I started racing an OSSA in 1969 at Louden, N.H. It was a Stilleto with
motocross bars and a 21 inch front wheel with a street tire on it.
I beat A1R,s: X6,S: and various yams. It was areal kick. I raced it for
two years in AAMRR then got an AMA license to race at Daytona. At that
point I got a Stilleto TT with the 5-speed close ratio.
I had a Dunstall seat, tank and fairing. I finished around 25th in 1971.
I raced it in Canada and all the AAMRR races at Louden.
My last AMA race was at Louden in 1974.
As usual I finished 25th. My present bike is the Motocross 4-speed that
I raced in 1969. It is in a 1970 Wildfire frame.
Porting Specs
OSSA 250 CC
50 HP Engine
Builder: Ken McGuire Specialities, P.O. Box 29, Monsey, N.Y.10952
Porting: Richard's Engine Development, Hollis, Me. 207-929-3344
[ Howard Richards]
Computer Designed Expansion Chamber:
VP Engineering, 2921 Patricia Drive, Des Moines, IA..50322
[515-276-0701]
Coatings: Britech, P.O. Box 371, Southbridge,Ma. 01550 508-764-6304
[ Jay Strait]
Pipe Builder: Bart Chamberlain; 207-929-3030; Hollis, Me.
[ you could frame this pipe and put it on your wall]
I raced this type of engine at Daytona in 1972. It produced app. 36hp.VP
Eng. pipe specs predict an increase to 46.8 hp and torque to 27.6 using stock
porting.. I rode it and I believe it. Then I got greedy and seized it. I simply
did not break it in properly. The engine was beautifully built by Ken McGuire
who installed the Yamaha rod conversion. Upon disassembly we found that Ken's
work was bullet-proof and all we had to do was reassemble the lower end.
As long as I had it apart I wanted to try to avoid further seizures and shoot
the moon on porting.
I bought a 73mm. piston from Hogan's Cycle[413-786-9170], rebored to .0035
and took the cylinder head and piston to Britech who ceramic coated the
combustion chamber and the piston crown.The piston skirts were graphite coated.
The graphite coating is at a thickness of .0005 which gives a cylinder wall
clearance of .003. To further enhance power, and to prevent seizure,Howard
Richards recommended 2 boost ports up the backside of the cylinder.
Full boost port specs are included in the following memorandum.
For those of you who are interested, I am including the original OSSA specs
so that you can compare
them to the modified specs.
ORIGINAL 1969 OSSA STILLETO SPECS 72mm bore x 60mm stroke
Exhaust port data:
Exhaust roof height: 31.22mm [1.229in.] BTOC
Exhaust width: 41.9mm [1.650in.]
Exhaust port open: 85deg. ATDC
Exhaust port fully open: 190deg. TOD [Total Open Duration]
Transfer port data:
Transfer port width: 32mm [1.260in.]
Transfer roof height: 43.41mm [1.709in.] BTOC
Transfer port opens: 71deg. BBDC/142deg. TOD
Transfer feed entry: 97.66mm [3.845in.] BTOC
Transfer port floor: 60.05mm [2.364in.] BTOC
Intake port data:
Inlet port floor: 92.38mm [3.637in.] BTOC
Inlet port opens: 79.2deg. BTDC/158.4deg. TOD
Inlet width: 48.01mm [1.890in]
MODIFIED 1969 OSSA STILETTO SPECS: 73mm bore x 60mm stroke
Exhaust port data:
Exhaust roof height: 30.68mm [1.208in.]BTOC
Exhaust width: 50.70mm [1.996in.]
chordial @ 1.385in. [35.179mm] BTOC down to 1.707in. [43.3578mm] BTOC. This width is approx. 70%
of bore diameter. 060in. [1.524mm] radiused chamfer
around entire exhaust port periphery @ cylinder wall.
Exhaust - top radius: 9mm [.354.]
Exhaust - bottom radius:12mm [.472in.]
Exhaust port open: 84deg. ATDC
Exhaust port fully open:192deg. TOD [total open duration]
Exhaust floor: 59.69mm [2.350in.] BTOC
Exhaust outlet I.D.: 43.77mm [1.723in.]
Transfer Port Data
Transfer port width: 32.77mm [1.290in.]
Transfer roof height: 45.64mm [1.797in.] BTOC which yields 66deg. BBDC
opening point. 132deg. TOD achieved by epoxying to
lower height. Roof entry angle is 90deg. to bore.
Transfer port opens: See above note on roof height.
Transfer feed entry: 93.27mm [3.672in.] BTOC.
Transfer port floor: 60.05mm [2.364in.] BTOC.
Intake Data
Intake port floor: 93.38mm [3.637in.] BTOC for a width of 35.38mm[1.393in.]
centered in intake opening. This yields 90.5deg. BTDC
opening point and 181deg. TOD with piston skirt "notched" 5.21mm [.205in.] upward x
50.7mm [1.975in.] width.
Intake corner and
roof heights: 63.14mm [2.486in.] BTOC for a width of 9.91mm [.390in.] -
centered in intake opening - 68.2mm [2.685in.]
height @ upper-outer corners:84..46mm [3.325in.] height at
lower corners.
Intake width: 50.17mm [1.975in.] upper to lower corners. This width is
approx. 69.3% of bore diam.
Intake port opens: See above [opens 90.5deg. BTDC with 181deg. TOD] specs on
"Intake Floor"
Boost Port Data [two added]
Roof tops: 44.75mm [1.762in.]BTOC yields opening @ 112deg.
ATDC [136deg. TOD].
Width: 9.65mm [.380in.]
Trough depth: 6.61mm [.260in.]
Trough floor height: 76.43mm [ 3.009in.]
Piston feed holes: 6.99mm [.275in.] below crown edge x 8.89mm
[.350in.] inside diam. cut to align with boost troughs.
Cylinder Head
Mill head gasket to 1.02mm [.040in.] thickness.
Radius cut head pocket to increase volume to 23.7cc [flatplated].
Yields 13.5:1 C.R./ 1.32mm [.052in.] squish clearance.
Timing
Conventional: 3.0mm BTDC
Modified: Many high performance OSSA'S are timed to 3.15mm BTDC
Base Transfer Ports
The base transfer ports must be substantially opened up in order to
perfectly match the cylinder transfers. This is most important in order
to get the fuel volume necessary to take full advantage of the above modifications.
The base transfers were designed around old two-stroke technology which
believed that velocity of fuel flow was of most importance instead of
current technology which looks to volume and breathing to achieve maximum performance.
NOTE
Using DMR or even GP 4 specs will not produce roadrace performance. The
above specs and the VP Engineering pipe are necessary to achieve high performance.
This is true of any dirt bike conversion to SOS competition or road race activities.
Last Note
The 73mm piston is produced by Keith Lynas [619]670-8966, the Calif. OSSA
expert. Bob Hogan of Hogan's Cycle has provided me with everything needed to complete
the racebike. Now it's on to the dynamometer! Last-Last NOTE: I have not yet put this
bike on the dyno as I am carefully breaking it in. It is a rocketship even though it
idles and has low and mid-range power. So far I have found no limits to the top-end.
I will go up a tooth on the countershaft to 14. The rear sprocket is 40 teeth.
There is no doubt that this motor can produce 10000RPM with this set-up but
I will probably red-line at 9000. Another interesting fact is that after 45 minutes of
operation you can put your hand on the cylinder head without discomfort - due to the
ceramic coating keeping the heat in the combustion chamber.
Howard Richards has done an incredible job. Most amazing is his exact
documentation of each and every modification. Most porters wont tell you
very much but Howard is an exception. I believe he is the best two stroke
porting specialist in the country[ he mostly ports 4 strokes now]. Don't
hesitate to call him at 207-929-3344.
author:
Gaylord Boutilier email:lordpat@megalink.net
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