Kubota Standard Bolt Grades

[Junkman302]

There are a lot of general purpose bolts on every Kubota tractor, but I cannot find much information on them. There are no grade markings on many of them that make any sense. For example, on my B7200, there are a lot of M10x1.25 bolts holding controls, accessory mounting brackets, etc. on. Most of these have a "7" stamped on top. There are also M8x1.25 bolts that have a "4" stamped on them. You can look any of these bolts up on a Kubota parts diagram, and it will reveal the Kubota part number, but no other information, not even size in many cases. The torque tables in the manuals cover critical fasteners like rod and main bolts, but I can't find any torquing info on standard bolts.

The reason I ask is that I'm a bit obsessive about torque specs, even on general purpose fasteners. Since I can't really be sure what grade a lot of these fasteners are, I don't know what I can safely torque them to.

Does anyone know anything about Kubota Standard Bolt Grades and Torque values. Should I just assume they're grade 8.8 and take my chances?

 

[Hugo Habicht]

Hello,

very good question, I would like to know myself if 7 for example corresponds to 8.8 10.9 or not.

Attached is a table from the operators manual of my G1900. It has the tightening torques of the bolts by diameter and.number stamped on it. It would not surprise me if Kubota did their own thing, they for example had smaller head size years before the ISO standard changed.

Kind regards,

 

[Junkman302]

Your attached photo indicates that "Figure 7 on top of the bolt indicates that the bolt is of special material," with the accordingly higher torque specs.

I did find a page in my WSM regarding "Tightening torques for general use screws, bolts and nuts."

It provides torque specs for three "grades" of bolts, "no grade", "7T", and "9T". (No distinction between thread pitches tho)

From both these pages, it is clear that the "7" stamped on top of the M10 bolts indicates a higher grade, 7T (quenched and tempered), and the accordingly higher torque specs. That tells me pretty closely what I need to know about identifying and torquing these bolts.

It would be nice to know whether that grade 7 corresponds to 8.8 10.9 or something else.
And what of the "4" stamped on top of the M8 bolts?

 

[Hugo Habicht]

As far as I know 4 and nothing on the head is the same, standard strength.

With respect to the ISO strength you have to compare the tightening torques and see if it matches. To be honest, I simply use the Kubota table for the torque setting.

 

[Russell King]

Here is what I can tell you from researching this a few years ago so I am relying on some memory.

In the USA there are three common bolt grades (2, 5, 8). Soft, medium and high(er) strength. There are high strength bolts above these and these are automotive grades. Then there are structural grades. Grade 2 have pretty much fallen out of use in mechanical design but are sold in hardware stores. They have no head marks (unless it shows the manufacturer mark). The grade 5 bolts (three radial marks) used extensively, and then grade 8 (five radial marks) used as needed for strength/safety. See this link for more information

Bolt Head Markings and What They Mean - Wilson-Garner

If you’ve ever wondered what bolt head markings mean, or if there is some over-arching logic to them, then you are in the right place.

wilsongarner.com

In the Metric world the use a different grading system that still has three grades but those show the actual strength of the bolt (but still a little confusing to those that don’t think in SI units).
See this link

Understanding Metric Bolt & Screw Grades — & Head Markings!

Interested in what metric bolt and screw head markings mean? It has to do with their grades. We clarify with definitions, examples, and photos in this article.

wilsongarner.com

BUT none of those would show the “fancy” 7 or 9 that you will see on Kubota bolts (and on Japanese cars). From my research those head markings were used by the Japanese Auto industry to provide the torque values to the workers. They had tools that were similarly marked and set for the 7 and different tools for the 9. So same size and potentially strength bolt could be torqued to different values of torque. But others were saying it was a Japanese strength standard that is no longer in effect. Here is a link to some charts and discussion on this subject

https://www.suzuki-forums.com/threads/bolt-designation-numbers-on-the-front-subframe-in-this-case.271672/

Metric bolt head markings

Suzuki, at least as recently as 2009, has provided a table of standard bolt torques in their motorcycle service manuals that refer to markings on the heads of metric bolts. Two values are listed for each bolt size. The first is for conventional (unmarked) or "4" marked bolts and the second for...

www.eng-tips.com

Bolt grade markings, 4 7 9 what are they equivalent to?

www.patrol4x4.com

 

[MOOTS]

I give random bolts 2 solid uggas and half of a dugga with my 3/8 rattle gun.

 

[TheOldHokie]

As far as I know 4 and nothing on the head is the same, standard strength.

With respect to the ISO strength you have to compare the tightening torques and see if it matches. To be honest, I simply use the Kubota table for the torque setting.

The G2160 WSM had a great crossreference tablr. IIRC

Kubota 7 = ISO 8.8
Kubota 9 = ISO 10.9

 

[TheOldHokie]

I give random bolts 2 solid uggas and half of a dugga with my 3/8 rattle gun.

That of course is the tried and proven German engineered GootenTite specification.

Dan

 

[MOOTS]

That of course is the tried and proven German engineered GootenTite specification.

Dan

Exactly!

 

[Junkman302]

I'm just trying to find some method better than the old method of tightening bolts up to one quarter turn before breaking.

Thanks for the responses.

 

[TheOldHokie]

I'm just trying to find some method better than the old method of tightening bolts up to one quarter turn before breaking.

Thanks for the responses.

You are way overthinking an ordinary everyday activity.

Dan

 

[Hugo Habicht]

I'm just trying to find some method better than the old method of tightening bolts up to one quarter turn before breaking.

I cannot follow you here. You have a table with the torque to be aplied for each bolt diameter and strength. You simply set your (quality) torque wrench accordingly. You do not need to know if the equivalent is 8.8 10.9 or 12.9. Or am I missing something?

 

[PoTreeBoy]

I'm just trying to find some method better than the old method of tightening bolts up to one quarter turn before breaking.

Thanks for the responses.

That's better than quarter turn too far, which I occasionally resort to.

 

[imarobot]

Tighten until it starts to break, then back off a half a turn