Metric threads

[Kedge]

Hi folks, new kid here, recently picked up a 1961 ca72 (date on wiring harness so may be a 62?). Any way, I had a 65 ca77 back in high school and thought this might be an interesting restoration project in my retirement years. My question is regarding thread pitch. I have bought the JIS screw driver bits, and have read here that the small screws used on our machine have a thread pitch slightly differing from today's metric standards. Does this JIS standard also apply to all the threaded fasteners on the bike? Thanks

[mrhonda-hawaii]

Honda used JIS thread pitch fasteners until about 1967. Honda TSB bulletin concerning the changes:

CHANGE OF THREADED PARTS
In an effort to obtain more universal uniformity of threaded parts, the JIS (Japan Industrial
Standards) threaded parts standards have been modified to conform with those established by ISO
(International Standards Organization). All Japanese industry, including Honda Motor Co., is
cooperating in this change. Although the modifications of the present JIS system are not
extensive, this change will require some new tools, different tool applications, and will introduce
the possibility of non-interchangeability of similar parts.
This changeover will be effective from the start of production of all new motorcycle models
first produced after the beginning of 1967. In all other models, those previously in
production, the changeover will be made gradually and on no set schedule.
The following will describe the modifications and the effect they will have at the dealer level:
1. Changes in Thread Form
The minor diameter of internal threads will be decreased slightly in all sizes. This change is very
minor and will not affect interchangeability in any way.
2. Height of Bolt Heads
There will be minor changes to the height (thickness) of bolt heads in several sizes. These
changes will not affect interchangeability.
3. Thickness of Nuts
The thickness of both common and thin nuts will be reduced in sizes 8mm and smaller. These
changes will not affect interchangeability.
4. Width Across Flats of Bolts and Nuts
The width across the flats of the hex (the wrench size) is changed for all nuts and bolts except
6mm.
NOMINAL DIAMETER WIDTH ACROSS FLATS (WRENCH SIZE)
(SIZE OF BOLT OR NUT) PRESENT JIS MODIFIED JIS (ISO)
3 mm 6 mm 5.5 mm
4 mm 8 mm 7 mm
5 mm 9 mm 8 mm
6mm 10mm 10mm
8 mm 14 mm 12 mm
10mm 17mm 14mm
12mm 19mm 17mm
14mm 21mm 19mm
16mm 23mm 22mm
18mm 26mm 24mm
20 mm 29 mm 27 mm
New wrenches in the 5.5 mm, 7 mm, 12 mm, 22 mm, 24 mm and 27 mm sizes will be required
for the modified bolts and nuts. All other sizes are currently in use. This change in hex size
affects only the wrench sizes; the interchangeability of bolts and nuts is not affected.
5. Changes in Thread Pitch
The standard thread pitches for all bolts, nuts, and screws in the 3 mm, 4 mm, 5 mm and 12 mm
sizes are changed as indicated below:
NOMINAL DIAMETER THREAD PITCH
(SIZE OF BOLT OR NUT) PRESENT JIS MODIFIED JIS (ISO)
3mm 0.6 0.5
4mm 0.75 0.7
5mm 0.9 0.8
12mm 1.5 1.25
These changes will affect the interchangeability of threaded parts in these sizes. Care must be
taken to use the correct pitch to avoid stripping. These changes will also require new taps and
dies. New tools, where required, will be available through parts department. To help avoid
confusion in the 3 mm, 4 mm, 5 mm and 12 mm size, wherein the thread pitch has been changed,
the modified JIS (ISO) threaded parts used by Honda will be marked such that they can be readily
identified. The marks used are as follows:
PART TYPE DESCRIPTION OF MARKING
Standard Hex Bolt A number, indicating the strength of the bolt, will be
embossed on the top of the bolt head. This mark will appear on modified JIS (ISO) bolts of all
sizes; but as explained above, all sizes except 4 mm, 5 mm, and 12 mm (there are no 3 mm hex
bolts) will interchange with the present JIS parts.
Slotted Hex Bolt There will be a hemispherical embossment (bump) on the top of the bolt
head in the 4 mm and 5 mm sizes (3 mm and 12 mm slotted hex bolts are not in use).
Cap Bolt There will be a hemispherical dimple on the side of the bolt head in the 4 mm, 5
mm and 12 mm sizes (3 mm not in use).
Stud Bolt There will be a hemispherical dimple or embossment on the nut end in the 5 mm
and 12 mm sizes (3 mm and 4 mm studs are not in use).
All Cross Screws There will be a hemispherical embossment on the top of the
Recess (Phillips) screw head in the 3 mm, 4 mm and 5mm sizes (12 mm not used).
All Nuts There will be a hemispherical dimple on either the top or side of the nut in all
sizes with changed pitch. This will also apply to nuts that are permanently welded to other parts.
All mechanics and parts department personnel should read and understand this bulletin,
to avoid confusion and damage to threaded parts when the changeover occurs. Additional
information relative to these changes (part numbers, tool numbers, etc.) will be published by the
parts department.
AMERICAN HONDA MOTOR CO., INC.
MOTORCYCLE SERVICE DEPARTMENT
Source: Honda Service Letter #74 6/8/67
Data supplied courtesy of American Honda Motors for use in my Honda Restoration Guide CDs for all
models of Honda 250-305cc twins. Details apply to all models from that era.
Bill Silver (aka MrHonda)
www.vintagehonda.com

[oldbikedude]

Thanks Bill. Lots of good info there.

[deuce_7]

JIS 3mm diameter and 5mm diameter cross-slot screws are commonly used on your CA72. These JIS screw sizes are a different, coarser thread pitch than ISO fasteners used on later Honda models after 1967. (6mm, 8mm, 10mm screw thread sizes are not affected and remain the same thread pitch before and after 1967).

For example, the RH and LH handlebar control clamp screws on a CA72 will be JIS 5mm diameter with a 0.9 thread pitch. CA72 petcock mounting screws are also 5mm-0.9. ISO 5mm screws have a 0.8 thread pitch (5mm-0.8) and will not work. They will damage threads if forced.

JIS 3mm-0.6 pitch screws are used on your CA72 for mounting tank badges and some other small applications. Likewise, ISO 3mm-0.5 pitch screws will not work.

New JIS fasteners are available from specialty fastener suppliers, with some searching. Usually they are not available in hardware, auto parts and motorcycle shop stores.

[Michael Stoic]

Pay attention folks! You're getting personal attention from some of the best informed people in this field!

--
Michael

[Muddy]

It certainly is an honour and privilege to have Mr Honda, Bill Silver, posting on our forum. Bill and Ed Moore are known experts on our range of motorcycles. Bill used to produce an invaluable package of information on the Honda 305 & 250 C(A)77 & C(A)72.

Welcome along Bill.