Well after taking apart a good running 312 for refresh I find the block is overbored 80 thou and has had a crack repair near number2 main journal ( not into the stud holes) Repair is good and has been fine for the 12 years I have run the car. So, decided to use block and sleeve it back to standard. Ordered loads of new parts at great expense. Everything cleaned, painted and ready to assemble, crank ground, new wrist pin bushes, new everything! First job, fit cam bearings. Went in nice, cam wouldn't even enter. Knocked bearings out, got another set. Fitted middle one first. Great cam goes in nice. Number 4 next......tightened up cam wouldn't go in. Knocked it back out, measured cam bearing bore......oval. Damn. Not an ideal introduction to the joys of building Y blocks.

I have had simler isues even in fe engines. my solutio is a have cam that i ground a grv in several bearing portions then rotat the cam into the tight bearings,usualy onley takes a skim to corect minor isue,which most likley caused by cam bearing being dmaged or slightly cocked.
 i am sur ther will be those that question this solution. has worked well for me.

Yeah, Ive cut grooves in the old camshaft to use as a reamer but unfortunately the old cam is 10 thou smaller on the journals than the new one. It just falls in! I think I have inherited a problem that somebody has tried to solve in the past by grinding the cam journals down. The problems just never end with this engine! Ideal solution would be to find another block, but there are none here in the UK and the cost of shipping one over from the USA is prohibitive.  

I think Ted has said that many 312 have that problem and the solution is to line bore the cam tunnel and bush the bores back to the proper size or have custom bearings made, something like that.

So with a 312, in addition to looking for cracked mains, you need to look for a straight cam tunnel.


Lawrenceville, GA

I can get the cam tunnel line bored but would have to ship the job out. This engine is a pain in the ass!!!  I pulled the motor on the 13th of February and I'm still nowhere near getting it built. 

It's definitely not the way most Y-Blocks are.

Since you already have so much in that block, sending it out is probably the best option.

The best alternative to that is to find a good 292 block and have the mains bored or to fit the crank or the crank turned to fit the block, with appropriate action taken with the rear oil slinger, for whiscruber option you choose.



Lawrenceville, GA

As one Mike to another, and understanding you're in the UK. I bought a running, recently rebuilt 312 as the basis for my F code motor. When we tore it down, we found a repair on one of the main caps, and the crank didn't pass the mag check. Ran fine cracked, but wasn't going to be a stroker with 8lbs boost.

The repair on the block was good, and I went to main studs to be sure. Been running 9 years with the blower, and a new crank, offset ground to 340cid.

Take a deep breath, and wait for Ted or John to chime in. Or make a phone call. My other stroker motor is a 292 block, with the 312 crank turned down to fit. The rear main is much less of an issue going that way. But they may have a way to salvage that block's cam journals. I know summer is short over there, like Seattle. Good luck.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Thanks Mike, yeah we get 4 days of sunshine for summer, it's June and it hasn't stopped raining for days. Just trial fitted the crank and all is fine so I'm guessing the ovality in the cam bearings is not a distorted block problem. It's only number 4 that is affected, I'm pretty sure we can sort that out .

I do run across a number of 312 blocks with cam tunnel issues.  Old school machine shops use to simply carve on the cam bearings until the camshaft turned freely within the bearings.  Not the best fix but it was the accepted fix at the time.  Those cam tunnel issues range from out of round holes or to holes that are not in alignment with each other.  Fords’ original fix was an undersized cam bearing that was installed and then bored or aligned honed to the correct size.
 
Why this problem shows up on 312’s and not 292’s is subject matter for another discussion.  One guess is that the 312 blocks were in such a demand that they were machined too soon after casting for the iron to take a set.  Another possibility is that the larger main bore size effectively weakens the block in the cam tunnel area to the point that some distortion takes place.  Regardless of the reason, the 312 blocks seem to be plagued with this problem while 272 and 292 blocks are not.
 
I have a fixture here that helps to identify those blocks with cam tunnel issues during the tear down process.  For the more serious 312 cam tunnel problems, the cam holes will be bored to 2.182 ±.0005 which is the 1954 cam tunnel bore size.  Cam plugs are available for this.  New cam bearings (brass) are fabricated which restores the block for use with standard journal cams.  While the normal clearance for a babbit cam bearing would be in the 0.003” range, the brass cam bearings require a clearance of 0.006-0.008”.  No issues with oil pressure as the oil hole is being pinched at the bottom of the journal anyhow.

Lorena, Texas (South of Waco)

Thanks Ted. What a pain in the ass this is turning out to be. Had another look at it today, number 1, 3, 5 bearings are fine, number 2 is out of alignment and no 4 is 6 thou oval. Gonna ship it out for line boring but where can I get oversize bearings?