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Classic, Vintage & Veteran For Coventry and Meriden Models. Anything pre-Hinckley goes.

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Old 11-30-2012, 07:33 PM   #31 (permalink)
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As a former pro mechanic I can tell you for sure the best place for a bead hone is in the nearest rubbish bin.

.
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Old 11-30-2012, 07:35 PM   #32 (permalink)
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Piston Ring Rotation

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Originally Posted by JohnA View Post
OK, so there is no point at all in spacing the ring gaps at 120 degrees when reassembling an engine? They might as well be put on the pistons randomly, gap-wise?

I take the point about 2 stroke rings being pegged to prevent movement, but I always thought this was just done as a precaution against the unlikely but distinctly possible event that the rings could one day rotate to the point where a gap coincided with a port, with obviously disatrous results (and also as a precaution against ham-fisted owners installing new rings without a thought about ports & ring gaps)?

If there's some kind of evidence that rings rotate freely in 4 stroke engines I'll willingly accept it, but in view of the fact that every manual I've seen, and every decent mechanic and engineer I've known recommends 120 degree spacing, this is what's ingrained in my brain and it will take some convincing to shift it!
Google the heading and it is all explained and shown how they rotate.
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Old 11-30-2012, 09:42 PM   #33 (permalink)
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The spacing of ring gaps is for start up after a rebuild when the cylinder walls probably have too much oil on them from assembly. If you oil the rings and cylinder then there is too much oil that needs to get scaped away by the rings rather than burned. It's good practice to space ring gaps on reassembly. Just becuase they don't stay that way doesn't mean you shouldn't still follow best practices.

I still agree with the article. Not becuase I have any knowledge of what happens to cylinder walls on a microscopic level but becuase that is what I was taught by a guy I trusted. That was also the practice we followed at the dealership. Hone only after a rebore.

Scott
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Old 11-30-2012, 10:48 PM   #34 (permalink)
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This is a great discussion. I've always wondered why 2-stroke rings were pinned and thought it was to avoid installing them in the wrong positions. But since I have now learned that rings do rotate in operation, the pins are there to prevent the ends from rotating to align with one of the ports, which would without a doubt allow it to pop into the port and break the ring.

I still maintain that when you quit learning, it's time to be fitted for a box.

regards,
Rob
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Old 11-30-2012, 11:22 PM   #35 (permalink)
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Quote:
Originally Posted by Old Cafe Racer View Post
As a former pro mechanic I can tell you for sure the best place for a bead hone is in the nearest rubbish bin.

.
Depends upon the intended use. For breaking a glaze and reestablishing a cross hatch pattern they are hard to beat. They follow the contours of the cylinder and are very forgiving of less than centered use.

Spring loaded hones (like the 3 stone job) can work well, but it is very easy to damage the cylinder by not maintaining perfect alignment. They also can create "waves" in the cylinder walls through uneven pressure. you can end up with a surface that is nice and concentric, but create an included angle to where the bore is no longer perfectly centered vertically.

The ideal is to use a boring machine where the cylinders are fixed to a machined base. Unless you have a machine shop, this isn't available to most. Even with this process, most machine shops will follow with a very mild hone to change the resultant spiral grooves from the cutting tip to the proper cross hatch pattern if the machine isn't of the plunge spindle type.

If you are in need of the next oversize, I would not try anything by hand, much to easy to goof it up. Most shops that handle this type of work will do so for around an hours labor per bore.
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Old 12-01-2012, 01:07 AM   #36 (permalink)
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Rings

Quote:
Originally Posted by Rookster View Post
The spacing of ring gaps is for start up after a rebuild when the cylinder walls probably have too much oil on them from assembly. If you oil the rings and cylinder then there is too much oil that needs to get scaped away by the rings rather than burned. It's good practice to space ring gaps on reassembly. Just becuase they don't stay that way doesn't mean you shouldn't still follow best practices.

I still agree with the article. Not becuase I have any knowledge of what happens to cylinder walls on a microscopic level but becuase that is what I was taught by a guy I trusted. That was also the practice we followed at the dealership. Hone only after a rebore.

Scott
But why would they have to much oil on them at re-assembly? After washing the bores to prevent oil being their in the first place, not oiling the rings on assembly (but a small drop on the skirts) it matters not. How are the rings meant to bed into the bores if their is oil present? Simple, they cannot! Therefore it does not matter where the ring gaps are placed on the piston as given time, they will rotate.
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Old 12-01-2012, 03:55 AM   #37 (permalink)
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Quote:
Originally Posted by RII View Post
Not really sure I want to wade into this one, but what the heck...... Original bore specs may be too loose for a modern forged piston to avoid psiton slap and other related issues.


OK, you can all take aim now.....

The only nit I would pick is that the replacement pistons commonly now sold for Triumph twins appear to be very like original Hepolites in expansion rates and I don't think they're made to any better or worse engineering standard. That is to say, assuming that a tighter clearance would be right for your new pistons leads to prompt seizure.

Also- many racing forged pistons are actually recommended to be used with a larger clearance than ordinary cast pistons.

Ok that's two nits.
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Old 12-01-2012, 09:58 AM   #38 (permalink)
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Quote:
Originally Posted by lovecuba View Post
Google the heading and it is all explained and shown how they rotate.
I've just finished doing that. You know what? I'm convinced! The piece of logic that worked most on me was that there are never wear marks (or to be more accurate, lack of wear marks) on old cylinders where the gaps have been. Not a trace of them, and that proves to me that the rings must rotate.

It also tells me that despite what all the manuals instruct, and what all the mechanics say, there really is no point in carefully arranging rings with gaps 120 degrees apart. You have to put the gaps somewhere when fitting rings/pistons, so may as well make it look neat, but no need to do more than avoid getting them dead aligned.

Its good, learning new stuff you thought you already knew about!
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Old 12-01-2012, 08:21 PM   #39 (permalink)
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Quote:
Originally Posted by RII;2438593 [B
Depends upon the intended use. For breaking a glaze and reestablishing a cross hatch pattern they are hard to beat. They follow the contours of the cylinder and are very forgiving of less than centered use[/B].Spring loaded hones (like the 3 stone job) can work well, but it is very easy to damage the cylinder by not maintaining perfect alignment. They also can create "waves" in the cylinder walls through uneven pressure. you can end up with a surface that is nice and concentric, but create an included angle to where the bore is no longer perfectly centered vertically.

The ideal is to use a boring machine where the cylinders are fixed to a machined base. Unless you have a machine shop, this isn't available to most. Even with this process, most machine shops will follow with a very mild hone to change the resultant spiral grooves from the cutting tip to the proper cross hatch pattern if the machine isn't of the plunge spindle type.

If you are in need of the next oversize, I would not try anything by hand, much to easy to goof it up. Most shops that handle this type of work will do so for around an hours labor per bore.
That's exactly the point, they true nothing, the balls are on the end of springy wire stalks that bounce around and do nothing but follow and then exagerate any out of true/out of round/small surface defects.. like I said the best place for a ball hone is in the nearest rubbish bin. (after you have rendered it unusable so no well meaning apprentice can rescue it and use it on his backyard jobs)
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Old 12-02-2012, 12:08 AM   #40 (permalink)
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No hone is meant to true a bore, that is done best using a machine. The bore needs to be maintained in the best possible alignment to the cutting edge to ensure ideal results. It's even better if you can simulate head torque, as this will allow for the correction of any distortion that occurs under normal installation/running conditions.

For those who are curious, here are plenty of sample images of these machines:

http://www.google.com/search?q=cylin...w=1280&bih=717

The one thing they all have in common is fixtures to hold the bores in place, and a large, infelxible, piston driven cutting edge. Setup is critical to maintaining a true bore and you will not get this type of accuracy with any handheld attempt. Hones (bead and otherwise) are designed in an attempt to alleviate the inherent inaccuracies involved in completing this process by hand, but again, none are meant to make up for any serious defects in the cylinders surface.
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