5. Degreeing the cam
|Finding TDC - dial gauge set up to measure height of the number 1 piston roughly zeroed at the top of piston travel - the crank will always be turned clockwise from here on....|
|Degree wheel bolted to crankshaft nose - reads 10 degrees ATDC when dial indicates 0.5 thou before greatest lift....|
|....and 13 degrees ATDC when dial indicates 0.5 thou after greatest lift...|
|Degree wheel turned anticlockwise by 11.5 degrees (the original 10 degrees plus half the difference between the two readings) - now reads 1.5 degrees BTDC when dial indicates 0.5 thou before greatest lift....|
|...and 1.5 degrees ATDC when dial indicates 0.5 thou after greatest lift- so the degree wheel is now set up precisely to indicate TDC.|
|The crank is turned clockwise to indicate about 90 degrees ATDC and the dial gauge is moved to rest on the top of a pushrod on number one cylinder inlet (second tappet back from the front of the engine). The crank is turned and the dial gauge set roughly to zero at greatest lift, on the next turn of the crank stop 1 thou before greatest lift.....|
|... and note the degree wheel reading - 101 degrees ATDC........|
continue turning the crank till 1 thou after greatest lift - note the
reading, 115.5 degrees ATDC.
The greatest lift therefore occurs at the mean of the two readings - 108.25 degrees ATDC. This is a Piper BP270 cam which should be timed at 107 degrees. The chain will stretch slightly in use and will retard the cam so I like the leave the cam slightly advanced initially - so I will fit a 2 degree offset key to bring the timing to 106.25 degrees.
|Just for comparison - here is a duplex chain & sprocket set up on the same engine, same cam - note that stopping just under a thou before max lift occurs at 105 degrees ATDC...|
and the same amount after full lift occurs at 116 degrees ATDC, so with
the duplex set up the same cam is timed at 110.5 degrees ATDC - 2.25
degrees later. In this case I would fit a 4 degree offset key to bring the
timing back to 106.5 degrees ATDC.