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Old 03-10-2008, 11:19 PM
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Camshaft moves too much - SBC 400

I'm building a mild SBC 400 and I noticed that the camshaft can move about 1/4" front to back with the timing cover on. I discovered this while installing the distributor. As the dizzy gear contacted the cam gear, it pushed the camshaft forward. After the dizzy was installed, I could move the camshaft front to back by turning the rotor.

I put on a new timing set, and everything is lined up there and the chain is snug, it's just that the cam can walk out from the block while pulling on the chain. Has anyone ever seen this? Will it stay aligned in the block while running, or do I need a button on it? BTW, the cam is not a roller.

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Old 03-10-2008, 11:28 PM
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You do need a button, easy to find at Summit, Jegs and some parts houses.
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Old 03-10-2008, 11:33 PM
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I just noticed that the timing chain will pull the cam back to alignment if I rotate the engine 1/4 turn or so.
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Old 03-10-2008, 11:40 PM
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Quote:
Originally Posted by 68NovaSS
You do need a button, easy to find at Summit, Jegs and some parts houses.
I've seen the buttons before. I have never noticed this before, but this is my first GEN I build. I've built an LS1 and a few SBFs.

My question then is this: On any SBC, doesn't the cam move a bit if forced front to back? The timing chain gives a bit, and the plug at the back should not come into contact with the cam. That means that any camshaft without a button should be able to move until the timing chain stops it. True or false?

If false, something is wrong with my build. The timing set is new, and the chain is tight. The timing gears are also lined up.
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Old 03-10-2008, 11:59 PM
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False, sorry, but it's not a problem with your build. Crane recommends .005 -.008 thrust with their buttons. You have .250, the chain isn't designed to hold the cam thrust and the dist push will wear the links in no time.
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Old 03-11-2008, 05:48 AM
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If this is a non-roller cam the taper on the cam lobes and the offset of the lifters will actually force the cam into the block. The only thing offsetting this load is from the oil pump. Stock pumps and conventional weight oil isn't much of an issue. High volume pumps with higher viscosity oils will tend to push the cam some.
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Old 03-11-2008, 06:11 AM
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Quote:
Originally Posted by engineczar
If this is a non-roller cam the taper on the cam lobes and the offset of the lifters will actually force the cam into the block. The only thing offsetting this load is from the oil pump. Stock pumps and conventional weight oil isn't much of an issue. High volume pumps with higher viscosity oils will tend to push the cam some.
Thank you! I just found this at Comp Cams as well:

"Flat tappet cams are ground with taper on the lobes to force the cam to the rear of the engine. Roller cam lobes are ground flat, so a thrust button must be used to keep the camshaft to the rear of the block."
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Old 03-11-2008, 09:48 AM
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don't use a button for flat cams...learned that the hard way
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Old 03-11-2008, 11:40 AM
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Quote:
Originally Posted by BBCMudbogger
don't use a button for flat cams...learned that the hard way

Why not?... if you set them up correctly the button is extra insurance also if you are running a gear drive timing set a button is mandatory regardless of what type pf cam it is.
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Old 03-12-2008, 05:43 AM
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For a few dollars, why would you not install a button? I'd be paranoid not to, having used them for so many years. What happened that you
Quote:
Originally Posted by BBCMudbogger
don't use a button for flat cams...learned that the hard way
that you 'learned that the hard way'?
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Old 03-12-2008, 10:08 AM
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It made the cam too tight against the back of the block. I guess thats what ground the bottom off of a few of the lifters.
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Old 03-12-2008, 10:53 AM
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Oh yeah that will happen... that is why you have to check camshaft endplay when you set up the button. It is not just a bolt it in and go type of deal.
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Old 03-12-2008, 11:11 AM
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Quote:
Originally Posted by Dino308gt4
I'm building a mild SBC 400 and I noticed that the camshaft can move about 1/4" front to back with the timing cover on. I discovered this while installing the distributor. As the dizzy gear contacted the cam gear, it pushed the camshaft forward. After the dizzy was installed, I could move the camshaft front to back by turning the rotor.

I put on a new timing set, and everything is lined up there and the chain is snug, it's just that the cam can walk out from the block while pulling on the chain. Has anyone ever seen this? Will it stay aligned in the block while running, or do I need a button on it? BTW, the cam is not a roller.
Chevy uses, or used, a taper on the lobes that caused the lifters to press the cam toward the rear of the engine. This took up the clearance between the thrust face of the cam's timing gear and the block. It's not a reliable process which has resulted in a many failures over the years, enough that GM has been Class Action sued several times over this issue. I might add they lost all of these suits. The advent of the roller cam came on the heels of the last and biggest suit. So the need to take ZDDP and other such similar high pressure lube compounds out of engine oil for reasons of extending catalytic converter life coincided with the need to do something about the bouncing cam problem that ate lobes and lifters. Ford went through this with the FE back in the late 1950s. But they didn't ignor the problem like GM, they went first to a production thrust button in mid 1958 and finally threw the whole Mickey Mouse solution out in 1962 and went to a thrust plate.

With todays low HP additives in motor oil, ZDDP being an example of organic- metallic compounds removed, the problem of lobe to lifter wear has grown. The aftermarket manufacturers are using better materials to counter the problem, but from an engineering standpoint using tapers on lobes to force the already pretty busy lifters to provide thrust control is really more like wearing gloves because your pen leaks. It just doesn't address the root cause of the problem.

I highly recommend using a thrust button to take this fore and aft load off the lobe and lifter interface. However, the thrust button is not an easy thing to set up. extremely careful attention to details must be paid to arrive at both the correct clearance and a reliable method to maintain that clearance over time. The former is an issue of initial set up which requires following the manufacturer's instructions very carefully. This may require you purchase a set of long feeler gauges so you can check this clearance from the bottom of the timing cover with everything but the pan installed to the engine. You can GOOGLE these things. The latter is an issue mostly of support and long term adjustment. The typical sheet metal timing cover is not strong enough to provide long term support, it will deform which usually opens the clearance, but it also can be deformed inward to reduce or eliminate the clearance. Although this condition places a preload on the cam gear to block interface it cannot cause the damage that another contributor suggests elsewhere in this blog until or unless the preload causes the cam gear to block interface to cut material from either part which would then allow the cam to move backwards and side load the distributor drive gear if sufficient preload contined to be applied. A common means of insuring the timing cover doesn't deform is to replace it with a cast aluminum part which is stiff enough to provide long term support. Or welding a piece of metal to the outside of the area where the cam button rides to stiffen the sheet metal cover. Or using a bolt that screws into a boss on the bottom of some water pumps for this purpose, where the bolt is extended to just touch the timing cover preventing it from distorting. The bolt is then locked in place with a binding nut.

BOL
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Old 03-12-2008, 01:41 PM
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Good info! Thank you all.
I'm waiting for my carb to arive, then I'm going to fire it up. I'll let you know how it goes.
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