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Old 02-16-2011, 09:49 PM
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What is the purpose of a camshaft thrust button ?

Considering that on a SBC or BBC the angle of the cut on the camshaft gear that drives the distributor and oil pump holds the camshaft to the rear of the engine , I don't understand the discussion I see sometimes about camshaft endplay or thrust buttons. All of the gear angle is holding the rear of the camshaft sprocket against the front of the block. What am I missing here ? With the rotational resistance of the oil pump I don't see how a cmshaft could walk forward.

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Old 02-16-2011, 10:36 PM
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on a flat tappet cam, the lobes are ground with a slight angle so that the valvetrain pressure on the lifter restrains the camshaft to the rear of the motor. but on a roller cam and lifter, the surfaces are ground flat and the lifter no longer puts pressure on the cam to restrain it, therefore a cam button on the gear end of the cam is used to take up clearance between the cam and timing cover to keep it centered. thats about the best i can explain it, hope it clears it up a little for you
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Old 02-16-2011, 11:15 PM
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The cam gear is not what holds it back. For that reason all roller cams need some kind of device to keep the cam in place, either a retainer plate if the block is configured for one, or a thrust button.

With flat tappet cams it is the taper ground on the lobes that provides the thrust control in conjunction with the slight crown to the lifter foot and slight off-center location of the lifter bores to the cam lobe location. Incidentally, this taper on the lobes, lifter foot crown, and lifter bore location is also what spins the lifter to prevent undue wear on the lobe and lifter foot.

I use a thrust button even on flat tappets, if you have ever noticed the timing advance jump around when using a timing light you will find that minimizing end play in the cam helps control this(along with minimizing end play in the distibutor shaft).
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Old 02-17-2011, 10:06 AM
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I may have phrased this wrong . I was referring to the distributor drive gear on the camshaft . The angle on it in conjunction with the rotational resistance of the oil pump holds the camshaft to the rear of the engine as well as holding the distributor shaft down. I was looking at this as I was assembling a 454 recently. I still don't understand how a cam could walk forward with the distributor drive gear cut at the angle that it is. I just throw this out there to make us think . Next time you have a camshaft in your hand , look at the distributor drive gear and see what you think. Could all the concern about camshaft endplay be for naught ? As we all know from priming a new engine there is quite a bit of rotational resistance to the oil pump. I believe this is the reason that manufacturers have gone to recommend using 5W30 oils . Lower viscosity = less resistance = less power loss = better fuel mileage. Of course this is just my observation and open to discussion. Who knows, maybe the moderators will move this to the discussion forum.
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Old 02-17-2011, 10:13 AM
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You are correct that on a CW rotating distributor, the shaft will be 'pulled' down. A Pontiac will be the opposite.

But on coast-down, like when decelerating the engine, this downward thrust can be momentarily lost- and this will allow the cam to "walk" around, IMHO.
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Old 02-17-2011, 10:28 AM
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Quote:
Originally Posted by cobalt327
You are correct that on a CW rotating distributor, the shaft will be 'pulled' down. A Pontiac will be the opposite.

But on coast-down, like when decelerating the engine, this downward thrust can be momentarily lost- and this will allow the cam to "walk" around, IMHO.
Not to argue, but even on a coast down you still have the resistance of the oil pump. JMO
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Old 02-17-2011, 10:36 AM
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Quote:
Originally Posted by adantessr
Not to argue, but even on a coast down you still have the resistance of the oil pump. JMO
No argument at all, Big.

I look at the oil pump as placing a load laterally on the drive shaft, not up or down, if you follow me.

So, to my way of thinking, the oil pump would only tent to 'hold' the shaft in whatever position (up or down) the shaft was in, until a greater force was exerted on it- but not to drive it upwards or downwards by the oil pump itself.
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Old 02-17-2011, 10:46 AM
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Quote:
Originally Posted by cobalt327
No argument at all, Big.

I look at the oil pump as placing a load laterally on the drive shaft, not up or down, if you follow me.

So, to my way of thinking, the oil pump would only tent to 'hold' the shaft in whatever position (up or down) the shaft was in, until a greater force was exerted on it- but not to drive it upwards or downwards by the oil pump itself.
For discussion sake, once again you have the same angle cut on the distributor gear, so as long as there is rotational resistance, the angle on the gear would provide the downforce. If the gears were cut at the opposite angle the camshaft would be held forward and the distributor upward. Also if the distributor was on the other side of the camshaft. We are combining the rotational resistance of the oil pump with the angle cut of the the distrubutor gear and the distributor drive gear, which holds the camshaft thrust to the rear of the engine and the distibutor shaft down . Just my observation.
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Old 02-17-2011, 11:13 AM
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In the case of Chevy V8's, the rearward thrust of the cam- when it's present- is checked by the thrust surfaces of the cam gear and block. On a Pontiac for example, (right side/opposite rotation of the dist.) there's a cam retainer plate used to counter the forward thrust of the cam.

But it's mainly when a Chevy V8 has a roller cam (with "non directional" lobes) that the cam can (and will) walk during periods where there's insufficient force applied through the cam/distributor gears to prevent it, or on any cam when the vibrations/harmonics are such that they overcome the other forces holding the cam rearward.
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Old 02-17-2011, 11:27 AM
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How do you attach a cam button on a SBC when using a flat tappet cam? Is there one size or type?
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Old 02-17-2011, 11:27 AM
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Quote:
Originally Posted by cobalt327
In the case of Chevy V8's, the rearward thrust of the cam- when it's present- is checked by the thrust surfaces of the cam gear and block. On a Pontiac for example, (right side/opposite rotation of the dist.) there's a cam retainer plate used to counter the forward thrust of the cam.

But it's mainly when a Chevy V8 has a roller cam (with "non directional" lobes) that the cam can (and will) walk during periods where there's insufficient force applied through the cam/distributor gears to prevent it, or on any cam when the vibrations/harmonics are such that they overcome the other forces holding the cam rearward.
It has been a very intersting and enlightening discussion and I have learned a little in the process. Now it is time to take the T-bucket for a ride. It is sunny and the temp is 69 . Not bad for Feb in WV. Have a great day ...Allan
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Old 02-17-2011, 11:37 AM
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Quote:
Originally Posted by Dougie
How do you attach a cam button on a SBC when using a flat tappet cam? Is there one size or type?
There are actually several types of cam 'buttons': aluminum and steel roller type, some w/a torrington-type bearing, others w/a simpler design. Simpler yet are the nylon buttons that are a simple "bumper", to name a few.

Using a button on a FT cam isn't something I'm familiar with doing, but there are two notable members here who do this, apparently routinely. Hopefully they will fill in the necessary details, if they differ from how a roller cam button is used.

I suspect the set-ups are similar for a FT cam as they are to a roller cam, but I do not know if the clearances differ, etc. For one thing, you would want a timing cover that gave a deflection-free inner surface for the button to register to.

Good luck.
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Old 02-17-2011, 12:39 PM
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The real reason for a cam button is to prevent unintentional timing variations. If there is any forward/aft motion in the cam while the engine is running, this will cause the timing of the distributor shaft to vary. The button, combined with a thrust surface or bearing on the face of the block, eliminates the "slop" that can allow this motion and thus prevents erratic timing.
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Old 02-17-2011, 01:37 PM
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I can tell you this for sure...The gear thrust is not enough to prevent cam movement, and I have a set of Isky solid roller lifters and cam from an engine put together by someone who's knowledge of this was lacking(wasn't me, I just got to fix it ).

The cam moved forward enough for the cam lobes to come in contact with the side of the adjacent roller lifter...with disastrous results. The struts of the lifters that hold the pin and roller wheel are chewed up and ground away .040" or so, and the cam is damaged along the side of all the lobes. It put a lot of metal shavings in the engine.

The thrust button used with a flat tappet cam is the same as a roller cam, fits into the center hole of the cam gear against the face of the ends of the cam. The small clearances to the front cover keep it from falling out once the cover is installed, but a cam bolt lock plates' center hole is also sized correctly to form a retainer for the button and will prevent it from falling off when the cover is not present.

Aluminum and nylon buttons work the same way, they are just a simple bumper. A roller button is just slightly fancier but really unnecessary as the friction present in an oil bath under the front cover is pretty minimal, I have never seen a aluminum or nylon button wore out. I'm more worried about the tiny torrington bearing needles in the roller button failing due to a material or manufacturing defect and polluting the engine, and so I don't care for the roller bearing buttons.
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