Looking for engine help ideas...

[cngodfather]

another issue. I have a howards retrofit cam. 91 gmc 5.7 block. The double roller timing chain I was going to use looks like it rubs the block if i install it. I am not sure what to get now.... any ideas?

 

[Schurkey]

Grind the block for clearance? What exactly isn't clearing the chain/sprocket?

The OEM timing set was probably a single-roller; might have been a narrow silent-link chain, but single-roller was used on the later engines.

 

[L31MaxExpress]

another issue. I have a howards retrofit cam. 91 gmc 5.7 block. The double roller timing chain I was going to use looks like it rubs the block if i install it. I am not sure what to get now.... any ideas?

Double roller generally will rub on the later blocks and need clearance grinding. The single roller chains will not. The 1-ton TBI engines use the single roller chain you need for the large/older bolt circle on the retrofit roller cam.

 

[L31MaxExpress]

Grind the block for clearance? What exactly isn't clearing the chain/sprocket?

The OEM timing set was probably a single-roller; might have been a narrow silent-link chain, but single-roller was used on the later engines.

The cast area on the later block where the cam retainer bolts are drilled and tapped. Basically the wide ears will rub the timing set without grinding.

Single roller was used on some flat tappet 350 TBI engines.

 

[cngodfather]

It is the cam sprocket. it hits the block.

 

[cngodfather]

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[cngodfather]

The cam sits flush with the block and bearing. The sprocket requires the cam to sit .125" into it. Is the cam to far into the block?

 

[Schurkey]

The cam sits flush with the block and bearing. The sprocket requires the cam to sit .125" into it. Is the cam to far into the block?

Yes. Pull the cam out where it belongs.

 

[cngodfather]

Ok. what is to stop the cam from sliding back to let the cam sprocket contact the block?

And another question, if the cam is able to move front to back, what is the point of setting a specific gap between the cam button and the timing cover? Seems like this measurement can be all over the place.

 

[Schurkey]

Ok. what is to stop the cam from sliding back to let the cam sprocket contact the block?

Nothing. The cam sprocket IS THE THRUST BEARING for the camshaft. That's why there's oiling grooves in the cam sprocket thrust face, and why the front of the block is machined smooth around the cam bearing. The sprocket is SUPPOSED to ride against the block.

With a flat-tappet cam 'n' lifters, the taper of the cam lobes acted on by the valve springs, rockers, pushrods and lifters, helps pull the camshaft rearward so that the sprocket is held against the block.

On any SBC with an oil pump driven by the distributor, the drag of the oil pump acting through the distributor gear also helps to pull the camshaft rearward, but with less force than the valve spring pressure on tapered cam lobes.

One of the really wonderful things about the OEM roller-cam system is that they use a machined thrust-plate bolted to the front of the block. The camshaft is absolutely controlled fore-and-aft to about .002; and the thrust plate protects the block so it cannot be damaged by excessive thrust. It's a really nice engineering solution compared to those stupid aluminum or nylon thrust buttons on the aftermarket system, that then has to have a reinforcement on the timing cover or a support from the back of the water pump. All that crap works--if set up properly--but the OEM system is unbeatable.

And another question, if the cam is able to move front to back, what is the point of setting a specific gap between the cam button and the timing cover? Seems like this measurement can be all over the place.

Roller cams don't have tapered lobes, so the roller valvetrain doesn't tend to pull the cam rearward; although the oil pump drag does, somewhat.

You need a gap so that the sprocket isn't "squished" between the timing cover and the front face of the block. That can cause galling of the block surface, and puts metal particles in the oil.

The gap between cam button and timing cover does nothing but assure that the cam cannot slide forward more than a few thousandths of an inch; while also assuring that the sprocket thrust face isn't overloaded by rearward thrust.

If the camshaft was sliding fore and aft more than a couple thousandths, the ignition timing would be all over the place as the cam gear teeth pulled or pushed on the distributor gear teeth in addition to merely turning together.