'66 L-79 Pilot Bushing - Too much interference?
Thanks in advance for you opinions and past experience.
Bob
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Thanks in advance for you opinions and past experience.
Bob -
Re: '66 L-79 Pilot Bushing - Too much interference?
Bob -
The design spec for the bushing O.D. is 1.094". Make sure the I.D. of the hole in the crank is clean and free of corrosion, and it should install without any drama. You might put it in the freezer overnight first. After installing the bushing, make sure the end of the input shaft fits in it without having to force it, before re-installing the clutch and bellhousing. -
Re: '66 L-79 Pilot Bushing - Too much interference?
[quote= You might put it in the freezer overnight first. After installing the bushing, make sure the end of the input shaft fits in it without having to force it, before re-installing the clutch and bellhousing.[/quote]
You could even heat the crank a little so the bushing pushes right in.Comment
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Re: '66 L-79 Pilot Bushing - Too much interference?
I felt the need to add to this thread based on experience so anyone searching the subject of pilot bearing press fit or interference might benefit from a little more info..........
I wound up having the new bushing turned down to about what I measured for the old bushing. When removed, the old bushing was still pressed snugly into the crank, and showed no signs of any motion relative in the crank. The new bushing installed without any drama with an interference of about .001-.002".
I was not comfortable installing a bushing into a crank with .004-.005" interference (even with the assist of bushing cooling and/or crank heating). Here's why:
Many years ago as a rookie engineer designing helicopter rotor systems, I was given the task of designing a 1/4 scale fatique test specimen. The one mistake I made was not reducing the interference by the 1/4 scale factor. The result of assembling with an interference of .004" per inch of diameter instead of .001" per inch of diameter was excessive hoop stress in the component that a bolt was pressed into and some split 4340 steel....pretty embarassing!
Cooling the bushing and/or heating the crank may make assembly easier, but won't have any affect on the final hoop stress in the 'female' part after the temps equalize.
Regards,
BobComment
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Re: '66 L-79 Pilot Bushing - Too much interference?
Bob,
When I replaced the pilot bushing in my 1963 engine, the OD crankshaft bore was .010 smaller at 1.086. I believe the crank had been replaced at one time because part # is 4577.
After careful measurements, I had the new bushing turned so there is a .002 interference fit and installed without problems. The bushing I took out was clearly distorted because of being driven into a hole that was too small.
I can't explain why the bore was not correct except to add the bushing after installation stuck out approx .125. I double checked depth and it is good so the complete input pilot shaft sits into the bushing.Comment
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Re: '66 L-79 Pilot Bushing - Too much interference?
The as designed pilot bushing interference in the crankshaft does not create a stress problem. The bushing is so soft relative to the crank that it yields to conform to the hole clearance. Any resulting hoop stresses in the crank will be minimal.
The only potential problem a large interference can create is that as the bushing conforms to the pilot hole, the bushing inside diameter will reduce and may make the initial installation of the transmission input shaft difficult.Comment
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Re: '66 L-79 Pilot Bushing - Too much interference?
Robert,
I find no reason to dispute what you are saying but after removing the bushing shown in this post I felt much better taking careful measurments and installing with .002 interference.
https://www.forums.ncrs.org/showthread.php?t=70160Comment
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Re: '66 L-79 Pilot Bushing - Too much interference?
Tim - The bore in my crank was well below 1.094" too. Go figure.....
Robert - Thanks for your input.
Regards,
BobComment
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Re: '66 L-79 Pilot Bushing - Too much interference?
Tim,
In your case from the other thread, the interference fit was very tight and the bushing deformation was excessive. So, it makes sense that you would want to turn down the bushing outside diameter from the standard dimension to give a reasonable fit in your nonstandard size crank pilot.
However, it's not necessary with standard parts, as in the OP's case. A reduced interference with the crank will actually reduce the amount of that the bushing inside diameter decreases in size during installation, and therefore give a larger clearance at the transmission input shaft. But it probably really doesn't make any difference for the numbers involved.Comment
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Re: '66 L-79 Pilot Bushing - Too much interference?
I agree with the 0.001 -0.002 inch interference and using an original OIL-LITE type brass bushing......just as original. The OD can be "turned down" a few thousands using a drill press and a fine mill file or wet/dry sandpaper on a metal backing plate. DO NOT, however, try to alter or enlarge the ID, as it will destroy the oil pores.
That being said, the July 2010 issue of Corvette Enthusiast Magazine (just arrived yesterday) has an ad on page 93 regarding a pilot bushing made of Kevlar. Said to have superior durability, excellent lubrication, and easier to install. Made by RAM Automotive.
This may end future installation discussions/issues for the original brass bushings.
LarryComment
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Re: '66 L-79 Pilot Bushing - Too much interference?
Robert -
I'm unclear whether my parts fit the "standard parts" category or not. Based on measuements, I'm convinced my apparent interference before turning down the replacement bushing would have been more than .0055". Is this within the as designed tolerance range for "standard parts"? (0.0055" is way tighter than what I've always understood is desirable for an interference fit on a 1.0" daimeter....in fact my old, maybe aged, 7th Ed Mark's Standard Handbook for Mechanical Engineers limits the heaviest interference fits, FN4 & FN5, to less than .002" for diameters in the range of 1.094".)
Is your statement that bronze is soft enough to deform rather than induce excessive stress in the crank based on theory or experience?
I can easily imagine that some bronze bushing o.d. material is sheared away during insertion depending on relative temperatures of the two components. Maybe the as designed configuration depended at least in part on this....?
Since the relative stiffnesses, yield strength of bronze vs ultimate tensile strength of steel, and geometry (nominal interface diameter, bushing i.d., surrounding crank section) all come into play in determining the final hoop stress, this just is not intuitive....at least not to me......thus my caution with a fresh numbers matching rebuild that I waited more than a year to receive....
Regards,
BobComment
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Re: '66 L-79 Pilot Bushing - Too much interference?
the crank will shave off the excess OD. i must admit i never checked the OD of the bushing or the ID of the crank bore and i never had one not go in or cause any problem with the bushing bore getting smaller and causing the trans input shaft not to go into the bushing. i don't know the manufacturing tolerances GM used in the crank bore or the the OD of the bushing but i bet on the assy line they just pressed them into the crank without any measuring. JMHOComment
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Re: '66 L-79 Pilot Bushing - Too much interference?
Bob,
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Re: '66 L-79 Pilot Bushing - Too much interference?
As another press fit example I just pressed on a power steering belt pulley onto the pump. The replacement pump input shaft was undersized and the old pulley slid on. (The reason for the measurements) The shaft was roughly 5/8 of an inch in diameter and the pulley had over .003 interference fit, with the original pump and with the second replacement pump. This was the new style power steering pumps that do not have any keyway or retaining nut. Frankly I was surprised at the amount of interference fit on this system. However one can't argue with the system as it seems to functions well as designed.
RickComment
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