J Model Piston/Cylinder Setup

[Tommo]

1.5 thou clearance per inch of bore size is how I set up most of my old air cooled motors.
Start them and allow them to warm up while you put your gear on and away you go.
Bedford pistons do have provision for a ring below the wrist pin (RBP ring below pin) like some of the original HD ones had and if a client was worried about piston slap noise I fitted a ring here and it steadied the piston up and eliminated the slap.
I preferred not to do this but some clients insisted and as a result you had to open up the oil pump quite a bit to compensate for the lack of oil on the bore.
I must say most of these engines were for bikes that have done very few miles since their restoration and are only started to impress those viewing the machine. Show ponies.

[Steve Swan]

Tommo, thank you for more great info, i did a google search for Bedford pistons and don't see a manufacturer website by the name Bedford, two or three vendors with limited Bedford piston collection. Do you have a web address or a specific resource for Bedford pistons?

just so we're talking the same thing, the bore of a 74 ci model is 3.437" x .0015" = .0051555" or 5 thousandths of an inch skirt/wall clearance.

[RichO]

Steve, I just ran the top two compression rings, cast iron of course, and nothing in the third ring land. My friends Dave Kafton and George Hood have ton's of J model motor builds between them and total loss works. Victor on his '14 and young Loewen on his dad's '26 J had perfect scores on this years Cannonball. Both out of Dave's.

[Tommo]

Sorry Steve I should have said 61's.
Very few guys ride JD's 74,s here in NZ, we find the 61 J a much more pleasant bike to ride and they go just as well as a JD.
It's only at the top end that the JD wins out

[Steve Swan]

Steve, I just ran the top two compression rings, cast iron of course, and nothing in the third ring land. My friends Dave Kafton and George Hood have ton's of J model motor builds between them and total loss works. Victor on his '14 and young Loewen on his dad's '26 J had perfect scores on this years Cannonball. Both out of Dave's.

Rich, i'm pretty much 100% certain the forged Venolia's in my '27 probably have a chrome ring, or the like. Where would one begin in the search to source cast iron rings to fit the Venolia's? and why could one not use the rings the Venolia's came with assuming they are most likely chrome and not cast iron?

[Steve Swan]

Sorry Steve I should have said 61's.
Very few guys ride JD's 74,s here in NZ, we find the 61 J a much more pleasant bike to ride and they go just as well as a JD.
It's only at the top end that the JD wins out

i'm always glad to hear from you Tommo! using the same simple formula for the 61 ci, 3.312" x .015" = .004968" basically .005 is the skirt wall clearance for the 74 and 61.

i imagine the 61 is a bit smoother overall machine.

[sswaney]

"I am making three different observations onset ups described so far: 1. Find a suitable compression ring where the oil expander ring normally fits. 2. Do not fit a ring where the expander ring goes. 3. With or without an oil control ring; either works fine."

Steve, I think you could add a fouth option, I have doubled up two 3/32" rings in the 3/16" oil land.
As for a ring source, I have had good luck with Hastings. They have a good website. But I don't think you'll find a 3/16"compression ring on there. It's not something they produce any longer.
https://www.hastingspistonrings.com/

And to answer your previous question, I'm no longer in Nebraska,
I'm familiar with the Hardington area. Tampa Bay area now.

[RichO]

Steve, As the gentleman above stated, I to have used Hastings in everything I've ridden except for an Evo. They like those micro seal things. I would NEVER run a chrome ring. They just never seal and are a cause of over oiling an upper end even in Knuckles and such. Cases I know of to numerous to mention. Cast iron guides and rings won't let you down. Cam ground alloy pistons and your good to go in anything of some age.

[Steve Swan]

Rich and sswaney, thank you for for your replies! i am going to copy and past this info to my email files!

[Tommo]

If you don't put something in the groove the oil ring usually goes into you will get a fair amount of piston slap and the bore will wear a lot quicker.
The higher the com height of a piston (The distance between the piston crown and the gudgeon pin) the greater the need to support the piston to avoid piston slap.
Thus in a lot of these old engines the piston skirt was very long when compared to modern pistons and this was done in an effort to eliminate piston noise.
That's the reason some pistons have a ring below the gudgeon pin to further support the piston in the manufacturers effort to control piston rock.
If you look at a modern say Ducati piston the skirt is very short but the gudgeon pin is right up under the piston crown and therefore reduces the possibility of piston rock/slap.
So what am I saying, I'm saying yes you need a ring or 2 rings in the oil groove to support the piston.
Here in NZ we have a manufacturer who will make any width ring we require so I use one ring but it is just as good to use two rings to fill the gap.
Hope this helps

[Steve Swan]

Thanks Tommo!

[Steve Swan]

one more question.... for now. The ring that goes in bottom gap or the so-called "oil ring" where one uses a suitable compression ring (or 2 rings), is this ring(s) also cast iron like the top 2 rings?

[RichO]

Yes, it should be.