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Old 28th October 2005
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Default Straightness and Roundness of Cylinders

OK, y'all are probably about sick of me talking about straightness and roundness of cylinders, how important it is and how bad some cylinders are. So I'm gonna stop. Soon as I do it one more time

Today I took some pictures of a new take-off stock cylinder, as we did some measurements on it (holding the dial bore gauge is my son-in-law and shop manager Jon). What we did is measure this cylinder in 1" depth steps. We did the measurement both in the thrust direction and 90 degrees to that direction at each depth. You do it this way to show out of roundness. Really, four postions is better than two. But two will get the point across with less complication here.

The dial bore gauge shows a relative measurement, don't try to figure out bore size from it's reading. Each numeral on the dial is .001", which we normally call a "thou". It has ten little lines between each numeral, each representing .0001", otherwise known as a "tenth", because it's a tenth of a thou and thousandths is always our default unit of measure.

So anyway, here's what we got:


One inch down. Very little out of round, maybe a couple tenths.


Two inches down. About 8 tenths out of round. Hmm, not good.


Three inches down. Yikes, 3 thou out of round. This is bad, very bad.


Four inches down. We're into the spigot, which has no material backing it up and is very prone to distortion. We're literally 8 thou out of round down here. Holy smokes.

This is not a highly unusual result. I've seen worse in fact, and I've seen better, too. It's something of a crap shoot when you grab a production cylinder. Might be good or bad or anywhere in between, you don't know unless you measure.

In all fairness to HD, some things need to be noted here:

- The spigot is really hard to get straight. But no way should it be anywhere near 8 thou off. Maybe 1 thou.

- The other cylinder of this set was only about half a thou (5 tenths) total out of round until the spigot and then it was about a thou out. It was definitely a better than average factory cylinder.

- The type of torque plates used and the way in which it's tightened also have an effect. It's not a totally repeatable process. I can make a cylinder perfectly straight and round, take it out of the torque plate and it'll be distorted as hell, put it back in the torque plate and it'll be close to what it was when I machined it, but not exactly the same. A few tenths off here, a few tenths off there. But this cylinder's distortion is way beyond the repeatability of the measurement.

So anyway, now I hope y'all understand where I'm coming from when I say you're better off to get your cylinders bored and honed than to buy new ones. There may not be much difference in cost, but there can be a huge difference in quality, and this is an area that really matters for both the power and longevity of your motor. Just make sure the shop you have do the boring knows what they're doing. Getting a cylinder straight and round and perfectly sized for the correct clearance takes a bit of skill.

And don't even think about measuring or machining a cylinder without torque plates. It ain't even worth your time to stick the bore gauge in it, it'll change so much. If a shop tells you they can measure or machine them without torque plates, run, don't walk, away. A well designed, hefty aftermarket cylinder (i.e. Axtell) will change only slightly in and out of a torque plate. A factory cylinder will move a mile. They're flexi-flyers.

Actual phone conversation I had once:

customer: just got my cylinders back and they don't measure right

me: what kind of torque plates did you use?

customer: what kind of what?

<followed by a long conversation explaining how bad a factory cylinder distorts and how essential the torque plate is>
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Last edited by aswracing; 28th October 2005 at 04:45..
  #2  
Old 28th October 2005
Takingabreak Takingabreak is offline
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Well that does proove the point, torque plates are a must.
  #3  
Old 28th October 2005
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Thanks JT. Tell you what, it can be a real eye opener to measure factory cylinders. Sometimes I'm amazed this stuff ever works.

When I get time I'll show how much a cylinder moves in and out of a torque plate.
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Old 28th October 2005
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I should point out, too, just how bad these numbers are. With the cylinder this far out of round, you're basically asking the rings to change shape as they go down the bore. That's not going to happen to a very large extent at all. Instead they're going to leak.

On top of that, consider that a cast piston is designed to be fit to about a .001" clearance in the bore. Forged pistons are typically .002 to .003 depending on the application. Too tight will cause scuffing, too loose will cause piston rock and poor ring seal. So with a cylinder that' s this crooked, how do you get the right clearance? On what measurement do you base your clearance? It's kinda ridiculous.
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Old 28th October 2005
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Aaron...

Don't stop providing this kind of information....some of us eat it up, and some of us just don't click, but for the sake of those who enjoy this depth of discussion, its really helpful...Thank you!
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Old 28th October 2005
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Yeah! What HE /\ said! Question: What experience have you had with the 1000 cc Ironhead cylinders 'fitted' with stock pistons sold as a set from V-Twin, if any?
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I bought new cylinders and pistons when I did my upgrade. Got them home checked them out and they read right at the one and a half thou, perfect fit pistons were nice. Then everyone on the forum said I should have them check with torque plates, so I did. With the torque plates installed the cylinders tightened up at the top and 2 thou taper at the spigot. Anyways to make it work I had to have brand new out of the box cylinders bored to 5 thou oversize to get the right fit.
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Old 28th October 2005
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Great Stuff. Thanks for the time you put into this Aaron, I'm learning more all the time.

Cheers,

Brett
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Old 28th October 2005
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Very much appreciated Aaron. Nothing like a truly round cylinder to seal the rings and get the HP you're paying for. I am more curious about how distorted the cylinders get over time from being constantly cycled? Is it even a concern? I know when we used to race stockers we never allowed the oil temps to get under 120 degrees F (used factory cold weather oil heater) whenever the car wasn't being raced. We felt this helped keep the cylinders from distorting virtually eliminating hot and cold cycles. The owner had held a few records over the years.
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Old 29th October 2005
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Great Info Aaron!!!

 

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