Indian piston fit

[exeric]

Up front, I appreciate the knowledge of members who have years of real world experience of vintage motorcycle engine building. That is something that comes with a lot of dedication, and hard work that isn't easily found in old manuals, and isn't something that most present day motorcycle shops would have experience with. I am curious about piston clearance in sidevalve, and IOE cylinders. I'm sure there are many variables, but my question would be more about the application of modern aluminum pistons in cylinders that would have been made for cast iron, or aluminum pistons of the day. I've heard that more liberal clearances are recommended for street use, or the occasional Sunday morning ride. Again, not looking for any dimensions carved in stone, but more of a general opinion about it. Thanks.

[Tom Wilcock]

I have found that my 1940 Indian 4 with over 75,000 miles on the rebuilt engine with modern pistons likes .008"-.010" clearance.
Tom

[chuckthebeatertruck]

Being a few thousandths "too big" is far less hassle than being one thousandth too small.


About the only thing you'll notice running most alloy pistons .005-.010 loose is a bit of noise at start up, sometimes, and a small increase, sometimes, in oil consumption. However, it can be hard to find motor builders or machinists who aren't scared of those numbers.

I am running .0065 loose on modern alloy slippper Pistons in a 90" big twin flatty.

[exeric]

Thank you for the information. I haven't the equipment to do cylinders so I'll be looking for a shop that looks capable. I'll have to give them the tolerance, but like Chuck said, there will probably be a debate

[gharper]

Eric,

I agree with all the above comments, a bit looser is good. A big consideration when comparing modern clearances with those in the book is new vs old piston types. That includes materials as well as bore type and piston shape. Older (teens and earlier?) used a tapered bore and round cast iron pistons. The theory was the cylinder will expand more at the top because of greater mass. That went away and alloy pistons came in which needed greater clearance. Then the cam ground piston came about. So, today any pistons you get commercially are cam ground. Bores today are straight, if your machine shop knows what they are doing!

For reference if you look in the '40's Indian specs they list .004-.0045 for Scout and .0045-.0055 for Chief, depending on what alloy piston used. I think these are a bit tight. For a reference I used .007" clearance in a cast iron bore on my 24 Chief Cannonball motor, with Greer cam ground pistons. This motor runs HOT and worked extremely hard, all across the country, hauling a sidecar and 2 big people. I had no problems at all. For my recent 24 Big X motor I fitted the pistons at .0055-.006" with a Nikasil bore. These are 61" knucklehead pistons and the spec for HD was .001, or .002" which I thought was crazy tight. However...... The OHV pistons are fitted to a cylinder of relatively even mass all around so expansion is even and they run cooler than a flathead or F head because the exhaust is in the head. Flathead and F head motors have a very uneven mass, side to side and have all the exhaust heat in the cylinder as well so need more clearance.

So, I would be fitting your Scout pistons at a minimum of .005". After you have someone hone them, check them yourself, top and bottom!

One thing often overlooked when building the engine is making sure the cylinder base flange is perpendicular to the bore. If it is not, the piston will be cocked at an angle and will wear things out pretty fast and the rings may not seat. You wouldn't think it could be an issue, but it is, especially if the cylinder has had any welding repairs done to it. One of the best tools I purchased is an adjustable mandrel that fits 2.5-3.5" bores. I use this once the bore is finish honed to square the base flange. There can be other ways to set the cylinder up in the lathe but this works easiest if you have a lot of cylinders to do. The first set I did in my small Logan lathe using a large pipe size live center in the tail stock and the center hole on the head in the headstock center. Any way you can get them set up true will work. Once its set up, check for runout on the base flange, it's amazing how far out some are, yet others are very close. Make the lightest cut needed to true the base flange. This will insure the bore is perpendicular to the base mounting flange. Now, if only the case base mounts are parallel to the crankshaft and the rods are straight.........

Truing X base flange.jpg

[T. Cotten]

Picking a number out of the air is conjuring, Eric!

How about a little information, like what model, and how big of an overbore is it?

How thick are the cylinder walls at the thinnest point?

THINBORE.jpg

T-slot pistons, or modern design with an internal stiffener? What does the manufacturer suggest?

Do you have a base stress-plate, or are you just gonna poke 'n hope?

chfpla~1.jpg


....Cotten

[gharper]

By the way, in looking at the above photo it looks like the cylinder is turning pretty fast but it's not! We ran them at the slowest speed possible, in back gear at about 50 RPM.

[exeric]

Great information, Gene, particularly taking a cut to the base flange for perpendicularity. Tom, the cylinders are for my '41 Sport Scout, and they are at standard bore, albeit worn out. I have made stress, torque plates and hope whoever does the work will use them. The pistons are new, + .010" T-slot pistons which came with the project but the pistons are at least 20 years old and probably came from Jim Sutter, Rocky, or Kiwi so that's as much as I know about them. I suspect the cylinders were from a race engine as the cooling fins were relieved for an oil pump mounted magneto, and are at stock bore.

[T. Cotten]

At low overbores, Eric,...

A stress-plate has little effect (unless sleeved).

Service manual spec for clearance seems appropriate.

....Cotten

[Tom Lovejoy]

I am going with .006 on my powerplus build for the pistons, forged aluminum new made. That was what the piston maker suggested and what a machinist/ engine builder agreed with. Very interesting and timely for me Eric, thanks for this post. I have not started on my engine yet, but I am gathering all the parts needed and having things done, cylinders cleaned up good, new flywheels from truet/osborn, I also have what I think are a good original set and my piston's are being made right now.

[PRG]

Howdy Eric,

This a general reply. Can depend on what piston configuration (piston material, 3 ring gas expansion style, 4 ring tension based) used and what RA (roughness average) one can achieve with an accurate rebore and/or hone. 4 ring tension pistons specified for this engine’s oil lubrication format are very high drag and therefore contribute to head generated by these flatheads. These engines run dirty enough already what with .004 unsealed valve guide clearances so didn’t want to contribute to this mess with overly large cylinder bore clearances compromising ring seal. In a traditional 4 ring tension piston, I went with just below .004 with the finest RA I could achieve. That was 11 years ago on this 346 operated in an excessively hot humid southern climate and it’s not whimpered or smoked since.