8ba rear mains

B

bigfeller08

Junior Member
hi members , i have had a problem with re-building my 8ba v8 , motor , will try and make it as short as possible being a self taught back yarder , i had a go at FULLY rebuilding my motor , new ground crank , 80th bore ,new piston etc, ,WHEN I GOT TO PUTTING THE MOTOR TOGETHER , IT FELT HARD TO TURN OVER ,EVEN BE FORE I PUT THE PISTONS IN , THOUGHT IT STANGE , AS I HAD ALL NEW MAINS ETC, ANY WAY WENT A HEAD ,GOT IT RUNNING , ,FOR A COUPLE OF HOURS , THEN SUDDENLY IT STOP WAITED FOR AWHILE , RE STARTED IT , AND RUN OK , ,WAS NOT HAPPY SO I PULLED IT DOWN AGAIN , AND FOUND THAT THE REAR MAIN BEARING WAS STUUFED , TOOK IT TO THE MACHINE SHOP , AND he said that the main seal was to tight , and that it heated up and cooked the main , i have never heard of this , ,SO , HAD THE CRANK CHECKED AGAIN , AND HAVE TO ORDER NEW SET OF MAINS , ,can some one help me on this question , as i dont want to stuff up a new set of mains , , front and middle mains ere ok , just the back one , need help guys ,

cheers BIGFELLER 08,
 
Put it togetter without the rope seal in it...if it doesn´t turn over easily the block is either out of alignment....unbolt one maincap at the time and see if it spins freely with one loose.
Or the crank was ground to big for the bearings.
The rope seal is real tight when installed...the new teflon style is a bit better...don´t forget some lube on it when assembling...so far never had one that injured the crank/bearing...what can happen is they burn up and leaks when installed dry....oil should cool the bearing if clearance is ok...was it a rear main bearing with oversize flanges by chance ?
Or some modification to the oiling system not letting the rear mainbearing get enough oil ?
To little axial play in the rear bearing gives you to low oilflow/cooling to...
 
What were the bearing clearances when you put it together? (You did check them, didn't you?) Never assume! Years ago I saw an engine put together with a fresh .010" under crank and bearings stamped .010. It knocked. Turned out the bearings were actually standard. Mistakes happen!
 
Plastigauge is the best way to check your clearance on the bearing shells without damaging the soft surfaces. That would give an accurate clearance on each bearing set.

The Best brand gasket sets have a graphite/teflon seal that works well if properly packed & trimmed. The originals work OK too but all have to be properly installed or they will be troublesome. The old ropes have to be soaked in oil for a while and then packed in the retainers. Sometimes they are too long and have to be trimmed a bit or they will be too tight. I think the Best brand ones have a cardboard trimming guide that is included with them. This is one chore that is not explained very well in any of the overhaul manuals. It can be a PITA. The Lincoln 337 engine had a special tool to seat & trim the real main but the 8CM merc engines didn't.
 
I bought a '51 Mercury motor a year or so ago. One piston was stuck so I disassembled it. It had been "rebuilt"--I think just the crank ground, pistons knurled and new rings, and didn't appear to have been run much BUT the side of the rear main were scored pretty bad. So..... I assume the clearance/end play wasn't checked when it was put together and it was too tight, which scored the thrust portion of the bearing. Which raises the question: when the end play on the crank is too tight, what is the solution????? Do you find a different bearing shell or sand/file down the one you have to reach the proper clearance? Rod
 
With the crank out on the bench, you can trial fit a thrust type set of bearing shells to the main journals and see what kind of preliminary fit you might have. Then put shells in the block & set the crank in with plenty of oil to check for play & smooth rotation with no caps. If the crank has been ground a person needs to look at the radius at the ends of the journals. Too much radius could cause an end clearance problem. A warped crank can also cause problems so make sure it's concentric too. It's good to know if it is a crank problem or a bearing shell problem first. The shells can be surfaced some but be careful to keep the work concentric and don't take off too much. The thrust faces are as soft as the journal bearing faces so use some fine grit paper with light oil on a surface plate or plate glass to hone them a bit. You may be able to hold them with the main cap but that may not be necessary. The Lincoln Mercury O/H manual states .004" to .008" for end clearance on the crankshaft. A crank grinder could take a little more off the thrust face on the crank but a person needs to know the initial clearance first before doing any machine work. The crank should have been right before it left the factory but you never know.

Once a person puts the main caps on and torques it up, it should still rotate but the design of the rear seal should have some friction to it. That's hard to gauge what the torque to turn should be but it should still turn with out too much rolling torque. It may take more torque to get it rolling but that is normal. Once the first piston is in then hand rotation decreases to nil pretty quick due to ring friction.
 
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