Technical Sbc undersized cam journal/thanks

Soooo, I can’t win!! After cracking my rebuilt 350 block I was able to get an ATK long block at a great price through a buddy. So I am swapping the heads and valve train from the cracked engine. The ATK cam journal is .010 undersized so my cam will not fit in the bearings. I cannot find anything on the net about an undersized cam journal so here I am reaching out to the experts. The ATK cam journals are 1.858 and all my searches come up with sbc journals being 1.868. What do I do????? Thanks, Dale

 

I’ve heard of this before. Why it happens? I don’t know. But replace them, or have a shop replace them.
I’d guess the only other option would be cutting the cam down-probably more expensive.
Other than that issues, all good with the ATK in whole?

Edit: Oh wait, you said journals, or did you mean the bearings themselves?

 

Sbc's have 3 different sized cam journal bores. If they got one of the larger OD bearings pressed in to one of the smaller OD bores. The ID bearing bore will be undersized for that journal.

 

Makes sense, at least I’m finding this out now before I have the engine in the car and decided to change it out later. Thanks!!

 

Soooo, another setback. Decided to change the cam bearings out to make cam changes for the future easier. Pulled everything apart and the number 5 bearing is different than the stock sbc one!!!! Anyone have any insight on this???

 

You said you obtained an ATK long block. Typically, (I’ve only been doing this for 34 years), long blocks come with a camshaft and valve train installed. If the cam bearings measured sizes other than stock, what did ATK install for a camshaft? Did you happen to measure the journals on the ATK cam, or did you measure the inside diameter of the ATK camshaft bearings? Secondly, since you mentioned the #5 cam bearing from ATK is different, why not knock it out and install a stock one?

 

How does changing cam bearings make cam changes in the future easier?
Different as in the second oil hole?
What block is that?

 

That is or how the 1955 & 56 265’s rear cam bearing look like. Why it’s there?

 

Yes it came with what I assumed to be a stock valve train. I just wanted to upgrade the cam a bit and hit the hurdle of the journals being undersized .010. If I can just replace number 5 with the stock one I’m ok with it, I just don’t know enough to feel comfortable doing it so that’s why I’m here.

 

Yes the second hole and the bearing is wider. The bearings are .010 oversized to fit the undersized journals on the cam. Instead of having my new cam turned down to fit I opted to change the bearings hence making future changes easier.

 

Hmmmm??

 

Here’s some pics of rear journal early vs late design.

Early

Later

 

Do you have the correct tool to replace the cam bearings? I suppose you could try using a regular bushing driver, but there’s a lot to go wrong doing it that way. The correct tool has a rubber cushion on the actual driver and a pilot for the mandrel.

 

From what I have found is that the replacement block seems to be a new casting from ATK so who knows what fits what? I guess I’ll knock out the bearings and see what lies underneath.

 

I ordered one through speedway as it will be probably the only time I will be doing this.

 

I think I would return the tool to Speedway and take the ATK to a reputable machine shop/builder to be sorted out.

 

Looking at the ATK website and their ATK#13ZB 350 CI Block machined and ready to assemble it states cam bearing bore is std. (1.8705” – 1.8710”). Seems your new cam with 1.868" Journals would fit with .0025-.003" clearance. I suspect some else put the wrong bearings in the wrong locations and then modified the cam journals -.010"? Pretty hard to imagine.

Taking it to a reputable shop is good advice in that they will have the proper measuring tools to inspect the block and insure you have the proper bearing press fit and final bearing clearances. Let us know how it turns out.

 

If ATK is casting their own SBC replacement blocks, it makes no sense (to me) for them to machine their blocks to accept anything other than of-the-shelf aftermarket SBC replacement bearings. Why would they machine their blocks for oddball cam bearing sizes then be required to turn down cam journals to fit their wonky bearing choice? As someone else mentioned, how does your replacement cam measure compared to the one you took out of the ATK engine?

I'd be verifying the origin of your block before moving forward. If it's a factory GM block, then there is something off with your cam bearings, if it truly is an ATK cast and machined SBC replacement block, then you need to talk to ATK for advice.

There is no way (I would hope) that ATK pressed the cam bearings in, in the wrong spots then wasted time machining the cam to fit, cam bearings are (or were) dirt cheap and it would have been easier for ATK to just punch them out and put in another set.

Does ATK even produce aftermarket blocks?

I just don't get it ... a SBC cam came out yet a different SBC won't fit back it? If someone just installed new cam bearings and the cam won't fit, THAT makes sense ... removing a cam and not being able to install another just doesn't make any sense. What am I missing?

 

Aftermarket blocks like Dart and Merlin have all of the cam bores the same size. Standard blocks have 3 different bearing bores, staggered in the block. Mis-matched block and bearings ? Did someone actually turn a cam to fit bearings ? Doubtful, it wouldn't be cost effective to turn and grind a SBC cam, probably the cheapest on earth. Mike

 

Trying to figure what I am missing too. The cam that came with the engine is .010 smaller at the journals than a stock cam.

 

As soon as I saw the 5th cam bearing, picture above, I knew something was off. I believe numbers 2 and 5 are interchangeable in a stock gm engine and these not. I can’t win!!!!

 

You can win. The path to victory is just going to cost a little more time and money. I would go through and check everything. Who knows what else they did. Very strange for sure.

 

So maybe a little light at the end of this tunnel. I assumed this wasn’t a gm block based on the small front pad with no numbers stamped in it? After running the casting number it appears to be a 77-79 350 and the block was probably decked eliminating the code on the front pad. I’m used to the long thin pad up front. Now that rear cam bearing still has me puzzled. Thanks for all the help!!!

 

So it's a GM 350 block...that's good. If the rebuilder is regrinding cams and is doing enough of them, maybe they grind the cam journals .010 under and have bearings made to suit. Not unheard of. If so, install stock cam bearings, slide in your new cam and all is well.

 

Did I miss something? Is it all the journals or just the rear one?

 

Factory block ... perfect. Punch out the existing bearings then install new ones made for your year of block. Do NOT dump the new bearings on the bench and just randomly start installing them, they need to go in, in a very specific order. It's been years since I've done SBC cam bearings but, if I recall correctly, the box they come in should state which bearing goes where.

Pay attention to the oiling holes in the bearings as well as they need to line up with holes in the block. Since the bearings you are removing don't seem to have been factory installs, spend some time Googling to determine the correct orientation of each bearing. When I did my SBC cam bearing installs, I remember using another block/factory installed bearings as my guide.

This ain't rocket surgery as I am pretty sure SBC engines are pretty popular

 

Quote:
"The ATK cam journals are 1.858 and all my searches come up with sbc journals being 1.868. What do I do????? Thanks, Dale"

This was in the very first post. Just seems like a rebuilder thing...like oversized valve stems so the guides can be reamed rather than replaced.

 

A little search to see if under size cam bearings exist…

 

Looking suspicious, isn't it? Thanks for the research, Johnny.