March 29, 2024, 12:57:13 AM

News:

For advertising inquiries or help with registration or other issues, you may contact us by email at help@harleytechtalk.com


Piston to cylinder tolerances?

Started by Adam76, February 20, 2019, 03:01:17 PM

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.

Adam76

 Hey guys,
Im replacing my stock 2018 sportser pistons for the SE  /  Wiseco  10.5 comp forged pistons.
My Question is  -  if the bike has only done around 200 miles (brand new) can I use STD bore size pistons and just slip them into cylinders with no honing or machining?

Or do I still need to buy .010 over size and get the cylinders bored and honed to the pistons. 

Thanks for any advice. 

Ohio HD

You need to measure the cylinders to see where they really are IMHO. Forged pistons will need more clearance. So "maybe" they can be honed while in torque plates to the proper clearance. But maybe the honing won't be enough to make the cylinder really straight, if it's not. Measure is always the best thing to do.

Adam76

Quote from: Ohio HD on February 20, 2019, 03:59:33 PM
You need to measure the cylinders to see where they really are IMHO. Forged pistons will need more clearance. So "maybe" they can be honed while in torque plates to the proper clearance. But maybe the honing won't be enough to make the cylinder really straight, if it's not. Measure is always the best thing to do.
Thanks Ohio for the info.

I mistakenly thought it was the other way around,  as in the forged pistons expand as they get hot  so in theory a touch more clearance between piston and cylinder wall would not be a problem.. my mistake.

I also assumed since the cylinder walls have only 200 miles on them so it would be ok to replace std size pistons,  rather than boring to 0.010 oversize?
Cheers

Ohio HD

Quote from: Adam76 on February 20, 2019, 06:44:41 PM
Quote from: Ohio HD on February 20, 2019, 03:59:33 PM
You need to measure the cylinders to see where they really are IMHO. Forged pistons will need more clearance. So "maybe" they can be honed while in torque plates to the proper clearance. But maybe the honing won't be enough to make the cylinder really straight, if it's not. Measure is always the best thing to do.
Thanks Ohio for the info.

I mistakenly thought it was the other way around,  as in the forged pistons expand as they get hot  so in theory a touch more clearance between piston and cylinder wall would not be a problem.. my mistake.

I also assumed since the cylinder walls have only 200 miles on them so it would be ok to replace std size pistons,  rather than boring to 0.010 oversize?
Cheers

The forged pistons do require more room. That's why maybe you get lucky, but maybe you need to hone to fit, or maybe there's too much clearance. Hard telling the true size of the bore without measuring. The best ring seal will come from a fresh new bore, bored straight by a competent machinist. I have about 2,000 miles on fresh standard bored S&S 4.125 cylinders. I decided to up the compression, so I ordered +0.005" pistons. My machinist will hone them to size. 

Piston bore sizes are listed in nominal sizes, like 3.875", 4.125" etc. Depending on the pistons actual measurement, the cylinders may be right or not. Forged Wiseco pistons generally require 0.0025" clearance. I wouldn't trust them to chance, I'd measure.

A good machinist will not bore cylinders without the pistons there to measure them first. Then size the bore per the desired clearance.

Adam76

Quote from: Ohio HD on February 20, 2019, 06:59:50 PM
Quote from: Adam76 on February 20, 2019, 06:44:41 PM
Quote from: Ohio HD on February 20, 2019, 03:59:33 PM
You need to measure the cylinders to see where they really are IMHO. Forged pistons will need more clearance. So "maybe" they can be honed while in torque plates to the proper clearance. But maybe the honing won't be enough to make the cylinder really straight, if it's not. Measure is always the best thing to do.
Thanks Ohio for the info.

I mistakenly thought it was the other way around,  as in the forged pistons expand as they get hot  so in theory a touch more clearance between piston and cylinder wall would not be a problem.. my mistake.

I also assumed since the cylinder walls have only 200 miles on them so it would be ok to replace std size pistons,  rather than boring to 0.010 oversize?
Cheers

The forged pistons do require more room. That's why maybe you get lucky, but maybe you need to hone to fit, or maybe there's too much clearance. Hard telling the true size of the bore without measuring. The best ring seal will come from a fresh new bore, bored straight by a competent machinist. I have about 2,000 miles on fresh standard bored S&S 4.125 cylinders. I decided to up the compression, so I ordered +0.005" pistons. My machinist will hone them to size. 

Piston bore sizes are listed in nominal sizes, like 3.875", 4.125" etc. Depending on the pistons actual measurement, the cylinders may be right or not. Forged Wiseco pistons generally require 0.0025" clearance. I wouldn't trust them to chance, I'd measure.

A good machinist will not bore cylinders without the pistons there to measure them first. Then size the bore per the desired clearance.

Thanks heaps.
Stupid Weston but is honing the same as boring? Is just a light hone cheaper than having to bore the cylinders?
Cheers

rigidthumper

Boring is removing material with a cutter, designed to remove material fast. think rough cut. must be followed by honing.
Honing is removing material with abrasive stones, designed to remove material slowly & precisely.
Shops will typically use the boring bar to get close, then rigid stones to abrade the last thousandth or two, to achieve final size. Plateau hones are final step, used to achieve proper finish.
Ignorance is bliss, and accuracy expensive. How much of either can you afford?

Don D

The cylinders are set at the nominal bore size and clearance required is built into the piston. Your cylinders are not worn, likely unless there was some problem. They may however be distorted.  This can be spotted easily by putting the cylinders in plates and measuring. Rarely are they very straight and true. Problem is to get most to come in it takes .0015 or thereabouts material removed with a fixed hone to arrive there. Now you have a loose fit on the piston to cylinder. Some say just ball hone them. Personally I don't do that, but it may work for some and can be done in a home shop.

Hillside Motorcycle

At our full-time, 6 day a week facility, we always measure piston skirt diameter, always, even on our own known, big bore kits.
Never assume.
Otto Knowbetter sez, "Even a fish wouldn't get caught if he kept his mouth shut"

Adam76

Quote from: rigidthumper on February 21, 2019, 05:40:42 AM
Boring is removing material with a cutter, designed to remove material fast. think rough cut. must be followed by honing.
Honing is removing material with abrasive stones, designed to remove material slowly & precisely.
Shops will typically use the boring bar to get close, then rigid stones to abrade the last thousandth or two, to achieve final size. Plateau hones are final step, used to achieve proper finish.
Ok thanks that's a great explanation.

So,  if I want to have standard size new pistons installed to a set of practically brand new standard size  cylinders..... Would a thorough measurement check and a light hone be all that is required?

In other words,  I don't need to have the cylinders bored out, so it's less work and less cost,  right?

Im just trying to weigh up the cost / benefit of fitting std pistons to stock std cylinders  VS  boring and honing my cylinders to. 010 larger pistons....

Thanks
Thanks

Adam76

Quote from: Hillside Motorcycle on February 21, 2019, 06:28:59 AM
At our full-time, 6 day a week facility, we always measure piston skirt diameter, always, even on our own known, big bore kits.
Never assume.
Thanks,  so what you're saying is that the first thing to do before ordering pistons is to properly measure the ID of the cylinders?  What should they be?  And what is within tolerance to fit std size pistons?
Thanks

Adam76

Quote from: Adam76 on February 21, 2019, 03:31:32 PM
Quote from: Hillside Motorcycle on February 21, 2019, 06:28:59 AM
At our full-time, 6 day a week facility, we always measure piston skirt diameter, always, even on our own known, big bore kits.
Never assume.
Thanks,  so what you're saying is that the first thing to do before ordering pistons is to properly measure the ID of the cylinders?  What should they be?  And what is within tolerance to fit std size pistons?
Thanks

Edit  sorry question has already been answered.

Thanks guys.

aswracing

February 22, 2019, 07:35:47 AM #11 Last Edit: February 22, 2019, 07:40:28 AM by aswracing
Quote from: HD Street Performance on February 21, 2019, 05:49:12 AM
The cylinders are set at the nominal bore size and clearance required is built into the piston. Your cylinders are not worn, likely unless there was some problem. They may however be distorted.  This can be spotted easily by putting the cylinders in plates and measuring. Rarely are they very straight and true. Problem is to get most to come in it takes .0015 or thereabouts material removed with a fixed hone to arrive there. Now you have a loose fit on the piston to cylinder. Some say just ball hone them. Personally I don't do that, but it may work for some and can be done in a home shop.

Adam, this is the correct answer. Especially the part I put in bold.

Believe me, lots of new or nearly new cylinders are distorted as hell. I've run my bore gauge down take-off cylinders from brand new bikes and found them to be a mess.

So it's not just a matter of wear. It's also a matter of distortion. If you want the rings to seal right, the cylinder needs to be straight and round, top to bottom, over the entire working area of the cylinder.

What he's saying is that you can take a near new, distorted cylinder, and hone it until it's straight, but by the time you do get it straight, it's too big.

And he's also saying don't count on the forged piston's greater clearance spec to save the day for you. That's what his first sentence is all about. The extra clearance required by the forged piston is built into the piston skirt, not the cylinder bore size.

Here's the bottom line. Without measuring the pistons, and without also clamping the cylinders into torque plates and running a bore gauge up and down them, nobody here can tell you whether or not it's advisable to try to fit standard bore pistons into your cylinders. It doesn't matter that your cylinders are nearly new, the question can't be properly answered without making the measurements.

Adam76

Quote from: aswracing on February 22, 2019, 07:35:47 AM
Quote from: HD Street Performance on February 21, 2019, 05:49:12 AM
The cylinders are set at the nominal bore size and clearance required is built into the piston. Your cylinders are not worn, likely unless there was some problem. They may however be distorted.  This can be spotted easily by putting the cylinders in plates and measuring. Rarely are they very straight and true. Problem is to get most to come in it takes .0015 or thereabouts material removed with a fixed hone to arrive there. Now you have a loose fit on the piston to cylinder. Some say just ball hone them. Personally I don't do that, but it may work for some and can be done in a home shop.

Adam, this is the correct answer. Especially the part I put in bold.

Believe me, lots of new or nearly new cylinders are distorted as hell. I've run my bore gauge down take-off cylinders from brand new bikes and found them to be a mess.

So it's not just a matter of wear. It's also a matter of distortion. If you want the rings to seal right, the cylinder needs to be straight and round, top to bottom, over the entire working area of the cylinder.

What he's saying is that you can take a near new, distorted cylinder, and hone it until it's straight, but by the time you do get it straight, it's too big.

And he's also saying don't count on the forged piston's greater clearance spec to save the day for you. That's what his first sentence is all about. The extra clearance required by the forged piston is built into the piston skirt, not the cylinder bore size.

Here's the bottom line. Without measuring the pistons, and without also clamping the cylinders into torque plates and running a bore gauge up and down them, nobody here can tell you whether or not it's advisable to try to fit standard bore pistons into your cylinders. It doesn't matter that your cylinders are nearly new, the question can't be properly answered without making the measurements.
Thanks,  I get it now and makes sense.  Pisses me of that the cylinders come from the factory so dodgy...

I guess with all the costs of checking and boring and honing my cylinders I might as well go with Aarons / Dan's  1250 package.

Thanks for the advice guys.   👍👍

Rsw

Use the Hammer 1275 kit . Very nice !!!

Adam76

Quote from: Rsw on February 22, 2019, 09:10:13 PM
Use the Hammer 1275 kit . Very nice !!!
Sure is nice !! 
My budget is not so nice.. 😨