Re: Crankshaft Drag Desired after Engine Rebuild

Posted by DJ.Voyce on March 04, 1998 at 10:42:41

In Reply to: Crankshaft Drag Desired after Engine Rebuild
posted by Gordon Biggar on March 04, 1998 at 00:18:32

G'Day Gordon,
It seems you may be confusing crank & piston/ring drag here as there should be very little resistance from the crank & rods or you're definately going to have problems. If your bearing to journal clearance is around 0.015" you'll be OK in this area & I don't recommend increasing it. The resistance you feel is only the oil drag (plus rear main seal if yours is modified) & once overcome, the crank should turn easily in the main bearings.I've never actually measured the drag on any new engine, but have learnt by feel during assembly whether its too tight or not & if you can't turn the crank by hand using the flywheel (even with a little resistance) once the pistons & rods are fitted, you may have a problem so here's some possible solutions.
The cause of your concern is probably the piston ring drag on the newly honed cylinder walls & could be due to a fairly coarse grade of stone having been used in this process. This is by no means detrimental as some engineering works believe it helps the rings bed in quicker to the bores & also helps retain more oil on the cylinder walls in the process, so aiding lubrication of the pistons during this critical period, but not doing much for initial oil consumption figures. To eliminate the possiblity of a tight rod bearing, loosen (do not remove) the conrod caps one by one, checking each time whether there's any change in the resistance to turn the crank. If there's no change & you can turn the engine with the flywheel, even with a hissing sound from the pistons as they travel up & down the bores, I'd suggest you carry on with the assembly & fire it up, then let it run until it's warm & check the crank again. Once the engine's been run a few times it normally loosens up a lot. If there's a noticeable difference on one rod there may be a number of causes, (the most common being a bent rod) but it could also just be the rod is a little tight on the gudgeon pin (wrist pin) bush & was off centre slightly during assembly causing it to bind. If the rods weren't checked on the crank-pins before assembly & have babbit bearings, the rod bearing may also be a little too wide causing it to foul on the side of the crank web. If you're still not happy after checking the above, then remove all the pistons & carefully remove all piston rings & re-check the resistance. Hopefully you won't need to go this far.
Piston to bore clearance is always measured at the bottom of the piston skirt at right angles to the wrist pin, as pistons are both tapered towards the top (because the crown is thicker & so expands more) & oval at the bottom as there is more material around the wrist pin holes. Good Luck.
Regardz
DJ.
OutOfAfrica A's

: I've been doing this engine rebuild "by the book" (Les Andrews' book, in this case). Les calls for a drag of 35 foot pounds or less on the turning of the crank when the engine has been rebuilt.I'm coming up with 58 foot pounds.
: The Model "A" block was re-sleeved, and standard pistons and rings employed.Using Plastigage, all of the main bearing clearances were set at approximately .0015 inches, based on 50 foot pounds of torque (final torque setting was 80 ft. lbs.).After all three main bearing caps were installed, there was not too much drag on the crank (probably 15 foot pounds or so).
: Naturally, with each piston and rod added, the drag began to increase.The rod bearing clearances were also set to .0015 inches at 35 foot pounds of torque.Although the pistons appear snug in their sleeves -- as they should -- each one has .009 inches of clearance between the piston and sleeve, measured at the top of the block.(There is no lateral movement of the pistons within their respective sleeves -- at all.)
: Is 58 foot pounds of drag too much?Where have I gone wrong?Where did Les obtain his 35 foot-pound figure?Do I go back and increase rod bearing clearances to .002 inches, which would be outside of specifications?
: It's a great learning process, but oftentimes frustrating!Thanks for any advice that one may have on the subject.
: Gordon Biggar
: Houston, Texas

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