The original plan was to build the engine minus the crankshaft and to then have Alpha Bearings make the entire crankshaft + connecting rods as with the AJcette engine. This turned out not to be an option and I had decided to use a pair of Harley Davidson EVO knife-and-fork connecting rods and big end assembly and to make the crankshaft myself. This decision had to be made early in the project to be able to machine the crankcases – in-line or staggered cylinders.
These con-rods/big-ends
can be bought remarkably cheaply on ebay from the USA. The cost of the actual
item was 60 USD (!!) and although shipping and import duty have to be paid when
sent to the UK it is still remarkably good value.
These parts are manufactured in Japan; the quality is excellent and I really don’t know how they can make them for the price.
The top-down approach I decided on was the following:
1) Determine
the stroke and calculate the compression ratio and swept volume
2) Weigh the connecting rod small ends/big end and pistons for balance calculations
3) Design the flywheels and mainshafts
4) Machine the flywheels
5) Machine the mainshafts
6) Assemble and check
The Harley Davidson HD connecting rod assembly that I had used in the design of the engine has a distance-between-centres of 7 7/16'' and the crankcase machining (distance between the crankshaft centre and the cylinder base) had been based on this.
In a previous blog I described how spacers had been added under the cylinder barrels of both cylinders to give the correct cam chain tension. This is really the only way in which the chain tension can be changed by any substantial amount as the alternatives would be either to remove metal from the cylinder barrel base flange or top face, neither of which is desirable, or to change the height of the cambox above the cylinder head, which is also not desirable as it would change the rocker-to-valve geometry. Shimming the cambox-to-cylinder head is an option for minor adjustment but not for gross changes.
The
crankcases + cylinder barrels + spacers + cylinder heads + camboxes were therefore the starting point and the first step was to place
the pistons at their TDC position in both cylinders and to determine the stroke
that would result on each cylinder. From these 2 separate determinations of the stroke the
minimum would then be chosen (to avoid the piston colliding with the cylinder
head on the other cylinder!). This begs the question: “why would the calculated stroke
be different on each cylinder?” Well, there are a number of possible reasons
for this. Although the crankcase machining had been carried out accurately –
the crankshaft centre – to – cylinder base flange was accurate to within a
couple of thou and the 500 V-angle was accurate to 0.10,
it transpired that there are differences between the cylinder head and barrel
that were used on the twin-port Velocette KTP engine versus the single port engine.
With the piston (plus the top 2 rings) positioned at the top of the spigot on the cylinder, shown below.
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The TDC volume was determined for both cylinder barrels/heads using a burette:
The volumes turned out to be quite different for the front and rear cylinders/cylinder heads with values of:
Vrear = 87 cc (KTP head)
Vfront = 74 cc (single port head)
Why would the rear KTP cylinder head have a higher volume? I had always assumed that the only major difference between the single port and twin port Mk 1 OHC Velocette cylinder heads was the number of ports. But it turns out that there are at least 2 significant dimensional differences (there are some more detailed differences that I won’t go into here). The first is the height of the spigot on the cylinder head, 0.25” on the KTP head versus 0.3125” on the single port head, and the other is the diameter of the hemispherical combustion chamber – see pictures below
Before progressing any further, the major dimensions of the piston are required. In particular, the distance of the gudgeon pin centre to the top land, ie to the point where the piston would touch the cylinder head at the top of the spigot, which has a value of 1.116” as seen in the sketch below of the important Ducati piston dimensions.
All dimensions are in inches.
Calculation of the stroke is then based on:
Lcr_to_spigot = S/2 + Lconrod + Lpiston
Shown schematically below
Where:
Lcr_to_spigot is the distance from the crankshaft centreline to the top of the spigot on the cylinder and measurements are: rear cylinder = 10.26''; front cylinder = 10.33''
S is
the stroke - to be determined
Lconrod is the HD connecting rod length (7 7/16'' = 7.4375'')
Lpiston is the height of the piston from the gudgeon pin centre to the top land (measured at 1.116'')
The measurements quoted above for Lcr_to_spigot were made with the barrels set up to give the correct cam chain tension and with paper gaskets in place – this is the “as assembled” dimension. It can be seen that there is a difference of 0.070'' between the rear and front cylinders. This is very close to 1/16'', which is the difference in spigot heights between both cylinder barrels, and this is the reason for the difference in Lcr_to_spigot between the front and rear cylinders.
Inserting the values into the above equation for the stroke gives rear (Srear/2) and front (Sfront/2) half strokes of:
S/2rear = 1.7075'' = 43.345 mm
S/2front = 1.776'' = 45.125 mm
To avoid contact between piston and cylinder head a value lower than the lower value of Stroke/2 must be taken. The lower value of 43.345 mm is therefore rounded down to 43mm and this gives a stroke of:
S = 2 x 43 mm = 86 mm
It is now possible to calculate the compression ratios of each cylinder and, of course, the swept volume.
To calculate the TDC combustion chamber volume there is now an additional volume that must be added to both rear and front cylinders resulting in the reduction of stroke from their maximum possible values to 86 mm. These are:
Vadd,rear = 1.57 cc
Vadd,front = 9.63 cc
And when these are added to the measured combustion chamber volumes for each cylinder we get the total TDC volumes as:
VTDC,rear = 87cc + 1.57cc = 88.6 cc
VTDC,front = 74cc + 9.63cc = 83.6 cc
It can be considered fortuitous that the difference between the maximum possible stroke for each cylinder and the actual stroke to be used has resulted in very similar values of combustion chamber volume. The original difference of 14 cc between the single port and KTP heads has been reduced to a difference of only 5cc at TDC.
With a bore of 76 mm the swept volume is calculated to be:
Vswept = 390 cc
Giving a total engine capacity of 780 cc
And the compression ratios (CR) of each cylinder are:
CRrear = 5.4
CRfront = 5.66
Whilst the relatively low value of compression ratio may at first seem disappointing the effect of this needs to be understood.
It originates from the use of Ducati pistons intended for road use. These have a lower dome height that a period Velocette piston. The picture below shows the Ducati piston (on the left) and one of the Velocette KTT pistons that I had made by JE Pistons and copied from an original Mk1 KTT piston. These have been positioned, as closely as possible, to align the top lands so that the dome sizes can be compared and it is seen that the Ducati piston is about 0.25'' less, thereby increasing the TDC clearance volume.
For the record, the AJcette, which uses an identical Velocette piston to that shown on the right above, had a (measured) compression ratio of 7.5:1 (see here and here).
I have also previously used one of these Ducati pistons in the K7 restoration. The picture below shows a comparison of the Ducati piston with the original K7 piston.
The compression height of the Ducati piston is 0.177'' lower than the original AJS piston. The compression ratio of a K7 fitted with a standard original piston, as delivered in 1928, is quoted as 6.25 (see instruction manual) (a race piston was available from the AJS works to give a compression ratio of 7.5). A reduced dome height of 0.177'' would increase the TDC volume by 19cc (approximately, because the domes are not identical) and the compression ratio would be correspondingly reduced to 5.1. Although I unfortunately did not measure the compression ratio of the K7 at the time of restoration it would have been around this value (5.1); nevertheless, the performance of the bike was excellent.
So, what is the effect of using a compression ratio of ~ 5.5:1 ? In reality and for the purpose which the bike will be used (enjoyment in riding ….I don’t plan a re-run of the World Speed Record attempt!) it doesn’t really matter. I would not expect the power to be reduced substantially compared to a ~ 7.5:1 compression ratio; however I would expect a lower efficiency which, in turn, would translate into poorer fuel consumption and a slightly higher exhaust temperature.
These Ducati pistons are intended for “Road Use”. There are also Race pistons available which could be used.
And maybe I’ll buy a pair of these at some time in the future for the “race kit”
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