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Message started by Dennisgb on 04/28/20 at 08:08:45

Title: Piston to Cylinder clearance
Post by Dennisgb on 04/28/20 at 08:08:45

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?

Title: Re: Piston to Cylinder clearance
Post by Fast 650 on 04/28/20 at 09:33:05

Depends on how hard you plan to run it too. DBM found out what the minimum ring end gap should be the hard way. If you plan to run it hard stay on the wider side of the clearances.

Title: Re: Piston to Cylinder clearance
Post by srinath on 04/28/20 at 09:42:39

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

Title: Re: Piston to Cylinder clearance
Post by Armen on 04/28/20 at 11:22:35

Remember that Norton ran that loose for two reasons:
a) they have cast iron cylinders which don't grow nearly as much as cast iron liners in aluminum sleeves.
b) those specs came from a time when Norton couldn't bore a hole that was even remotely round and straight. Modern machining has come a long way.
A 70's air cooled BMW (Airhead) was running about .0015" when Norton was doing .004-.005".
I've owned and worked on both Nortons and Airheads.
Follow what the factory Suzi-Q manual says.
Talk to Drag Bike Mike about torque plates if you are going to a serious oversize.

Title: Re: Piston to Cylinder clearance
Post by Dennisgb on 04/28/20 at 12:02:18

5D6E7179721C0 wrote:

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. [ch128512] 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.

Title: Re: Piston to Cylinder clearance
Post by Dennisgb on 04/28/20 at 12:20:40

The Norton and other bikes in my shop before the fire..

Title: Re: Piston to Cylinder clearance
Post by DragBikeMike on 04/28/20 at 16:53:06

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.

Title: Re: Piston to Cylinder clearance
Post by Dennisgb on 04/28/20 at 17:49:12

Thanks [ch128522]

I should have been clearer with my description.

My plan is to use a stock Suzuki piston and cylinder. I purchased a used cylinder that came with a piston. The piston and bore looks to be in good shape, so my plan is to glaze bust the cylinder and put new rings on the piston. I know I could go further on this since I’m going in there but just want this one to be a decent runner with the side car rig.

I was asking the question so I could do some measurements when I get back to where the bike is. I have proper bore and piston measuring tools. Need to check out the bore for wear and the piston/bore clearance to determine if usable or if I should bore and replace the piston.

Title: Re: Piston to Cylinder clearance
Post by justin_o_guy2 on 04/28/20 at 18:58:12

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?

Title: Re: Piston to Cylinder clearance
Post by Dennisgb on 04/28/20 at 19:14:50

Slightly wider gap not the end of the world. Don’t think it would loose much compression. Doubt if it would score cylinder.

It’s usually not too hard to file gaps to spec tho. A good file and a little patience. The problem I’ve had is tiny burrs on the gap edges.

Title: Re: Piston to Cylinder clearance
Post by justin_o_guy2 on 04/28/20 at 19:47:39

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.

Title: Re: Piston to Cylinder clearance
Post by Armen on 04/28/20 at 20:16:52

On BMWs I used to order 1st over rings to use with standard pistons and file to the minimum gap. On my R65 BMW (alloy jugs with iron liners) I set them up with .0008" piston to wall clearance. I had to buy the cylinders a few drinks to get the pistons in :-0
Never had a problem with tight clearances. Ran quiet and cool.
Now I use Nikasil cylinders on the Airheads and don't even stop to think.

Title: Re: Piston to Cylinder clearance
Post by DragBikeMike on 04/29/20 at 00:08:47

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).

Title: Re: Piston to Cylinder clearance
Post by DragBikeMike on 04/29/20 at 00:15:59

Although I had ample butt gap for a stock engine (as I recall I had .020" on the top ring, spec is .012" - .018"), it wasn't enough for what I was doin. Run it on the lean side and you end up with the rings getting so hot that they close up the gap and bind. You can see from this picture how the top of the piston was forced upward, it wasn't detonation, had to be ring binding. Windin it up waaaaay too tight with stock rings and a lean mixture. I needed more butt gap.

Title: Re: Piston to Cylinder clearance
Post by DragBikeMike on 04/29/20 at 00:34:06

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.

Title: Re: Piston to Cylinder clearance
Post by justin_o_guy2 on 04/29/20 at 07:00:22

I Really Really HOPE that what I now know about rings and ring gaps, I never need again,,And Thanks to all who contributed.. I totally get it!
And now I know I wasted HOURS fretting over a biggo Nuthin..

Title: Re: Piston to Cylinder clearance
Post by Armen on 04/29/20 at 12:53:23

Back when I was fettling a buds race bikes, and spent a lot of time the track, I got to talk to a few smart folks.
Next to us once were a guy and his son running a Yamaha 4 cylinder. I asked about ring gaps and squish band clearance. Turns out the dad worked for (owned?) a piston company and had use of the dyno. He says he'd build the motor, run it to 500 RPMs past redline (safety margin) and pull apart. Skim the cylinders and do it over and over til he'd see s faint shiny ring on the perimeter of the piston crown, indicating piston to head contact. Measure things up, and cut the next cylinder to a few thou taller.
Then he'd do ring gaps. Run with tighter and tighter ring gaps until he saw marks on the ends of the rings indicating that the ring ends were starting to to fret. Add a few thou to the clearance and write it all down.
Amazing what you can do with unlimited time and money and equipment and tools and skills.
Closest I ever got to that kind of R+D was doing a few dozen dyno runs in one day trying every combination of timing and jetting.
I've read stories about Irimijari and the development of the 250/6 Honda. Ordering connecting rods in 1mm increments, cams with a few degrees difference in duration, valves in 1/2mm increments, and building a single cylinder test mule of the 250 (so, I guess a 40cc 4 valve motor?) and running every possible combination until he found the best one.
Always in awe of those folks.

Title: Re: Piston to Cylinder clearance
Post by justin_o_guy2 on 04/29/20 at 13:39:58

Yep,, cubic money and time equals performance
BUT,, The smarts to know how to determine what works,, mehh, not for sale

Title: Re: Piston to Cylinder clearance
Post by Dennisgb on 04/29/20 at 13:48:23

Lots of really good info. You guys are very knowledgeable. I’m on a lot of different motorcycle forums, this one is one of the best. The help and knowledge is appreciated.

Title: Re: Piston to Cylinder clearance
Post by norm92de on 04/29/20 at 14:25:00

Reading all this about piston rings reminds me of the days when I flew Pratt and Whitney R2800 engines.

We had a procedure where we were instructed to not under any circumstances reduce the manifold pressure to less than the RPM, which meant since the RPM was almost always 2500, no less than 25 inches of mercury. We were told by Pratt that if we did we could cause top ring land failure and possible engine failure. Apparently, the top ring can flutter in the groove and cause piston failure.

I was flying with a Captain who one day said I am not supposed to do this and then closed the throttles to comply with an ATC request to descend rapidly.
I was flying that aircraft as Captain shortly after that incident when we scattered R2800 parts over the city of Houston. Anyway a single engine landing back at Hobby followed.

One of the 18 cylinders had disintegrated and it was described by the mechanics as the worst failure they had ever seen. The connecting rod failed after the piston and it took out the complete front row of 9 cylinders.

I don't know why Pratt aero engines would be so critical in this respect. We close the throttle all the time on our bikes.

Seeing that top ring land failure of Mikes reminded me about it. :'(

Title: Re: Piston to Cylinder clearance
Post by Armen on 04/30/20 at 06:43:39

Hey Norm,
Thanks for sharing that. Great story. Must have been crazy scary when it happened. I looked at some of those motors last year at the Curtiss museum in NY State. Amazingly beautiful. Can't imagine one fragging the way you describe. Wow!

Title: Re: Piston to Cylinder clearance
Post by justin_o_guy2 on 04/30/20 at 06:54:34

Air Force boot camp. Once in a while we would get a free hour. A buddy and I would run over to the museum. I never quite got my fill of staring at those radial engines.

Title: Re: Piston to Cylinder clearance
Post by norm92de on 05/01/20 at 11:54:21

Armen,

Yes it blew the cylinder/ head right off. It dropped down on to the lower cowling, glad the cowling stayed closed or we would have had a very serious problem indeed. The cooling fins looked like a slinky by the way.