Can you comment on the cylinder bore measurements on my recently opened up 850

D

DennisMo

VIP MEMBER
Joined
Jun 6, 2023
Messages
375
Country flag
The pistons are marked + 0.040 and are not visibly scuffed or scratched
I set my calipers to 3.072" and then set up my bore gauge.
Here is the outcome with all measurements relative to 3.072"
I would say accuracy is +/- 0.001" (but could be 0.002')

Left Cylinder Right Cylinder

Unworn bore top 0 0
and bottom

Front to back maximum - 8 thou -10 thou
wear in bore

Side to side maximum - 10 thou -15 thou
wear in bore

Maximum top
ring gap 56 thou 53 thou
(about an inch down)

Top ring gap in 20 thou forgot to do!!
unworn bore

Your comments on whether I can't away with a new set of rings or not would be appreciated!

Thanks

Dennis
 
Format did not work. Let me try again


................................... Left Cylinder ...................................Right Cylinder

Unworn bore top .............0 .....................................................0
and bottom

Front to back
Maximum.......................- 8 thou ..........................................-10 thou
wear in bore

Side to side
maximum........................ - 10 thou....................................... -15 thou
wear in bore

Maximum top
ring gap ...........................56 thou............................................ 53 thou
(about an inch down)

Top ring gap in ....................20 thou..................................... forgot to do!!
unworn bore
 
Ate you reading "thou"??
Or tenths??

If your "thou" is correct, it's wildly wiped out.

I'd wager on the latter unit of measure.

Respond please
 
Hi Concours,
Oops. I wrote minus instead of plus Corrected is below.
My bore gauge is metric so I converted the dial reading in 0.01 mm increments
to thou
All relative to 3.072" bore.



................................... Left Cylinder ...................................Right Cylinder

Unworn bore top .............0 .....................................................0
and bottom

Front to back
Maximum....................... +8 thou .......................................... +10 thou
wear in bore

Side to side
maximum........................ +10 thou....................................... +15 thou
wear in bore

Maximum top
ring gap ...........................56 thou............................................ 53 thou
(about an inch down)

Top ring gap in ....................20 thou..................................... forgot to do!!
 
DennisMo said:
Hi Concours,
Oops. I wrote minus instead of plus Corrected is below.
My bore gauge is metric so I converted the dial reading in 0.01 mm increments
to thou
All relative to 3.072" bore.



................................... Left Cylinder ...................................Right Cylinder

Unworn bore top .............0 .....................................................0
and bottom

Front to back
Maximum....................... +8 thou .......................................... +10 thou
wear in bore

Side to side
maximum........................ +10 thou....................................... +15 thou
wear in bore

Maximum top
ring gap ...........................56 thou............................................ 53 thou
(about an inch down)

Top ring gap in ....................20 thou..................................... forgot to do!!
Click to expand...
still way off.

What is your day job? You seem confused with these measuring tools.
 
Hi Concours,

Can you help me out and walk me through this.
It is the first time I have pulled my 850 apart.
What is wrong?
Thanks
Dennis
 
That's by far the worst set of numbers I've seen so I'm guessing you have not measured correctly.

If any portion of the area swept by the rings is more than .005" larger than the unswept area, then you need to bore. Those are massive ring gaps as well. The top ring gap should be between .010" and .012" and you can get away with a little bigger but .056" is ridiculous.

To double check, measure the ring gap near the bottom of the skirt where the rings never touch and in the middle of the bore and report back.

To see if they were bored correctly to start with and as a double check of your measurements, put the pistons in the bottom flush with the edge of the cylinder with the wrist pin crosswise. Measure the cylinder to piston skirt gap at the front. Then move the piston to the center of the bore vertically and measure again.

The shop manual tells you how to measure.

Your bore should be between 3.0715 and 3.0725 for +.040" so I don't understand starting with 3.0720 or how you are measuring.
 
Hi Greg

Thanks so much. I really appreciate your time on this.

I will re-measure tomorrow but what I did today is as follows.

I set my callipers to 3.0720 inches. Then I took my bore gauge, put it between the jaws of my calipers (already set at 3.0720) and turned the bore gauge dial to zero while it was between the calliper jaws.

I then put it into the bore and did all the measurements.

For the left cylinder..
At the very top of the cylinder, where the rings do not wipe, I cleaned well and then I measured and it was basically the same 3.0720 (ie the bore gauge dial did not deviate from zero)

Then I went down the bore (oriented front to back in the bike orientation) looking for the biggest increase and measured 8 thou more. (Probably an inch or so down the bore)
Then I did the same for left to right side (of bike) orientation of the gauge and measured 10 thou.

My bore gauge is metric and when I set it to zero, it says that each division (ie mark on the dial) is 0.01mm so if (just for example) I measured a maximum
deviation down in the bore of 20 divisions from zeroing the bore gauge, this would be 20 times 0.01 mm equals 0.20 mm and it would convert to 8 thou.

I will do it again, report, and I hope that I have made a mistake in the math or the process. I hope I misread the dial divisions or something.......

Lastly, I was not clear but the top ring gap was 56 thou when about an inch plus down in the bore. When I put the top ring in the very top of the bore (where there is no wear,) the gap was 20 thou. This gap should be the 10 thou (you mention) so clearly the ring is worn.

Grateful I am and I will report and I hope to say I made a big mistake....

Dennis
 
The change in ring gap shows 10 thou of bore wear, but that amount of wear on an 850 with oil filter means that bike must have either serious mileage or the air filter is useless.
 
DennisMo said:
Hi Greg

Thanks so much. I really appreciate your time on this.

I will re-measure tomorrow but what I did today is as follows.

I set my callipers to 3.0720 inches. Then I took my bore gauge, put it between the jaws of my calipers (already set at 3.0720) and turned the bore gauge dial to zero while it was between the calliper jaws.

I then put it into the bore and did all the measurements.

For the left cylinder..
At the very top of the cylinder, where the rings do not wipe, I cleaned well and then I measured and it was basically the same 3.0720 (ie the bore gauge dial did not deviate from zero)

Then I went down the bore (oriented front to back in the bike orientation) looking for the biggest increase and measured 8 thou more. (Probably an inch or so down the bore)
Then I did the same for left to right side (of bike) orientation of the gauge and measured 10 thou.

My bore gauge is metric and when I set it to zero, it says that each division (ie mark on the dial) is 0.01mm so if (just for example) I measured a maximum
deviation down in the bore of 20 divisions from zeroing the bore gauge, this would be 20 times 0.01 mm equals 0.20 mm and it would convert to 8 thou.

I will do it again, report, and I hope that I have made a mistake in the math or the process. I hope I misread the dial divisions or something.......

Lastly, I was not clear but the top ring gap was 56 thou when about an inch plus down in the bore. When I put the top ring in the very top of the bore (where there is no wear,) the gap was 20 thou. This gap should be the 10 thou (you mention) so clearly the ring is worn.

Grateful I am and I will report and I hope to say I made a big mistake....

Dennis
Click to expand...
Show a picture of your "bore guage".

If any part of the cylinder is more than .005" (.127mm) larger than the area not swept by the rings, you must bore. .008 does equal .2032mm
 

Similar threads

D
850 Head, cylinders and pistons are out. Can you comment on photos of piston tops and combustion chambers?
Replies
4
Views
103
marshg246
marshg246
Back
Top