Stock cast pistons: ring end gap?

[billbuilds]

     My Haynes manuel shows the top compression ring end gap spec as being .010 - .020. My cylinders have been freshly bored to .020 over. Using Hastings chromoly rings I measure an end gap of a tight .021 on one cylinder and .022 on the other. Seems like a lot to start a fresh top end off with (import cast pistons 8.5:1 CR). I don't have a dial bore guage so I have to take the machinest's word for it though the pistons seem to fit in the bores a little on the loose side.
     I saw in a previous post on ring gap  that .003 per cubic inch was recommended however doing the math you'd already be out of spec since 74 X .003 = .222.
     So now I'm really confused. Help would be appreciated. Thanks, Bill

[neten-o]

A little on the large side is far better than tight Bill ..

[rbonner]

its a lot more critical on a hot rod.  When the motor gets hot the piston and rings will expand considerably.  BUT sheesh, .020 is way the fricken heck out there even for an air cooled tractor motor.

I would read the pamphlet with your pistons or call the company.  They will tell you the recommended piston to wall clearance.  Make sure you aren't going over the recommended amount.  You'll have psiton rock and scuff like crazy.  Not to mention oil issues.

Then its best to be .001-.002 over the minimum ring gap, not way past the max.

Get your hands on your shop's dial ga and check them.

BOB

[moosemush]

I thought the standard rule of thumb on ring gap was .004" per inch of bore. There are pistons like KB that use special ring gaps. It still leaves you way out of spec. Bob has the best advise for you. Something seems amiss.

[rbonner]

Oh hey Bill, on top of that CAST pistons usually run about 1/2 the clearance as forged pistons.  Forged expand much more when hot.  Also no offense at all, but Rule of Thumb formulas are great for "Hey does this seem right" eyeballing but when it comes to running correctly you HAVE TO HAVE the spec sheet that comes with the pistons and bore and hone to the correct clearance.

First important item is bore straightness and roundness, then piston clearance and then last is the ring gap.  The piston clearance will make all the difference in the world as how the engine lives.  The ring gap will have to do with maximum engine power and not really effect much more. (unless too tight)

On our car racing engines, we try to run the ring gaps slightly closer than specs...  What you want is the ring to close to within about .0005" when the engine is hot.  This basically closes the gap.  Then you place the rings 180 degrees from each other which completely eliminates blow by.  (water cooled and steel blocks)

The hotter the engine runs the more the rings expand.  So these air cooled potentially parade operation air cooled jobbies need a bit more clearance in the piston to wall and end gap.  But you need to know the minimums first to determine the clearances you want to HONE and FILE the rings too.  Yes I always filed my rings.

SO anyway, I hope your guy didn't get crazy with the honing procedure, but that will take the dial bore gauge and the spec sheet.

Oh just incase you wondered, high RPM race motors usually around .002" with cast and .004 with forged pistons and ring gaps depend on bore size (rule of thumb ha) and the type of material and the estimated expansion rate through experience.  We always started with .005" over sized rings except the oil rings.

Just shooting from the hip these would probably double with a motorcycle air cooled.

BOB

[billbuilds]

     Bob,
     These are simple oem style replacement pistons made in Taiwan for Drag Specialties. There is no spec sheet with them or with the Hasting rings. AFIK you are to use the Moco's specs. I opened up my 78-1/2-84 FL/FX Moco shop manuel  and see that it shows a compression ring clearance of .010 to.031 for 1983 and earlier motors. Piston to wall clearance shows as .002 to .005.
     Rich, yes loose is better than tight for that gap but I hate to start out on the far end of the scale before the motor ever runs.
     I'll give Hastings a call later today. There's a neat lady in customer service who's been there for about thirty years and really knows her stuff. Having learned that the guy who bored my cylinders used a 300 grit stone for the final hone I gave her a call to make sure that this was ok for chromoly rings and she said usually 280 but 300 is fine.
     Bob,
     I have KB 8.5:1's in my 1983 FLHS 88" big bore. I sent the cylinders down to Zippers for boring since I was buying the pistons from them. They set them up right - ring gap and all. But that was back when I had a good job and was fairly well paid
     I will let you know what I hear from customer service at Hastings. Thanks for your help, Bill   

[billbuilds]

     Just wanted to let you know that I think I got the "rule of thumb" ring gap equation wrong. It's supposed to be .003 per inch of bore NOT per cubic inch. So 3.4375 X .003 = .0103
There, that sounds a little better doesn't it? Now if I add in the extra thou or two as Bob suggested that would put me at .011 to .012.  OK, now to call Hastings...

[hightop]

I think the white paper wrap hastings rings usually come in advocate .004" per inch of bore, so approx. .014" for your application. I think the cast Tiawan pistons are all that is available for Shovels now in cast stock compression, but they seem well made, have the steel reinforcment underneath and all. I believe recommended clearance is .0025" on these, I've bored a few cylinders for them, and the ring gaps are usually on the wide end of specs. A quick test of piston fit is to use a feeler guage between cylinder and piston. Don't force anything though.

If it makes you feel better, remember the whole gap isn't allowing combustion gases thru, only the 2 or 3 thousandths that is exposed in the bore, the rest is sealed on the lower land of the piston by pressure.

[billbuilds]

     I just got off the phone with Dawn in customer service at Hastings. She says that I should be fine with the .021 gap since the SM gives the spec as being from .010 to .031. As you said, Rich, better to be a little on the loose side than on the tight side. She did say that if I wanted to I could get .030 over rings and file them down to get a clearance of .011 or .012 but that in her opinion this was a waste of time and money. "The motor wil never know the difference" was how she put it.
     In reference to piston to cylinder wall clearance she pointed out that for every thou in bore size that you go over minimum spec on you will get a corresponding .03+ increase in ring gap.
     Hightop,
     The paper that the Hastings rings come in does say minimum ring gap on 1st and 2nd compression rings should be .004 per inch of bore size. Thus they're saying the minimum ring gap s/b .0133. It doesn't say anything about a maximum gap though. Dawn told me that in her opinion .021 was just about perfect.
     Thanks guys, I fell better now.  Bill

[billbuilds]

     One last thing. I did take your advise, Hightop, and got a measurement of a tight .003 piston to cylinder wall clearance. Guess I'm good to go. Thanks Bill 

[prodrag1320]

call total seal and get a gapless 2nd ring

[rbonner]

OK cool, glad you got the good news from her.

Like I already mentioned the piston to bore clearance is the most important for engine life.  The rings are just there.  If they are loose you'll have a higher leak down number and too tight cylinder damage if the ring ends come together under temp.  You always need some clearance there.

That's why they say better to be loose in the ring gap than too tight.  I just always fudged slightly tighter to close the gap without damage.

On gapless ring sets, I've used total seal back in the 70's on hot rods.  The gapless ring belongs in the top land.  They generally cause too much piston drag in the cylinders.  That's why we went to 1/8" rings with a step cut in the piston.  Those worked the best.

Anyway off topic.

The .004" is what I figured for piston to wall clearance, basically twice the cast piston car clearance.

You can't use a feeler to measure piston to wall on the thing.  You need a dial bore gauge that slides down and centers in the cylinder.  The piston needs measured on the middle of the skirt below the ring land not at the top or bottom. which is smaller, pistons are shaped like a barrel.  The gap check on rings is put the ring into the bore without piston and push it to the height it will sit when the piston is TDC.  The measure with feeler ga.  File to fit.

Anyway, sounds liek you are set.

BOB