Top of skirt vs bottom of skirt?

As this question seemed to get lost in the other thread I will repost it here.

Honing for piston clearance is about where I am at on a project engine I am building and having been doing some research.
One of my references is Modern Motorcycle Mechanics by J.B. Nicholson.
The clearance stated there for Combat Commando 750's with solid skirt is .0065" (top of skirt) and .0045" (bottom of skirt).
750's with thermal slotted pistons are listed as solid skirt is .005" (top of skirt) and .0035" (bottom of skirt)
Similar for an 850 with solid cast pistons. .0065" (top of skirt) and .0045" (bottom of skirt).

What is the reference (bottom of skirt) about and how does it work with the clearance called top of skirt?
Would this not cause a taper in the bore?

This is what I think is being described. This is a 850 that was bored for 40 over pistons that later seized up.
These are new 40 over JCC pistons and how they fit. Right now a very snug .004 at bottom of cylinder and a looser .005 at the top.

or
http://s225.photobucket.com/albums/dd50 ... ter=images
These feeler gauges are just under a 1/2" wide and conform nicely to the bore. As this method also involved using a spring gauge to pull the feeler, hence the hole, the non conformity of the gauge in relationship to the clearance would be calculated in when the specification was developed I would think. My guess is most folks who use this method go by feel and experience and not the spring gauge. This is after all all 1930 to 1950 tech.

Is my understanding correct?

Thanks

As only the piston maker knows the alloy used and production method eg cast or forged only the piston maker can tell you the clearances needed for their pistons. The dealer who sold you the pistons should have access to this info. The taper is in the piston not in the bore,

(/) Piston Taper.
To compensate for the unequal expansion due to maximum temperature
at crown and minimum at the bottom of the skirt, the piston is tapered along its axis, the crown
end having the smallest diameter (Fig. 3.82C). The degree of taper varies along the piston length,
greatest being in the ring-belt. The clearances between the piston skirt and the cylinder wall is
about 0.05% and 0.10% respectively at the bottom and top of the skirt.

http://www.the-crankshaft.info/2009/09/piston.html

so just use the 0.005" clearance between the widest point on the piston and cylinder and let the piston take care of the rest?
much easier.

Yep, googling JCC gives 5 thou, check front to back and side to side.

Hi

I have just put my bike back on the road 4 months now and when I did my reboar 850 Comando motor 40 thou over size Hepililte pistons, I only did 2 thou on the final hone, the motor was very tight but if you give the first run at night when its a lot cooler and take it real easy for the first 100 miles it will be OK, I have done 3,000 miles on it now and haven't had no troubles with the motor and the motor is still tight but feels good when running, I did this to my old Truimph many years ago when I did the reboar on it only went 2 thou on the final hone and it is still going after 70,000 miles and is stil a strong motor...

It all depends how you look after it when running it in, just take it easy for the first 200 miles...

Ashley

JCC pistons for Nortons are cast, so you would expect to use similar clearance to the original cast Hepolites. Norton's workshop manual gives the original tolerance for 850 piston clearance at the bottom of the skirt as .0020" - .0029". Aftermarket service manuals will give you different specs. Clymer says to use .0035" - .0040". I have a digital workshop manual from an unknown publisher that says to use .0045". All are measured at the bottom of the skirt, which is how almost all modern piston clearances are specified.

When I was racing Commandos using standard cast Hepolite pistons, I honed the cylinders to .0040" - .0045" for both 750 and 850 engines, and never had any problems. For a street bike that is going to broken in more carefully than most race bikes, anad where you would like the rings to last longer than a race season, you could certainly use less clearance.

I'm not sure how much I would trust the info from JCC's web site. They list the 750 Norton as having two cylinders, and the 850 as having four.

Ken

lcrken said:

JCC pistons for Nortons are cast, so you would expect to use similar clearance to the original cast Hepolites. Norton's workshop manual gives the original tolerance for 850 piston clearance at the bottom of the skirt as .0020" - .0029". Aftermarket service manuals will give you different specs. Clymer says to use .0035" - .0040". I have a digital workshop manual from an unknown publisher that says to use .0045". All are measured at the bottom of the skirt, which is how almost all modern piston clearances are specified.

When I was racing Commandos using standard cast Hepolite pistons, I honed the cylinders to .0040" - .0045" for both 750 and 850 engines, and never had any problems. For a street bike that is going to broken in more carefully than most race bikes, anad where you would like the rings to last longer than a race season, you could certainly use less clearance.

I'm not sure how much I would trust the info from JCC's web site. They list the 750 Norton as having two cylinders, and the 850 as having four.

Ken

Click to expand...

Smallest bore 3.0315
largest piston 3.0280
min clearance 0.0035
be carefull using feller gauge as you can distort the measurement easy, it is really only a very rough check.The safest way is to take the min bore size ,add the oversize, check that the clearance falls with in the spec , and bore to that size.