Reproduced by the kind permission of the Editor, Nexus Specialist Interests: RCM&E April 1977.
Look! Could this sandcast prototype herald a British revival in the 10cc engine field? Dick Fisher's impressive 'Redshift 60' is a thoroughly up to date Schnuerle scavenged design in the HP61FS/OS-60FSR/Webra Speed 61 tradition but with some very interesting internal features of its own. Engine has been tested with various carburettors: final choice yet to be decided.
We were recently invited to examine a prototype of an entirely new British engine that
looks extremely promising and could very well prove to be the U.K.'s answer to the foreign
engines currently dominating the international high-performance R/C engine market.
This new motor, provisionally named the Redshift 60', was designed and built by Dick Fisher of Beighton, Sheffield, who hopes to get it into production during the next few months. Technically, it is one of the most interesting single-cylinder 10 c.c. R/C engines to come our way for a very long time.
Its most unconventional feature is its crankshaft and front-end design , the object of which has been to permit a
very large induction passage through the shaft and, at the same time, to compensate for the
increased crankcase volume that this incurs, by adopting a new method of sealing off the
crank web and bearing assembly.
The 'Redshift 60', in fact, uses a 17mm o.d. shaft compared with the 15mm diameter of nearly all other modern shaft valve 6Os, the only exceptions here being the 0.S. 6OFSR which has a 16mm o.d. and the recently announced Rossi R.60FI which also has a 17mm size. Dick Fisher's design, however, has the largest gas passage of all at 13mm bore, some .40 per cent bigger than the 11mm i.d. generally, used with the usual 15mm o.d. shaft.
In theory and in comparison with most rear rotary valve designs, crankshaft induction engines suffer the disadvantage of having a considerable excess of primary compression chamber volume - the volume added by the hollow crankshaft. The larger the shaft bore, the bigger the excess volume becomes until the beneficial effects on aspiration are nullified by loss of crankcase pressure and a consequent
reduction in the weight of charge transferred to the combustion chamber .
A familiar method of reducing the effective crankcase volume is to use full circular crank webs with counterbalancing by means of peripheral slots either side of the crankpin, the rim being sealed with a shrunk-on aluminium or brass rim. However, Dick Fisher has adopted a new approach which is obviously more effective. Here the front housing spigot is extended much farther into the crankcase barrel and actually encloses the crank web. The crank web is counterbalanced by milling each
side of the crankpin slightly forward of the rear face in order to leave a 360° disc 32mm dia. and with a 1.0mm thick rim. This rim runs in the front housing with a clearance of only about 2 thou. and thereby, in effect, seals off the crank chamber from the bearing spaces. The hollow 7mm o.d. crankpin has an aluminium plug for
the same reason.
A consideration that most model engine designers have to bear in mind is the restricted choice of standard ball bearing sizes available. There are, perhaps, four or five model engine manufacturers in the world who are willing and able to have special bearings made to their own requirements. Generally speaking, however ,
engines have to be designed to suit standard size bearings - hence the wide use of
15x28mm or 15x32mm ball journal bearings for current 10 c.c. shaft valve motors. No bearing to suit the 'Redshift 60's' 17mm shaft was available and Dick Fisher therefore took the unusual step of incorporating a short 20mm dia. rear journal in his shaft and mounting this in a standard 20x32mm 14-ball brass-caged ball race. At the front, the shaft is carried in a standard 3/8 X7/8 in. ball journal which also
takes the thrust loading.
The 'Redshift 60' has a bore and stroke of 24x22mm and is, of course, a Schnuerle
scavenged engine. Porting follows the usual Schnuerle pattern and ports were very accurately cut on the prototype. Checked port timings were 150° of crank angle for the exhaust; the fore and aft transfers remaining open for125° and the upwardly inclined third port for 118° of crank angle. Nominal rotary-valve timing, incidentally, is 30° ABDC to 50° ATDC.
The engine uses a ringless aluminium piston running in a brass liner but
instead of the chromium-plated cylinder bore of the usual ABC types, the 'Redshift 60' has something
new again, namely a Drayloy coating having a Vickers hardness number between 1100 and
1300 (better than chrome) and which is said to offer a lower coefficient of friction. The piston
is flat crowned, has rectangular cutaways fore
and aft to avoid masking the transfer ports and is fitted with a lightweight 6mm o.d. tubular gudgeon-pin retained by wire circlips. The machined connecting-rod is bronze bushed at both ends, with oil slits and is slightly longer (42mm between centres) than most .60 rods - necessary to provide adequate clearance for
the piston skirt at BDC but having the beneficial effect of reducing conrod angularity and piston side-thrust. Reciprocating weight is quite modest, the piston weighing a checked 9.8 grames, the gudgeon-pin 1.9g and the conrod 6.0g.
The cylinder-head, machined from bar stock, has a shallow bowl shaped combustion chamber bounded by a 3.5mm wide squish band. Measured nominal geometric compression ratio of the prototype was 11: 1. The head is secured with six Allen cap screws, no gasket being used. As is usual with prototypes, all the castings were sand castings in the engine examined but the production model will have investment castings, offering a neater appearance and, possibly, a small saving in weight. As shown in our photos, the prototype weighed 460 grams or just over 16.2 oz, less carburettor.
Production engines will probably be equipped with a proprietary carburettor. Various carbs have
been tried and, when fitted with an E.D.Multi-Carb, the engine shown checked out at
510 grammes or 18 oz.
In our conversations with the designer, we learned that several prototype units have been built and that the engine's novel features have shown every indication of contributing a worthwhi1e bonus in terms of performance.
Dick Fisher told us that he set out to produce an engine that would be fully competitive with the O.S. 6OFSR and Webra Speed-61 and, by all accounts, he has succeeded. Moreover, it is anticipated that the 'Redshift 60' wi1l sell at a lower price than current imports. All of which s very welcome news and we wish Dick Fisher every success with this new venture.
The Redshift 60's exclusive "ABD" piston/cylinder set-up uses Drayloy cylinder wall coating that is claimed to be harder and to have a lower coefficient of friction than chromium.