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Bob uk
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Posted on Thursday, 25 September, 2014 - 10:52:   Edit PostDelete PostView Post/Check IP

All crankshafts are best made by forging so the grain follows the shape.

The oil ways are straight and drilled at an angle. Some cranks have centrifugal sludge traps.

Triumph twins had sludge traps.

In the manual there is a lot of measuring and clocking up to check a crank using stuff the average garage or diyer is unlikely to have.

So the easiest way is to get the crank reground which costs less than 100 and the crank will be as new. The undersize bearing shells will cost about the same as std size. To check a crank only and not grind will cost 30 so no savings because the engineer will find wear on a 100,000 mile shaft it may be within tolerance but maybe 50,000 mile it may be on the limit. RR cranks last well and with careful driving ( not necessarily slow driving) will last 300,000 miles and a quick regrind go another 300,000 miles. Unfortunately at 300,000 miles the liners will be causing concern and the crank will be a mere passing interest.

My view is once having got as far as taking the crank out one might as well go the whole hog. Otherwise why did one take the crank out!?. Because it's in the way of pulling the liners and another 100 is nothing compared to the cost of the liner job.!?

Obviously the crank has to be clean. Which is obvious how to do. Clean before it's ground and before its fitted. The shop that grinds the crank must get the brgs before they grind, because the size they grind to my be on back order and waits of 6 months are not unknown.

Balance is done at initial manufacture and a reground won't effect the balance.

I was told by B Leyland that their cranks are balanced and will run at 20,000 rpm without rods. After market balancing is only necessary for high rpm because of the clutch assembly.

My way to fit a crankshaft is to fit all rods and pistons and then using a floor crane lower the crank in to position guiding the rods into place and popping the big end cap on with loose nuts.
Once the crank is lying in the mains put the main brgs caps on.

Then tighten to 5 ftlbs.

Using a small pinch bar lever the crank to check for end float which is usually between 0.002 and 0.010, the shadow is 2-6 thou.
If the crank moves easily back and fro then it's probably correct. The big ends should also move sideways by hand if they do then torque the brgs up as per specs then check again then the crank should turn complete with rods using the flywheel as hand leverage.

I like to fill up the oil ways and holes with engine oil as I assemble.
The main brgs bolt holes must not be filled with oil because it will hydraulically lock the bolt and give false torque readings and can crack the crankcase.

If a tight brg is found then it's either dirt under the shell or the crank is machined to big.
Or bent rod.
If this happens do not think that the engine will run in. It won't it will seize. So take the bits to the shop that ground The crank and they will sort it out.

If the crank is for a race car then get the journals ground at bottom tolerance on the loose side and fit a higher volume oil pump. This is done to Ford crossflow engines and it allows the crank to flex a bit and rev freely without binding. Std 70 bhp tuned up 120 bhp and 10000 mile life before another rebuild.

To run in a reground crank keep the revs down for 5 mins or so then drive as normal. Once a few miles say a hundred then it's ready for max revs.

If there is concern about the oil system an engine without heads can be turned by hand to get oil coming out of the feed for the head or if you are lazy use the starter motor. Which on RR is a good time to check the mesh of the starter pinion.

Pistons and bores do need bedding in but is for a different posting.

(Message approved by david_gore)

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