Curious tech question about Honda cb450 oil drain plug thread size

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hondaman160mph
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Curious tech question about Honda cb450 oil drain plug thread size

Post by hondaman160mph »

Restoring a cb450 I found the oil drain plug threads in bad shape. Ordered a new drain plug but I would like to clean up the threads in the crankcase
before installing the new drain plug. Measuring the threads and also with some info online from Joker Machine I found the thread size to be
M33.7x1.5. This eliminates the possibility of using a tap to clean up the threads in the crankcase. I'm a curious guy and like to know the what, how,
and why of things, why would Honda make the threads M33.7 and not M33 or M34 like a common tap size?

EMS
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Re: Curious tech question about Honda cb450 oil drain plug thread size

Post by EMS »

One wild guess is, that it is not a standard metric thread, but something like a "NPT" plug, with some self sealing capability.

hondaman160mph
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Re: Curious tech question about Honda cb450 oil drain plug thread size

Post by hondaman160mph »

It screws in like a machine screw and seals with a o-ring
to a flat sealing surface around the opening so it's not
a pipe thread. If it were a standard M33x1.5 instead of
M33.7x1.5 the fix with a tap would be easy. Pulling the
engine and splitting the cases takes it to another level.

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Re: Curious tech question about Honda cb450 oil drain plug thread size

Post by EMS »

The fact is, there is no such thing as a M33.7x1.5 standard thread.

hondaman160mph
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Re: Curious tech question about Honda cb450 oil drain plug thread size

Post by hondaman160mph »

EMS wrote:
Sat Mar 27, 2021 8:01 pm
The fact is, there is no such thing as a M33.7x1.5 standard thread.
That's my point. One would think for manufacturing
Honda would have used a standard M33x1.5 size
thread. It's just a hole that oil drains out of, why would
it need to be .7mm bigger? I find that curious and
confusing. Not to mention much more work to repair.

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Re: Curious tech question about Honda cb450 oil drain plug thread size

Post by Rick Pope »

When your new plug arrives, take a three cornered file and cut some reliefs in the old plug. Use it like a chaser tap and clean up the threads. Old farmer trick.....
Rick Pope
Either garage is too small or we have too many bikes. Or Momma's car needs to go outside.

hondaman160mph
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Re: Curious tech question about Honda cb450 oil drain plug thread size

Post by hondaman160mph »

Rick Pope wrote:
Sun Mar 28, 2021 9:35 pm
When your new plug arrives, take a three cornered file and cut some reliefs in the old plug. Use it like a chaser tap and clean up the threads. Old farmer trick.....
Thanks Rick, great tip. I did that and installed the new
plug. The mystery of why Honda made the hole .7mm
larger continues....

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Re: Curious tech question about Honda cb450 oil drain plug thread size

Post by EMS »

Did you measure this yourself or did the plug come with that size spec?

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Re: Curious tech question about Honda cb450 oil drain plug thread size

Post by Larry Zimmer »

Just curious -- what does the thread of the new plug measure over the thread OD?
Larry Zimmer
cbxlarry@sbcglobal.net

hondaman160mph
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Re: Curious tech question about Honda cb450 oil drain plug thread size

Post by hondaman160mph »

EMS wrote:
Mon Mar 29, 2021 10:34 am
Did you measure this yourself or did the plug come with that size spec?
Both, I measured it at M33.7, a replacement plug from
Joker Machine is listed as M33.7, also contacted them
and verified that measurement. I also bought a M33x1.5
nut and tried it on the replacement plug, doesn't fit.
The drain plug size is definitely M33.7x1.5
One would think there's a reason Honda made it .7mm
larger than a standard size tap, what that reason is I
love to know.

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Re: Curious tech question about Honda cb450 oil drain plug thread size

Post by hondaman160mph »

Larry Zimmer wrote:
Mon Mar 29, 2021 11:05 am
Just curious -- what does the thread of the new plug measure over the thread OD?
I bought 2 plugs from 2 different vendors, both measure
M33.7 thread od.

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