BBC cam alignment

[Caveman Tony]

So, this question is geared towards flat-tappet cams, but it could apply (to a certain extent) to rollers too.

Q: How do you know that your cam is aligned properly, front to back? I.e., do your tappets ride on the center of the lobe, or a few hundreths towards the front or back of the lobe? Which is correct?

- rear cam freeze plug can screw this if installed too deeply

- shims under cam gear? Anyone do this?

- I've heard of some guys putting a straight edge on the cam gear & crank gear for chain alignment, but that still doesn't guarantee lobe-to-tappet alignment

- I know flat-tappet cams are "pushed" towards the rear of the block, iirc, does that even matter in a stock/street performance build?

- Should I care? Or just slap it in and break it in?

[Scott Liggett]

My bbc lifters ride towards the back of the lobe. I dont think they are supposed to ride centered as they are spinning on top of the lobe.

I tapped by my cam freeze plug in and then slapped the cam in. If the timing chain isnt rubbing the block or the timing cover and its aligned over the crank gear you are golden. This isnt about getting the exact placement. GM didnt have that kind of tolerences on these engines.

The break in is more important. If you have a radical cam with stiff springs, I would get old stockers for the break in. Getting the engine started quickly, not cranking a bunch, and getting it up to 2000 rpm and keeping it there for twenty minutes is the most important. The engine doesnt have to run perfect. The timing could be off, whatever. If there are leaks cant fix while its running, shut the engine off. I ran one sbc with two wires off and the 5-7 mixed up. Didnt matter.

[squirrel]

With a normal timing set (for a non-roller cam), the cam sprocket rubs against the block, and that controls front-to-back location of the cam. Install the cam plug flush to the block, and it should not touch the cam.

Scott covered everything else very well.

[CDMBill]

With a normal timing set (for a non-roller cam), the cam sprocket rubs against the block, and that controls front-to-back location of the cam. Install the cam plug flush to the block, and it should not touch the cam.

Scott covered everything else very well.

Not knowing BCC's, is there a stock specification for cam walk or thrust clearance? If so that is the specification I'd look to have during installation. You would check that without the chain in place but with the cam gear seated properly. Looking at the cam you remove s a good time to see if the cam bearings are in good shape. A lot of builders install the cam first when its a complete rebuild in order to be able to check lobe clearance, proper height for the lifters in the oil gallery band etc. IIRC for wach cylinder BBC's have one lobe that where the lifter is square to the cam centerline and one where there is slight angle which creates different side loads on the lifter bore; all good stuff to look at if there is an opportunity to do so and catch potential problems before they manifest.

[squirrel]

There is no spec for cam walk or clearance, because there is nothing limiting how far it can move forward. (aside from running into the timing cover, which they never seem to do)

The lifter bore alignment is kind of crappy from what I hear, which is probably one reason why they eat flat tappet cams so well.

[CDMBill]

Does the thrust form the cam gear/distributor gear/ oil pump drive pull the cam back making the rear freeze plug the primary cam walk limiter? Sorry for the hijack, just curious as to how things are done in the other camp. BBF's like most muscle ear Fords have a cam retention plate that bolts to the block between the cam and the cam gear.

[squirrel]

The taper on the cam lobes is what keeps it pulled back. the cam sprocket has a thrust surface that rides on the block. There is no thrust plate in there.

Later engines with the roller cam did use a thrust plate.

[anotheridiot]

dont over think it. The distributor shaft and oil shaft keep track of the rear, the timing chain or gear in the front. if you run a gear drive a nub on the front will keep if from going too far forward. you can center the cam dead nuts in the middle, but as soon as you put the timing chain or gear on the front and drop the distributor, the cam is going to go where it belongs.

[Mr4Speed]

I had a block that had the cam thrust area torn up. Got it machined flat and used a Crower Tru-Roller set that had the back of the gear machined to accept a torrington bearing and trust washers. IIRC it came with an assortment of different thickness thrust washers for proper alignment. I also put a cam button on it just to make myself feel better.

[Caveman Tony]

Ok, here's a wrinkle:

Squirrel, et al, chime in here if you think this even matters:

The block is a '91, which originally came with a roller cam.

All the references I read said all Mark IV, Gen V/VI blocks can use either roller or flat tappet, so I just went with a flat tappet.

That should have zero importance, correct?

OR did GM place less importance on machining alignment for roller tappet blocks vs. 'back when' flat tappet blocks? Could that have killed the lifters?

OR was it just shitty break-in?

[A/Fuel]

I had a block that had the cam thrust area torn up. Got it machined flat and used a Crower Tru-Roller set that had the back of the gear machined to accept a torrington bearing and trust washers. IIRC it came with an assortment of different thickness thrust washers for proper alignment. I also put a cam button on it just to make myself feel better.

I just worked on an engine that had an issue where I think the cam had too much play. First of all it broke 3 intake adjusters on two passes with the lash caps. We just dug out all the parts. The second time there was part of an adjuster missing, I thought maybe it ended up in th oil pan. When we tried to lash the valves, #1 would be tight everytime, like a lifter was sticking, but it kept happening.
We had some time to take the intake off, and found the block was cracked right over the cam with a big bump. The adjuster found it's way in there, but while we were looking the cam was sliding back and forth like the torrington bearing and all the shims were missing. We took everything off of the front to see and it was still there. The cam was moving so much you could hear the mags click and because it has a gear drive the cam was rotating. That engine ran horrible from the get go. I know the guy that put that engine togather. It had been taken out of service because it chewed a lifter up, and he was spicifically asked to check out the end play and do what ever he had to do to fix it. By then we had no choise but to take it out and put another one in. It had to effect the cam opening and could be one of the reasons it didn't run good.

[squirrel]

Roller cams need thrust control. Flat tappet cams generally don't (at least not if it's a big block chevy)

On the factory big block roller cams, there is a thrust plate that fits over the end of the cam. The cam has a step at the front end, so it can't move forward, and the cam sprocket prevents the cam from moving backwards.

If you have a block for a roller cam, I'd dang sure run a roller cam in it.

[anotheridiot]

Ok, here's a wrinkle:

Squirrel, et al, chime in here if you think this even matters:

The block is a '91, which originally came with a roller cam.

All the references I read said all Mark IV, Gen V/VI blocks can use either roller or flat tappet, so I just went with a flat tappet.

That should have zero importance, correct?

OR did GM place less importance on machining alignment for roller tappet blocks vs. 'back when' flat tappet blocks? Could that have killed the lifters?

OR was it just shitty break-in?

I think its less important for the flat tappets since the rollers are linked together and need extra machining done in the lifter area, but I've been wrong plenty before.

[Caveman Tony]

Come to think of it... I rebuilt this motor back in 2007... and when I tore the crusty thing down for a rebuild, the cam was a flat tappet. And it was flat then too.

I thought Gen V blocks were 1990-1995 and were roller?

Wikipedia doesn't specify, and the GM dealers I called said their computer system didn't specify. Didn't even have a photo. (...roll eyes *now*...) I spec'd a '91 454ci, which is what I thought the serial #s corresponded to.

Rock-Auto had a photo that showed a flat-tappet.

I'm gonna check the #s on the block when I pull it.

[Deaf Bob]

You gonna fly back out there, rebuild the motor and drive back to ?New England?