five7kid

iTrader: (14)

The first question was https://www.thirdgen.org/techbb2/sho...hreadid=337339 . Looks like the XS282S solid flat-tappet gets the nod for the 396.

Now, I've been using a double roller timing set with the roller thrust bearing since the engine was rebuilt back in 1997. At the time, I had to get the bearing separate and have the timing gear machined to accept it. I replaced the cam in 2002, at which time the timing set looked fine so I just put it back together. This time, I think I should go ahead and replace it for insurance sake.

These days, timing sets are available with the thrust bearing such as pictured below. I've heard varying stories about the wisdom of using the thrust bearing, some saying it's a very good idea on BBC's, others saying why bother, to a few saying the cam button required for it is not a good idea if you don't need it (roller lifters needing it, for instance).

What say you?

Damon

I've never used a torrington/roller bearing behind and of the timing gears I've installed with ANY flat tappet cam, big or small Chevy. I don't see any harm in going that route if you like, but I wouldn't call it "necessary" by any stretch of the imagination.

I also don't use cam buttons with flat tappet cams, either.

F-BIRD'88

On a flat tappet cam the cam lobes are ground with a slight taper. The lifters sit slightly off center on the tapered lobe. the lifters are also made with a slight crowned face.
This makes the lifters spin in their bores.

The cam has to float free in the block with no thrust preload, Like you would have with a thrust bearing behind and a cam button in front of the timing gear.
The lifters' offset and the cams' tapered lobes find their own sweet spot only if the cam floats in the motor otherwise.

The lifters slight crown face and the cams slight taper keeps the cam from walking in the block. Don;t need a thrust bearing or a cam button.

The cam button and thrust bearing are meant for a roller cam not a flat tappet cam. A flat tappet cam does not suffer from "cam walk" like a roller does.

A thrust bearing will interfer with the flat tappet cam and flat tappet lifters finding their own center. you don;t need or want a cam thrust button or a thrust bearing for a flat tappet cam. Leave it out.

Sell it to someone with a roller setup.
Was any material machined off the front of the block to accomodate the thrust bearing?

five7kid

iTrader: (14)

The block wasn't touched for the thrust bearing install.

As evil as the thrust bearing and cam button are with a flat tappet cam, makes one wonder how it's survived these 9 years through the thousands of miles and hundreds of 1/4 mile runs.

Guest

The taper of the lobes forces the cam to walk, to the back of the block. The timing gear prevents that, by rubbing against the block. What you'll save by a bearing instead of direct friction? Probably not much. I've also never seen a timing gear get so much wear that it becomes a problem. I suppose if its apart, available instead of machining, and you feel the urge... it cant hurt anything.

F-BIRD'88

probabily because there was not thrust preload between the timing gear and block that would have caused the cam to not find its natural center in the block (cam/ tappet face alignment.) Of course witout any bearing prelead the bearing is just along for the ride. (roller bearings need some preload) I guess you just got lucky.
When you get your new normal timing gear just leave the bearing out.

five7kid

iTrader: (14)

That describes the majority of my successes in my life, I suppose.

I would think manufacturing tolerance differences between bores and lobes would have more of an influence on the lifters not finding their own centers - even if the cam "walks", they'll be 16 lifters & lobes competing for their individual rights.

The theory I recall with the cam button has to do with timing accuracy - if the cam stays put, the helical gears of the cam & distibutor will always run in the same place, and not vary with the axial location of the cam. It also saves wear on the timing chain by not allowing movement that it isn't designed to withstand. Although the thrust bearing could be argued as not necessary, the lifters and distributor do push the timing gear against the block. Overkill, perhaps, but cheaper than a new block (I forget where or when I heard the story about that happening - probably out of date or a mis-diagnosis).