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Bored&Stroked · 08-31-2023, 07:32 PM

You're building a 1940 engine, with a 4" Mercury crank and 8BA rods. You do not touch ANY of the bearings with a rubber hammer! Since it sounds like you've already applied the hammer to potentially BOTH (rods and mains), you'll need new bearings for both.

The 8BA rods use insert style bearings - so there is no hammering involved. Each 1/2 shell should sort of snap into the rod cap (they can only go one way) and rod end and be snug - they will not be loose or move around in the least. The same goes for the mains - the rear one will be quite tight (sometimes I use a little wooden block to knock it down on the edges - never on the bearing surfaces).

On the rods, the chamfers on the edges need to be toward the outside of the journal - in case you need crankshaft fillet clearance. I haven't seen your rods or crank, so I don't know how things were prepped/machined. When in doubt, post some clear close up pics of a couple rod caps as well as the rod journals on your crank - then I can tell you more.

Checking Clearances: You should at least PlastiGauge the mains and rods to see what your approximate bearing clearances are. I like to have .001 or so of clearance per inch of journal diameter. You do this checking without the rope seals in place.

So .002 to .00225 on the rods and .0025 to .00275 on the mains. PlastiGauge isn't super accurate, but you should be able to ascertain if you're in the ballpark. Do not skip this step!

Tip: Take some heavy duct tape and cover the ends of the crankshaft main studs - that way when you man-handle the crankshaft into place (multiple times), you don't nick the bearing journals by hitting them on the sharp edges of the studs. The same goes for the rods - when you're installing the pistons/rods it is very easy for a rod-bolt to hit the crankshaft journal, making a nice ding in it. Protect your journals at all cost! LOL

It is critical that your bearing clearances are correct and that your crankshaft end-play is within spec (about .004 to .008 or so). When you use PlastiGauge, you do NOT put oil or lube on the bearings or journals and you must be careful not to turn the crank or move the rod when taking things back apart. Carefully take things back apart and use the little paper guide to see what your clearances are. I'd check all three main bearings and all 8 rod bearings - it is a good practice to be sure!

If they are too tight, you'll score and wipe out the bearings . . . to loose, you'll knock and then wipe out the bearings.

When you believe you have everything correct, install the main bearings (and the crank) and lubricate them well (again, without the rear seal plates). Then use 3 torque cycles to creep up on the torque spec on the main bolts/studs - and then see how the crankshaft feels - it should easily spin around just by grabbing the snout and twisting by hand. If is does not - do not proceed, something is wrong and you should not assume it will "fix itself" when you run it.

If the crank spins easily, then take it back out and put the seal plates in the block and the cap and reassemble it. Now you're ready to put in the pistons/rods. Lubricate the heck out of everything.

Okay, enough babble out of me for now . . . I'm sure there will be more later on! LOL

B&S

08-31-2023, 07:32 PM	#5
Bored&Stroked Senior Member Join Date: Jan 2011 Location: Central Ohio Posts: 5,910	Re: Rebuilding my Flathead random questions You're building a 1940 engine, with a 4" Mercury crank and 8BA rods. You *do not touch ANY of the bearings with a rubber hammer!* Since it sounds like you've already applied the hammer to potentially BOTH (rods and mains), you'll need new bearings for both. The 8BA rods use insert style bearings - so there is no hammering involved. Each 1/2 shell should sort of snap into the rod cap (they can only go one way) and rod end and be snug - they will not be loose or move around in the least. The same goes for the mains - the rear one will be quite tight (sometimes I use a little wooden block to knock it down on the edges - never on the bearing surfaces). On the rods, the chamfers on the edges need to be toward the outside of the journal - in case you need crankshaft fillet clearance. I haven't seen your rods or crank, so I don't know how things were prepped/machined. When in doubt, post some clear close up pics of a couple rod caps as well as the rod journals on your crank - then I can tell you more. Checking Clearances: You should at least PlastiGauge the mains and rods to see what your approximate bearing clearances are. I like to have .001 or so of clearance per inch of journal diameter. You do this checking without the rope seals in place. So .002 to .00225 on the rods and .0025 to .00275 on the mains. PlastiGauge isn't super accurate, but you should be able to ascertain if you're in the ballpark. Do not skip this step! Tip: Take some heavy duct tape and cover the ends of the crankshaft main studs - that way when you man-handle the crankshaft into place (multiple times), you don't nick the bearing journals by hitting them on the sharp edges of the studs. The same goes for the rods - when you're installing the pistons/rods it is very easy for a rod-bolt to hit the crankshaft journal, making a nice ding in it. Protect your journals at all cost! LOL It is critical that your bearing clearances are correct and that your crankshaft end-play is within spec (about .004 to .008 or so). When you use PlastiGauge, you do NOT put oil or lube on the bearings or journals and you must be careful not to turn the crank or move the rod when taking things back apart. Carefully take things back apart and use the little paper guide to see what your clearances are. I'd check all three main bearings and all 8 rod bearings - it is a good practice to be sure! If they are too tight, you'll score and wipe out the bearings . . . to loose, you'll knock and then wipe out the bearings. When you believe you have everything correct, install the main bearings (and the crank) and lubricate them well (again, without the rear seal plates). Then use 3 torque cycles to creep up on the torque spec on the main bolts/studs - and then see how the crankshaft feels - it should easily spin around just by grabbing the snout and twisting by hand. If is does not - do not proceed, something is wrong and you should not assume it will "fix itself" when you run it. If the crank spins easily, then take it back out and put the seal plates in the block and the cap and reassemble it. Now you're ready to put in the pistons/rods. Lubricate the heck out of everything. Okay, enough babble out of me for now . . . I'm sure there will be more later on! LOL B&S Last edited by Bored&Stroked; 08-31-2023 at 07:51 PM.