Cylinder bore and JSM lightweight pistons

First off I’d like to say that what I appreciate the most is how much smoother my Commando runs with the lighter JSM pistons. The reduction in reciprocating weight really makes a difference. I do a lot of freeway cruising at 4000 RPM and comfort and reliability are most important to me. I decided to fix a weeping case oil leak and so I’m taking a close look at the bore. I have put 30,000 miles on these cylinders. I had them measured and they were found to have one thousands of an inch wear just below the top. You can still see the hone scratches from when the cylinder was bored. I removed some carbon at the top of the cylinder so I could feel for a ring ridge. Its barely noticeable.





The short lightweight pistons from Jim Schmidt show some use that you can see on the skirts but it was measured at less than one thousands of an inch and I’m going to keep using them and install new rings. I'll probably rough up the bores a little by hand with some emery cloth so the new rings will break in. I’d like to see 50K miles on this motor before doing a complete rebuild.

You will get more than 50,000 mile out of your motor, there is not much wear in your bores and your botton end will last well over 100,000 miles if you run good oil and keep up with your oil changes, I know as mine did and I am not running Jims lightweight pistons but do have a balanced botton end.

Ashley

cyclepsycho said:

I have put 30,000 miles on these cylinders. I had them measured and they were found to have one thousands of an inch wear just below the top.

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Great information, I had asked about mileage on these parts but there had been no real answers until now. (That I had noticed)
That is 30000 miles on a fresh rebore and the JSM pistons ?

Thanx for showing us the good stuff lasting details you get to enjoy so much.
I just had my long suffering mower twin Bore Tech Ni Si Carbide coated and Peel's Maney bores done too to add some endurance on top of now proven good life time of JSM elite piston - pin- rings - rod kit. If Bore Tech don't have to do the bores each bore costs ~ $165. Swain and others offer ceramic top coat & chamber-valves plus anit-friction piston side layer for like another $200 per jug. I had the pistons & rings cryo tempered too. I bought a bore gage to check mower and Peel over time.

cyclepsycho said:

I'll probably rough up the bores a little by hand with some emery cloth so the new rings will break in. I’d like to see 50K miles on this motor before doing a complete rebuild.

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Those bores look just fine and certainly acceptable to new rings. Please don't artificially wear them by hand scrubbing with emery. Its just not necessary. You'll get another 20,000 miles out of them no problem with careful installation.

All goodly news re:longevity. Please do not use emery. :|

What piston to bore clearance did you start with?

Those bores look just fine and certainly acceptable to new rings. Please don't artificially wear them by hand scrubbing with emery. Its just not necessary. You'll get another 20,000 miles out of them no problem with careful installation

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Amen don't mess with bores. I'd Bore Tech em now. What is known to work best is dry install with a slight abrasive power dusted in with the new rings. This from TotalSeal tech phone call on what powder they sell for this. I use Bon Ami and even put a dash in initial break in oil. Rings should seat in a min or less smokeless when done this way. Seen em seal up 3x's now smokless in about 50 sec. Then shut down to cool bore ridges so don't melt gall over - double check stuff then do it again a couple more min to make sure. I never idle a new engine but try to get instantly to cam surf rpm but not a whole lot higher till time to change oil and take a spunky ride like I stole it. These are not Jim Comnoz level dyno pull break in sessions till 100-ish miles later next oil change.

What was the ring gaps on removal?
Could ya tell if rings sharp corners rounded off?

hobot said:

I use Bon Ami and even put a dash in initial break in oil.

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Abrasive powder to run the shell bearings in no doubt.
Some confusion perhaps regarding Caterpillar equipment, chrome rings, rebadged Ajax tins and Norton Commando's.

Ajax doesn't work any more,since they changed to no-scratch formula about the mid-'90s.
I use a mixture of BON-AMI and liquid detergent to bed rings.Use a dummy conrod to push the pistons through about 40 strokes and then inspect the ring seating.Wash off the barrel and pistons/rings with water,without removing the rings from the piston.

Then take everything to the engine and assemble it.Minimum oil on cylinders (as much wiped off as possible),dry rings/ring grooves and a drop of oil smeared front and rear of the piston skirts.

OK, I'll skip the emery cloth. Pistons were originally installed at the recommended clearance of .0055” on a fresh bore. I’m happy with this rod and piston combination. The smooth running is very nice. And it runs crisper than with the OEM parts. What else should I look for while its apart? Everything seems ok.

cyclepsycho said:

OK, I'll skip the emery cloth. Pistons were originally installed at the recommended clearance of .0055” on a fresh bore. I’m happy with this rod and piston combination. The smooth running is very nice. And it runs crisper than with the OEM parts. What else should I look for while its apart? Everything seems ok.

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That's nice to hear. With the head off and valves installed it worthwhile doing a valve seat leak test. Put the head exhaust side down so the inlet ports face upward. Get an egg cup of fuel and tip it into each port so the valve head is submerged. Look at the combustion chambers and see if any fuel is weeping past the valve seats. If nothing happens after 30 seconds, no more than a minute, then they are good as is, otherwise pop them and re-face. Do the same for the exhausts.

If you check the existing rings End Gap in the Bores , youll find what wear the Old Rings have - if any .

Whatever , remember to check the NEW ring gaps , before nailing it up .

Abrasives in the Injun . ?
Some halfwits recomended AJAX to a VW type , on a fresh tight motor .
After pouring a packet through :x the clearances were certainly opened out .
400.000 miles worth iin two minutes . As its not made in Bangladesh on grindstones
with worn files - stay clear of ABRASIVES . Brasso in the fuels another ! running in fuel .
Long term result is running out fuel . The abrassives become imbedded .

Moden Machineing ( after the war ) is generally suffuciently accurate
that pouring minature rocks throgh it is not recomended .
Thats why Air Filters are fitted , along with oil filters . To stop all that nonsense .

The ' Bust the Glaze ' trick is just that . if its not GLAZED . . . it dont need busting .
maye 600 to 1200 ( actually 800s maybe good ) wet and dry , 30 deg. cross hatch .

Check the EXISTING rings for clearance / float in lands , and end gap . please . :|

Duh dudes Ajax and such never worked inside running engines, just the non abrasive "never scratched anything" plastic lens type polishing compound like good ole Bon Ami. Alex I never heard of detergent and Ajax but at the light loads/no heat of hand stroking - actually makes appealing sense, if barrel already off. Another hick way to check seats is use finger or spring pressure to hold valve closed and fill up the port side with thin heated fluid, or push down under water trough with trapped port air to bubble or not. I had Peels lower cooler bore opened ~.001" more that upper bore to fudge a bit more heating. Jim Schimdt say Hang em out to cool - if ya got em.

Let us know when 50,000 miles come around on the pistons.

I got a bore gage now and have learned how to use it just to check into Peel's bores rather often at first as I increase the heat and pressure and rpm and kind of do expect 50K miles out of it to get some payback over many years. Thanx for helps our dreams come true Jim.