calculating CR – a simpler way

Here’s a method that involves little math compared with the conventional method as described here

The following method involves bolting the head and cylinder together as an assembly on the bench using all-thread. Then carefully placing the piston in the cylinder at exactly where it would be if it were at top dead center (TDC) in an assembled motor. The  volume of the combustion chamber (Vtdc) can then be measured directly with the burrette. It’s then simply a matter of dividing this volume into the entire volume above the piston at bottom dead center (BDC) to determine the compression ratio as illustrated in the diagram below.



Step 1. Determine piston position at TDC: We do this using the scribed line on the feeler gauge that was used  in method 1 to determine the deck height ‘dh’ and repeated her:

To measure dh, clamp the cylinder to the block using tubes on the studs, turn the motor via the back wheel or an 8mm allen key on the alternator rotor bolt to until the piston is at TDC. Slide a squared off 0.1mm feeler gauge between the cylinder wall and piston so that it rests on the 1st compression ring. Use a very pointy scribe to mark the top of the cylinder – some prussian blue would be handy here. Make sure you slide the gauge in line with the horizontal axis of the gudgeon pin (ie at 3 and 9 o’clock looking directly at the piston) as this is where piston rock is at a minimum. If excising care, you can measure dh with this method to within +/- 0.1mm.


Bolt cylinder to case using tubes

Bolt cylinder to case using tubes

Scribe line with piston at TDC

Scribe line with piston at TDC



Step 2. Off to the bench: Move cylinder off the bike and onto the bench then carefully place the piston at TDC using the scribed line on the feeler gauge. Grease the rings a little first in order to make things water tight. Before bolting everything together, use a vernier caliper to take a checking measurement from the bottom of cylinder skirt to the bottom of the cylinder to give me a means of checking that the piston hadn’t moved position after bolting everything together and buggerising around.

Step 3. The bench top assembly: Bolt cylinder, gasket and head together using 10mm all-thread then use checking measurement at Step 2 to check that piston hadn’t moved during the bolt up.

Using scribed line to set piston to TDC

Using scribed line to set piston to TDC

Taking measurement to skirt bottom

Taking measurement to skirt bottom

Bolting it all together

Bolting it all together


Step 4.  Measuring the combustion chamber volume (Vtdc) with a burrette: If you examine a head you’ll, see that when the plug hole is vertical, a part of the outer periphery of the chamber is higher than the bottom of the plug hole and around a corner a bit like under the lip of a toilet bowl. So if you fill when the plug hole is vertical, a sizable air pocket will form at this higher peripheral part.

The way to get around this is to first fill to the bottom of the plug hole with the cylinder vertical on the bench. This allows the peripheral volume to completely fill. Then, tilt the assembly so that the plug hole is vertical and continue filling until the fluid level just reaches the bottom of the plug hole.

BTW: The burette is the cheaper acrylic type (100ml) and cost me $27.00. Graduations are 0.2ml. (The glass ones are over $100). Use water or metho with food colour or some other means of tinting.

Final filling with plug hole vertical

Final filling with plug hole vertical





So the final number for my particular R100 BMW airhead? Combustion chamber volume Vtdc = 66.7 cc’s (or ml).

Step 5. Calculating CR: All that remains is a fairly simple calculation. In the case of my particular R100 airhead: Stroke = 70.6mm and bore = 94.25mm (first oversize)


Not terribly high for a large valve early 80’s R100 that supposed to be 9.5… but that’s another story…

Adapter flange twix airhead & oilhead

An adapter flange is needed between the output flange of the airhead gearbox and the drive shaft of the oilhead swing arm.

The adapter has a round spigot that is press fit into the female spline of the uni-joint on the R1100 drive shaft. Then the adapter will be welded to the uni-joint. The adapter is turned and milled from a piece of 75mm K1045 round bar.





ready for welding



Moto Guzzi Lemans Mk111 Agostini

Fixing busted head fins

I got a couple heads from a 1989 R100GS for the salt racer. These are the ones to have since they have the so-called ‘D ports’ and have larger intake spigots since they take 40mm Bings. Got them cheap because one of them was busted up: 4 broken fins, a cracked 6mm hole that takes the rocker cover stud and a stripped spark plug thread. Other then that, they were  OK!

Aside from the broken fins, you can see where I welded up the rocker cover hole ready to face and re-drill:


The process involves TIG welding a bead on top of  the broken fin edge, then another bead on top, then another, then another….until a thin wall is built. Then into the mill to have the sides shaved to the correct thickness then another couple beads and repeat.



And after quite a few hours of welding, milling and filing:





Dyno coastdown test


salt racer rear end

DIY dyno comipete!

dyno almost complete

TThe dyno is almost complete. The Datamite 111 dyno system by Performance Trends is hooked up and loaded into the laptop and I have taken the first trial runs. All that remains is to calibrate the dyno and hook up the brake caliper. Oh, and the small matter of building a room around it!



airhead salt racer gets stiffed!

TThere’s a certain satisfaction in hacking tons of detritus and redundant bits off a frame – a bare canvas on which to plonk your own detritus and redundant bits!

Effectionally known as the ‘rubber cow’ the airhead frame is well known for it’s elastic nature.  BMW, in their wisdom, took the lengendary Norton Featherbed concept but rather than continuing each loop over the top and under the tank to the head stem, the BMW loops converge at a single spine that starts at the rear end of the tank and continues to the head stem.

This is where a good deal of the elasticity lurks…. and waits to be exited by the various pendulum effects from the front end and rear swing arm sections amongst other things.  I love a simplistic look but this can be hard to achieve. I could have gone the way that so many do and add tie rods that run from the head stem down across each side of the motor to the swing arm pivot area, but to my eye, this really stuffs up the simple lines of the bike.

I stood and stared and orbited around the frame for hours before settling on a stiffening plan and opted to horizontally stiffen the spine area and tie the top loops through to the head stem in a way that replicates the original Featherbed idea. Some gusseting of the head stem area was also added. Measuring and drawing in CAD for laser cutting makes things a whole lot easier.

airhead slat racer frame stiffening.


BMW airhead frame stiffenin

The oil cooler is a hand- me -down from my shed mate Ross’s Triumph Bonne salt racer build



laser cut stiffening plates


dyno deck

TThe deck frame is made from 40x40x3 angle and 40×3 box tube and the deck itself is 5mm aluminimum chequerplate. The legs can be bolted to the floor.




dyno frame – the module

The frame for the roller module gets a look-in now. The frame is made from 75x75x6 box section and will be eventually bolted to the floor. A bracket for the brake calliper was fabricated and the pillow block bearings are standard 60mm UC212 units. The deck is 5mm aluminium chequerplate.




The bearings are induction heated and slipped over each shaft end, then the assembly is lowered into the completed frame.