves from the guides into the combustion chambers and tried a go-no go gauge in the guides and found the guides were not properly reamed. I reamed all the guides, changed the bent push rod, and the engine went over a thousand hours thereafter without problem. In later years all the Lycoming 360 Series Engines on both Fixed and Rotary Wing Aircraft wound up with Service Bulletins requiring a valve guide clearance check due to coking valve guides. The HIO360 Series, especially the HIO360D1A Engines required a check every 300 hours.
Good luck,
Jim

 

[firefighter jim]

Now that I think about this some more, if the valves were adjusted off TDC, the clearances would be greater at TDC. Assuming this engine has solid lifters, I still believe that improper rocker adjustments could bend a pushrod or at the least, would prevent valve from closing all the way possibly causing the piston to come in contact with the valve.

 

[jlrsn]

Now that I think about this some more, if the valves were adjusted off TDC, the clearances would be greater at TDC. Assuming this engine has solid lifters, I still believe that improper rocker adjustments could bend a push rod or at the least, would prevent valve from closing all the way possibly causing the piston to come in contact with the valve.

Hi Jim,
As I stated on my first reply I don't know the first thing about the engine on that tractor but I know piston engines in general. I've worked on everything from the 28 Cylinder R4360 Radial Engines to simple lawn mowers since the 50s and even though anything is possible I don't believe being off of TDC would cause that as the engine had run for a while. If that engine had a case of the valves striking the piston that would have shown during the first run.

A Differential Compression Check (If he has the equipment) should show if there was piston contact especially if that engine had angled valves but if the complete valve head made full contact with the piston it would have bent the push rod. No matter if it was a seized valve in the guide or a piston slap something bent that push rod. In reading the original posting it appears that the engine had been running for a while since the overhaul.
Time will tell,
Jim (A VFF)

 

[BotaBigot]

Bent pushrods are "usually" the result of valve train bind. Culprits include: Bent valve stem, coked valve stem, tight stem/guide clearance, valve spring bind, or rocker arm bind. Another cause could be a weak valve spring, allowing valve float with piston contact. Unless pushrod(s) replaced when rebuilt, not likely defective. Since this is a recent rebuild, I'd focus on stem/guide clearance...

 

[firefighter jim]

Hi Jim,
As I stated on my first reply I don't know the first thing about the engine on that tractor but I know piston engines in general. I've worked on everything from the 28 Cylinder R4360 Radial Engines to simple lawn mowers since the 50s and even though anything is possible I don't believe being off of TDC would cause that as the engine had run for a while. If that engine had a case of the valves striking the piston that would have shown during the first run.

A Differential Compression Check (If he has the equipment) should show if there was piston contact especially if that engine had angled valves but if the complete valve head made full contact with the piston it would have bent the push rod. No matter if it was a seized valve in the guide or a piston slap something bent that push rod. In reading the original posting it appears that the engine had been running for a while since the overhaul.
Time will tell,
Jim (A VFF)

Hello Jim, Interesting problem. Hope the OP can figure it out. Boy those old radial engines that you worked on were neat. Nothing sounded better taxiing down the tarmac! Still had those when I served in the USAF (68-72) (C-123,C-124,C-47). I was a crew chief on an F111A. Have you ever read or have a copy of Dykes Automobile and Gasoline Engine Encyclopedia? An old publication and the best engine book I've ever read (includes radial recips). Just curious. Jim

 

[jlrsn]

Hello Jim, Interesting problem. Hope the OP can figure it out. Boy those old radial engines that you worked on were neat. Nothing sounded better taxiing down the tarmack! Still had those when I served in the USAF (68-72) (C-123,C-124,C-47). I was a crew chief on an F111A. Have you ever read or have a copy of Dykes Automobile and Gasoline Encyclopedia? An old publication and the best engine book I've ever read (includes radial recips). Just curious. Jim

Hi Jim,
I'm also waiting to see the cause of that bent push rod. I was in the USAF (58-64) and worked on most of the old radials at the time. I initially went to school on the B-29 and as time past got to know most of the Radials so well that I was assigned to a Mobile Maintenance Team based out of Madrid (Torrejon) sent all over Africa, Asia, Europe to get AOG Aircraft back in the air. Finally transitioned to Jets. I flew a lot of the old Radials after I left the USAF. Never read Dykes Automobile and Gasoline Encyclopedia.
Take care,
Jim