Keeping power loss to a minimum during build

Crank, rods, sealing, pistons, block, flywheel etc
Guy Croft
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Joined: June 18th, 2006, 9:31 am
Location: Bedford, UK

Keeping power loss to a minimum during build

Post by Guy Croft » June 26th, 2006, 9:23 am

During the build we need to keep mechanical losses due to friction to a minimum, because not only can rubbing friction generate power loss, it can also cause heat to build up, with consequent damage, I'm talking here primarily about crank, rods, pistons, rings, cams, though of course ancillary things like water pump, oil pump, alternator all have a bearing on this.

Two of the pictures below show me checking the turning torque on a block - crank assembly with no rods, a short engine with pistons and rods fitted. The turning torque with no rods is almost too low to measure, in fact the crank turns by hand. Without seals it will spin by hand easily, and always should.
With the rods and pistons fitted the engine is harder to turn over, but that's normal. With the belt drive fitted we have the added and quite high turning torque of the valvetrain, especially with multivalve engines. The engine at right is Thorsten Scheffner's 2 liter 'Fast road' MonteCarlo, you can see that with racing rings the built turning torque is still only 13lbf ft.

If you're worried the thing is too tight, go to TDC and just rock the crank to and fro a few degrees, the engine should feel looser because the pistons are not moving.

The picture of the crank shows mods that I recommend to the oilways to improve the feed and pickup on the crankpin (rod journal) and main journal respectively. I am certain that this makes a real difference to the survivability of the rods and bearings. The chamfer on production cranks can be non-existent, or rather poorly formed.

After grinding a crank you must always polish these regions to eliminate completely any sharp edge - it can cut thru the oil film and slice the bearings to pieces.

Pistons must always go in at manufacturer's recommended skirt-bore clearance. Of course, you can run them looser, and provided that the ring end-gaps are OK she'll be fine. I would point out however that running cast pistons over-loose runs the risk of cracking the skirts.

I use more race rings than I used to these days, either from my race piston supplier or 'Total Seal'. The lighter the ring the better, and this usually means a thinner section ring. Lighter rings are more likely to stay on the lower (sealing) face of the ring groove at high rpm, and if they are low -outspring, they have less friction in the bore and the all sealing comes from the gas pressure. Total Seal rings feel very tight when new but they soon loosen up.

Rods must never bind on the crank. Here is a good check - I often build the block up with rods torque and turn it over then strip it to check the bearings, making sure there is no scoring or rubbing. Overkill? NO, peace of mind. Insofar as checking running clearances I refer to the Vandervell manual and use micrometers to check the crank and bore gauge to check the rods. It always gets done, usually the first job after strip and clean. You can tolerate a bit of undersize - if in doubt call me.

Cams MUST turn perfectly freely in their housings with your fingers before they are offered up to the head or the valves are fitted - or they will overheat and seize solid. As with main bearing caps and rod caps the cam housing caps (eg 16v engines) are line bored and MUST stay in the original position, if you swap them round you could end up with seizure or worse.
Slide29.JPG (63.21 KiB) Viewed 13017 times
RD brg damage 2.JPG
main bearing scoring due to razor-sharp edge left on modified crank, the debris is bits of bearing, this is straight out of the engine.
RD brg damage 2.JPG (50.4 KiB) Viewed 12982 times
RD crank oilway before.JPG
razor sharp main journal supposedly modified to improve pickup of oil to feed crankpins but actually cutting the main bearing to bits
RD crank oilway before.JPG (41.18 KiB) Viewed 12976 times


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