I’m wondering what the experts experience is with piston to cylinder clearance is. In my experience the clearance on air cooled engines needs to be more than on water cooled. On Norton’s we use .004-.005” Does that hold true for these engines?

Yea its called putting it together "loose".
This bike saves you a lot of worry with the ball bearing cranks.
But yea loose - the down side you may use oil a wee bit, but the counter to that is, you wont gall and score and experience sudden death.

Cool.
Srinath.ts called

Armen wrote on 04/28/20 at 11:22:35:


Thanks you guys.

Yeah I thought about the cast cylinder on the Norton after I posted. There is more expansion and contraction as I remember.

Poor Norton and those machines from before the war and the wooden fixtures. 😀 I hate to pick on them because I love my Commando.

I’ve read about the torque plates but don’t plan on doing anything too radical. Will stay pretty stock but clear out the exhaust ring while I have the head off.

Dennis, are you just curious or are you trying to setup your piston to cylinder clearance?

The clearance you choose depends on the piston and the application.  Typically, cast pistons are set up with tighter running clearance than forged pistons.  The cast pistons don't expand as much as forged pistons.

There are other factors that should be considered such as cylinder material (cast iron, aluminum with cast iron liner, aluminum with plasma coating, etc.), but lets stick with the Savage.  The Savage uses an aluminum cylinder with cast iron liner.

Factory clearance on the Savage is .0020".  While the wear limit is .0047", I personally wouldn't run it past .0035", but it will work OK up to .0047".

If you are throwing in a forged piston, you want the clearance a bit looser.  The standard 94mm Wiseco specifies .0025".  

Note that the 97mm Wiseco specifies .0019".  That defies logic.  I'm setting mine up about .0025".

If you are freshening up your top end and are installing new rings, a simple deglaze with a spring or ball hone will be fine as long as you are only deglazing (i.e. not enlarging the bore measurably).  If you are trying to straighten up your bore, or add a touch more clearance for a forged piston, toque plates are mandatory.  Trust me.  This cylinder changes dramatically when you squeeze it with torque plates.

The .004" to .005" you are considering is too much (IMO) if you are looking for good performance and longevity.

Good luck.

BTW. Sweet workshop.  Sorry about the fire.

Maybe now isn't the time, but while I understand the minimum ring gap, but the maximum bewildered me.
When I was doing a Yamaha Raptor top end I bought rings and a piston from the Yamaha dealer. Came home, slipped rings on, slipped it in, nope, too much gap. Return, test, return, test, and every one was outta spec.. And they said
Those are what we use...
But I wouldn't, because the book said they were outta spec. Anyway, the owner had seen me so many times he asked what was going on. He told the guy to order me a matched up piston and ring set. I didn't even Hafta pay the kinda uncomfortable difference.

So, what would be the problem with a bit wider gap? Kiss off some compression?
Would it tend to score the cylinder?

Yeah, I just didn't think I should do much filing, seein as how the gap was already too big. Though I have cut boards that was already a mite short a time er two..
I don't recommend it.

Ring gap is usually set to suit the application.  The gap is there to prevent the ring butts from touching, which will cause the ring to bind in the cylinder, and ultimately rip the top off your piston (like this).

Ring manufacturers generally specify butt gap based on application and bore diameter.  You usually want the second ring to have a little more gap than the top ring.  That prevents blow-by from building up in the space between the top & second ring, which could cause the top ring to lift off the ring land.

For example, high-performance street butt gap .0045" per inch of bore diameter.  The Savage bore is 3.700 inches so gap should be about .017".

Naturally aspirated drag race butt gap .0055" per inch of bore diameter, so gap should be about .020".

Nitrous race only butt gap .0070" per inch of bore diameter, so gap should be about .026".

The more you beat up on the engine the bigger the gap has to be.

It won't hurt much if the gap is too big, but it hurts like hell when its too small.

Keep in mind that the gap is a function of the circumference.  Circumference is 3.14 x diameter.  So, if your bore is .001" larger, the butt gap increases .003".  The piston doesn't have anything at all to do with butt gap.  The gap is solely dependent on bore diameter and ring circumference.  As things heat up, the ring circumference increases a lot more than the bore diameter increases.  The rings are the only avenue for heat transfer from the crown of the piston to the cylinder wall.  Those skinny little rings are moving a ton of BTUs from the piston to the cylinder.  They are one HOT tamale.