A rant about cam buttons for retrofit roller cams
10-18-2014, 04:57 AM
#21
That's a very interesting tech note, and the explanation makes sense. Apparently the theory is that improper machining of the lifter bores (making then other than perpendicular to the cam) can create a force that pushes the cam either forward or backward, depending on which direction the error was made. Rearward bias is seldom detected unless the force is high enough to wear out the thrust surface between the cam gear and block, but forward bias can push the cam forward.
The statement that this force "can easily create several hundred pounds of cam thrust pressure" sounds a little scary.
My impression is that most cam buttons, especially the solid nylon ones, are not intended to run under continuous load. I think the intent is simply to limit the cam's forward travel during a brief forward bounce.
The Isky tech note implies that the mis-aligned lifetr bore condition is pretty rare, and it recommends junking any block that tests positive for it. So, that still leaves us with the case of a correctly machined block. For a correctly machined block, I remain perplexed about what mechanism would push the cam forward, particularly since the oil pump is actaing to push the cam rearward.
For my build, I used a solid nylon button because I didn't want the risk of a needle bearing version coming apart inside my engine. So, my nylon button should be fine for an occasional forward bounce, and hopefully that's the most it will ever see.
The statement that this force "can easily create several hundred pounds of cam thrust pressure" sounds a little scary.
My impression is that most cam buttons, especially the solid nylon ones, are not intended to run under continuous load. I think the intent is simply to limit the cam's forward travel during a brief forward bounce.
The Isky tech note implies that the mis-aligned lifetr bore condition is pretty rare, and it recommends junking any block that tests positive for it. So, that still leaves us with the case of a correctly machined block. For a correctly machined block, I remain perplexed about what mechanism would push the cam forward, particularly since the oil pump is actaing to push the cam rearward.
For my build, I used a solid nylon button because I didn't want the risk of a needle bearing version coming apart inside my engine. So, my nylon button should be fine for an occasional forward bounce, and hopefully that's the most it will ever see.
My thinking is that the stock type cast iron camshaft gear along with the oil sling from the timing chain is sufficient to lube this surface.
I am curious about everyone's thoughts, I would not want to modify and install any roller bearings etc...
10-18-2014, 02:25 PM
#22
Drifting
Thread Starter
It seems to me that for a motor like you built you got it covered for the camshaft forward movement, how did you address the rearward movement.
My thinking is that the stock type cast iron camshaft gear along with the oil sling from the timing chain is sufficient to lube this surface.
I am curious about everyone's thoughts, I would not want to modify and install any roller bearings etc...
My thinking is that the stock type cast iron camshaft gear along with the oil sling from the timing chain is sufficient to lube this surface.
I am curious about everyone's thoughts, I would not want to modify and install any roller bearings etc...
I think rearward motion is already well controlled by GM's design that uses a machined surface on the back of the timing gear to bear against a machined surface on the front of the block. Millions of Chevy small blocks did just fine with this arrangement for 30 years of production.
If you are starting with a used engine and these two surfaces look fine, there should be no need to upgrade to a roller bearing.
Note that if anything, the rearward force on the cam becomes *lower* when you substitute a retrofit roller cam into a block that previously had a flat tappet cam.
I suppose that using a high-volume and/or high pressure oil pump will increase the load on this bearing surface compared to using a stock-type oil pump, but I'm not sure how to quantify the change.
BTW, an explicit bearing that is added between the timing gear and the block does not have to be a roller bearing, and it does not have to require machining of the block. While I did not realize it at the time I ordered it, the Cloyes 9-3600TX3 True Roller timing set that I bought comes with an explicit bearing that fits into the back of the timing gear. This bearing is not a roller bearing. It is a free-floating ring with a special bearing material bonded to the surface.
Note that most of the double-roller timing gears on the market (whether or not they have a separate bearing) use a chain that is wider than the factory chain, and usually a little bit of grinding needs to be done to the bosses of the oil gallery plugs to clear the wider chain. This is easily accomplished with a hand grinder.
10-20-2014, 10:30 AM
#23
Melting Slicks
When I built my 406 I also read all those 'opinions' cited by Gearheadjoe and decided that regardless of what some 'experts' believed, the best course of action was a button. And since I wasn't trying to retain an exact appearance, I simply used an aluminum timing cover to handle whatever load was applied by the button.
I did find through this exercise that one does have to be a fairly experienced mechanic/machinist to do this properly and can imagine many 'builders' skipping this step altogether. It takes a bit more skill to properly set up camshaft endplay that it does ring gaps and bearing clearances.
I did find through this exercise that one does have to be a fairly experienced mechanic/machinist to do this properly and can imagine many 'builders' skipping this step altogether. It takes a bit more skill to properly set up camshaft endplay that it does ring gaps and bearing clearances.
