[nbrown77]

I've got a 77 XR75 that has a piston marked with a "1" on the right side and a "153" on the left. I thought those numbers meant it was a STD or stock piston size. The Clymers manual tells me the piston skirt diameter should be between 46.97-46.99. With a max wear of 46.90. But I measure 47.95 on the piston skirt with a vernier caliper. I'll be getting a micrometer and bore gauge this week but am I missing something? Is this engine already bored over or is it stock?

 

[sgtbilco]

bore size for 5 speed xrs

I've got a 77 XR75 that has a piston marked with a "1" on the right side and a "153" on the left. I thought those numbers meant it was a STD or stock piston size. The Clymers manual tells me the piston skirt diameter should be between 46.97-46.99. With a max wear of 46.90. But I measure 47.95 on the piston skirt with a vernier caliper. I'll be getting a micrometer and bore gauge this week but am I missing something? Is this engine already bored over or is it stock?

View attachment 173685 View attachment 173686 View attachment 173687

check the bore size with your veriers std bore is 48mm for 1977 xr
4 speed and 5 speed motors have different bore sizes
also check engine # it will tell you the year of manufacturer of engine
the 7th digit of the engine number is the year of manufacturer
first xr75 1973 KO seventh digit was a 0
1974 was 1
1975 was 2
and if yours is a 77 should be 4
if not has had a engine change or swap around of parts
hope this helps
cheers

 

[nbrown77]

I appreciate your response. I went to a machine shop yesterday afternoon. They weren't all that familiar with vintage motorcycles, or all that pleasant to speak with, but they did have lots of bore knowledge and micrometers. The guy there believes it has been bored 1 over, which he argues may be the reason for a number 1 on the piston. The 153 number we both think means its just a Honda piston. I checked the engine number like you said and the 7th digit is in fact a four.

I'm still a bit confused. I thought being bored 1 over meant it was bored 10-thousandths (.25mm) of an inch over stock. The book tells me the largest stock piston skirt should measure 46.99mm. Adding .25mm to that just brings me to 47.24mm, not the 47.95 I measure with my caliper. That would be more like 0.96mm difference, which is more like forty-thousandths of an inch over stock size. I'm still lost and apparently have lots yet to learn about boring. I was hoping someone on here had been down this road before.

I'll measure the cylinder with my caliper tonight. I'm still trying to find a bore gauge locally.

 

[sgtbilco]

bore size

I appreciate your response. I went to a machine shop yesterday afternoon. They weren't all that familiar with vintage motorcycles, or all that pleasant to speak with, but they did have lots of bore knowledge and micrometers. The guy there believes it has been bored 1 over, which he argues may be the reason for a number 1 on the piston. The 153 number we both think means its just a Honda piston. I checked the engine number like you said and the 7th digit is in fact a four.

I'm still a bit confused. I thought being bored 1 over meant it was bored 10-thousandths (.25mm) of an inch over stock. The book tells me the largest stock piston skirt should measure 46.99mm. Adding .25mm to that just brings me to 47.24mm, not the 47.95 I measure with my caliper. That would be more like 0.96mm difference, which is more like forty-thousandths of an inch over stock size. I'm still lost and apparently have lots yet to learn about boring. I was hoping someone on here had been down this road before.

I'll measure the cylinder with my caliper tonight. I'm still trying to find a bore gauge locally.

the way it works from imperial to metric
.25mm is .010" 10 thousandths over {first over size}
.50mm is .020" 20 thou over 2nd
.75mm is .030 30 thou over 3rd
1.00mm is .040 40 thou over 4th

measure the top of the barrel and the bottom of the barrel and post the results in mm
that will tell me what the bore size is std or oversize
the old piston could be worn or the skirt may have collapsed in size due to overheating
the bore size is what you work off first then check the piston for clearance this will tell you if you need to re bore or get away with a set of rings these old XRs most of the time will be ok with new set of rings even if the clearances are at there max
hope this helps

 

[piranito]

Have a look what the Honda manual says to '77 XR75

Alex

 

[nbrown77]

That does help. I've got a set of bore gauges coming. They should be here tomorrow by the end of the day. I will post my findings then.

Thanks for the help.

 

[nbrown77]

Piranito: According to the supplement you've posted for me, my piston would be just a little worn. I guess the Clymers manual is wrong? That's irritating...

 

[piranito]

Clymer is not Honda.
Here are some other pics:

 

[nbrown77]

My micrometer came in the mail today but I'm still waiting on the bore gauges. The piston skirt is 1.8875". Based on the very useful supplement you shared earlier, I guess the piston is .0056 within its wear limit. Maybe I'll have to bore over to .25mm? Regardless, I'm waiting to measure the cylinder like sgtbilco suggested.

Thank you for the help. Very much appreciated.

 

[piranito]

As seen from your pics above, maybe a fresh hone and ringjob will do it.

Good luck anyways, Alex

 

[sgtbilco]

bore size

As seen from your pics above, maybe a fresh hone and ringjob will do it.

Good luck anyways, Alex

Alex is right ,as i said earlier these XR engines will freshen up with a hone and ring job OK
even if the wear limits are at there max
if your going to go the way of bore and hone it becomes expensive and you will have to deal with the grumpy machine shop dudes
cheers

 

[nbrown77]

I've got rings on the way. Hopefully my bore gauge will shop up sometime today and I'll post what I find.

 

[nbrown77]

Just a quick followup-

I finally got my bore gauges this weekend. Long story short is the cylinder has an average bore of 48.04mm after being honed. I've got new standard rings on the way. Once the rings get here I'll be assembling the engine back together and building a bench test rig of some sort to break it in. Thanks for the help.