CJS Customer Projects

one of 3 853R's

This is one of 3 748-853R conversions completed recently . Itís a lovely bike and has only covered 1400 miles but the owner insisted for smoothness and outright power without compromising reliability.  I recommended a full engine blueprint re-building the motor from the bottom up which I always suggest to my customers.  People who insist on bolting big bore kits to an unknown or stock block usually ask for trouble.

With the engine in bits the main bearings were changed, the crank and transmission was re-shimmed.  Re-shimming the crank is important. The cranks usually come with too much preload as standard. Reducing 0.10 from the crank preload enables the crank to spin with little noticeable drag.  Some bikes have really tight cranks which gives poor starting (starter struggles to turn as crank too tight) and can cause premature main bearing failure.


The crank/flywheel assembly was rebalanced with a Kamna evo flywheel, journal surfaces polished and refitted with the correct oil clearance. This is found using plasti-gauge combined with using the correct grade shells.  Shells come in different sizes red, blue, yellow etc.  Too tight a clearance and the journal overheats causing scoring and too loose means the engine will need to be overhauled sooner than later.  A crank blueprint includes honing the rods and deburring. This helps the sizing process and also eliminates any possible burrs one the conrods that can quash the shells out of shape when torqueing. Crank oil ways are also key-holed as the factory's radius on some cranks are substandard.  Many builders omit these important steps and slap in basic sized shells.


I sent the primary gears back to the customer who decided to lighten all the gears in his machine shop. I must say the job looks pretty cool. The engine has now lost a little weight, will spin up and accelerate from point A-B faster.


The cylinders were bored out and re-plated to 94mm with a pair of quality pistol pistons making 853cc's.  I donít use anything else as the design fits the chamber and valve reliefs perfectly.


Next up the cylinder heads given the CJS ultimate treatment.  One thing I noticed was how bad these heads were (all 748r are bad) but this took first prize. The port floors were ripped out on the exhaust ports of one head.  I know what shape an exhaust port is supposed to be in stock and modified form but this took the p*ss.  We had 2 choices, either buy another head or re-work the strange port and make the untouched head the same.  Whether this was a good or bad move I donít know but the exhaust flow has not been affected at all by its strange shape. This strange shape has been found on some of the early corse bikes and still exists in some early 996sps bikes and they all seem to go well.  If the problem was in the intake ports I would be very worried about performance.


Colsibro valve guides come with every head I make for reliability reasons.  They are tighter meaning I can size the valve to the guide using different reamers of selected sizes and a fine hone to finish the clearances. I gave up on installing new sloppy fitting Ducati OE  guides 4 years ago. They are loose, wear quickly and cause valve walking with high lift cams ruining my superb performance seat work.  My seat machine is like a portable serdi (seat and guide machine) with multi angle tips. The seat finish is amazing and very concentric which give superior valve sealing.  Valve jobs can make or break a port job. Too many times I have seen heads that were ported elsewhere with no improvement.  Some of the better ones had good ports but poor valve jobs (stock angles blended together)  resulting in poor low and mid lift flows. Correcting releases the flow and power. It takes 3x8 hour days to make a pair of heads here and the price does not reflect the amount of effort they see.  Enough ranting and back to the project in hand.

The intake ports were properly sculptured along with the chamber and modified, polished stock valves.  Flow testing revealed a substantial gain followed which was a little more than my  usual numbers.  I ran the test quite a few times making sure it wasnít an error. It wasn't, so I spent an hour meticulously measuring all the port and chamber dimensions until I found where the gain came from.  The ports velocity is also very high meaning the midrange is going to be very strong. Too many misconceptions out there about porting (peaky, unusable) - total rubbish I say if the job is done correctly.


All the valve clearances were set to the tighter spec of shimming. The old or tighter spec of shimming gives less worn rockers, less broken half rings and  smoother running as the valves seal so much better and also give extended service intervals. This shimming spec is now becoming common also being used by my friend Tom at Hypertrick.

Finally the cams were degree'd, squish set using different thickness base gaskets and the engine was assembled.  It looks nice and I'm in anticipation the dyno results of all three of these identical 853r builds.



Here is this 853r vs chrisiballs 853r on the other project page . The difference between these 2 engines are that this one has a full 50mm system where the other bike has a 45mm sytem . The 50mm system gives a 4-hp loss in this case . This is also true of quite a few other models . Some info on exhaust sizing , applications coming soon

our customers come first...