Kubota L4330 - Loader bolts missing / threads mashed / stripped threads

[Coyote machine]

Continuing my earlier post, I forgot to mention, in order to determine what length new bolts to use, I used a dial caliper to determine the depth of the hole and then measured the thickness of the loader frame and added for a flat washer. I don't remember if I had to custom cut the bolts to length or not.
I do remember the holes in the frame where the original bolt holes were tapped at the factory were elongated by the stresses of the bolts backing out and the loader frame pushing down on them in an attempt to snap them off over time.
Fortunately there were clean unused threads at the bottom of each hole allowing for about 1/2 inch of grab for the new longer bolts.
Hope this helps.

 

[rScotty]

My M59 lower loader frame shows sledgehammer marks/dents which are then covered with the original Kubota paint. My guess is that when the frame was made it was then "adjusted" on a jig to fit the engine width exactly. When a plate like this frame is bolted to a another plate (the engine), the analytic assumption is that the fit is perfect. If so, then the number and size of the bolts can be calculated such that the bolt tension will prevent the plate from sliding laterally. Always nice if the bolt holes in the plate are snug, but more important that the fit is flat.

Torque on the loader frame bolts (Metric size M18 on the M59) is 235 ft*lbs. I interpret the head marking to mean that these are special high strength bolts. BTW, each bolt head has a slash of silver paint that is just the kind of thing I would do to indicate that I'm satisfied with the torque. Hopefully that's right. I see no loosening. There's nothing specific in the operator's manual about re-torquing any thing other than a general statement that all the hardware should be checked daily to see if anything is coming loose.
rScotty

 

[tjkubota93]

What type of situation is a helicoil not appropriate? If installed properly it is stronger than the original threads.

 

[rScotty]

What type of situation is a helicoil not appropriate? If installed properly it is stronger than the original threads.

It's not the helicoil; it's the job. I don't know that helicoil has ever published a claim that they are stronger than the original threads. Although frankly I like them too...nothing better in the hands of an experienced metalworker for fixing a worn jig. And Scott might still end up doing it that way. But this isn't a job of drilling and tapping a coil into some easily worked material. It's a bunch of big bolts going horizontally into a cast material without a handy way to hold everything steady and perpendicular while doing all the prep work for a good helicoil. I sure wouldn't want the job. And then there's the expense. Big coils and the specialized stuff to install them ain't cheap.

Let me turn the situation around...What's wrong with trying to clean and chase the original threads first? Either they will hold the torque or they won't. Either way he will know what it is he is dealing with.
rScotty

 

[tjkubota93]

Ok. Your correct. I would not want to do it either. They are always a last resort. It would be a pain even to someone who knew what they were doing. And a crooked one wouldnt apply good pressure. I just always figured they are stronger because its the same as using a size larger bolt. I didn't know about the advertising (I live under a rock)

 

[crazyal]

A machine shop can do the heli-coils for you if you feel it's more than a job you want to tackle. They would most likely have a size that could be used and just charge you for the labor and the inserts vs you buying the drill, special tap, and the insert tool. If there is a reason why they can't be used in this application I would like to know as I'm not an expert.

 

[tjkubota93]

Id never pay anyone to do anything except a house roof because it would take too long. But ive been a master mechanic for 12 years.

 

[SPYDERLK]

They have been used by others here that have had the same problem with success. We use them at work all the time and I've never heard of a limitation. Do you have documentation? I would like to read up on it.

With careful installation and use they are a strong and viable repair. They demonstrate well in the lab and other situations where assembly is well controlled. Their nature of "no inherent structural form" is much more demanding of this than are threads in parent metal.

Working during assembly and disassembly with slight offsets, misalignments, dirt, dings in the bolt, etc can easily damage the 1st thread or two, or cause binding at some point down in the hole to spin and unseat the threadform. The repaired hole is then in jeopardy and very likely to surprise you later - maybe trying to go out or in with the bolt. The helicoil needs replacement and youre likely working thru a hole that is too small to replace it thru. A conventional bolting situation into parent metal is more durable, tolerant of much greater damage, and easier to correct promptly [often by just firmly enforcing the original correct lead in with an undamaged hi grade bolt lubed with moly].

Preserving this real world advantage, even greatly widening it is often easier than you might think. Making use of pitch crossovers within or between sizes in the US or metric, or the US and metric systems can allow this. Just go to the next bigger bolt with same pitch. Between system conversions match ~ exactly at 20tpi [1.25], 16 [1.5], 10 [2.5]. ... So you can find cases that allow you to pilot into the original helix with a slightly larger std tap. Some are; #] 1/2 x 20 into 12 x 1.25M, #] 7/6 x 20 into 10 x 1.25, #] 3/4 x10 into 18 x 2.5M, #]12 x1.25M into 7/16 x 20, #] 10 x 1.5 into 3/8 x 16 ...

Going larger, and hence more rigid is good as a rule in the service we are dealing with here. Some cases, such as where thermal expansion affects preload, you do not want to force rigidity in place of compliance offered by the original bolt size. Here, use of a helicoil into a usually softer more mobile metal like aluminum preserves compliance and extends the threadform to engage enuf of the parent metal for good strength. If helicoils ever can be universally deemed good it is in this type of application involving weaker metals.
larry

 

[SPYDERLK]

Now the choice is whether to use a new tap and carefully chase out the damaged thread. As to how to do the thread restoration.....
I've actually had better luck restoring damaged threads by not using a commercial tap at all - commercial taps are too sharp with too much cut and not enough thread support. Too easy to get them started wrong and they will cut regardless of right or wrong.

What I do is better and costs less. Take a hardened bolt of the right thread and size and cut half a dozen slots with a hacksaw perpendicular to the threads. Cut the slots about an inch long and somewhat deeper than the minor thread diameter. Now carefully taper the bolt for the first couple of threads. If you want, I can probably find an old bolt I've done this to and post a picture. This becomes my handmade thread restoration tap. It is way better for the purpose than a tap. Start it straight, go very slow with lots of light lube, and the real trick is to back it out to clean the particles with a hand wire brush quite often. Sometimes I back out every 1/8 turn I gain....or even more often. Constantly cleaning the particles out of the hand-made restoration tap is critical to doing this job. Each time you back the hand-made tap out, Use WD-40 and a nozzle to foam out the particles left behind in the thread bore.

When done, try to thread in the longer bolts you bought - best source for those might be at McMaster. With luck, there will be enough good thread to hold the torque. That's not quite as good as new, but I can't think of a better solution. Be careful not to use too long of a bolt. You'll probably have to cut them to special length. Use hardened washers. Measure to leave a little free space at the end. You sure don't want to go too deep and crack the casting.

What a pisser. Yes, the original bolts were more than likely poorly installed. This is as good as I can come up with. Don't give up on the threads you have until you've tried every way to get them into use again. Anyone else?
luck, rScotty

Very good suggestions for preserving damaged but salvageable threads

 

[rbargeron]

A machine shop can do the heli-coils for you if you feel it's more than a job you want to tackle. They would most likely have a size that could be used and just charge you for the labor and the inserts vs you buying the drill, special tap, and the insert tool. If there is a reason why they can't be used in this application I would like to know as I'm not an expert.

The reamer and tap for preparing a hole to receive a helicoil is several inches long and is generally not suitable for blind holes. A common application for helicoils is renewing spark plug threads, which are through-holes. A sizing reamer first pilots on what's left of the old threads, and reams out the hole. The next section of the tool taps the new threads using the end of the reamer flutes to guide and center the tap. All of this takes a tool that for a 16-19mm bolt size would be about 4" or 5" long, too much for a blind hole that's likely only 2-1/2" deep, if that.

Here's a slightly different idea that builds on other posts here:

To take full advantage of the existing holes, the threads can usually be extended somewhat. This is because the original threads are not fully formed for the full depth of the hole. Taps are tapered so in blind holes they leave a full diameter or more of incomplete threads. Machinist have a special tool called a "bottoming tap" that can be turned into a threaded hole to finish the incomplete threads at the bottom of the hole. Then a full length bolt can use all the threads, helping solve the problem. The bottoming tap may need to be guided and/or carefully advanced, but it will work much better than trying to oversize the whole thing from scratch. Best to have a machinist look at this with you.

Another thing I'd mention - be real sure of the diameter and thread pitch of any taps and/or new bolts you use. Take a bolt out that hasn't given any problems and carefully compare the size and thread pitch. Some bolts on my tractors are 1.25 mm pitch, others are 1.5 mm, even though they are the same diameter. They look the same at first glance and can create a real mess if mixed up. You might even find that someone has forced a mismatched bolt into a hole already.