OMC 15A Class Engine
OMC 15 A CLASS ENGINE
1. It is the intent of the Race Commission the this engine be raced without modification as received from the manufacturer.
2. Permitted modifications herein are for either safety or repair purposes. Inspectors may compare questionable parts to new parts to assist in inspection decisions. Use of these rules to gain an unfair advantage is not permitted and will be grounds for disqualification.
3. The driver is responsible for the condition of the engine being raced. Errors on the part of the manufacturer, mechanic, previous owner, or any other in non compliance with the rules in not permitted.
4. Height restriction, There shall be a minimum distance of no less than 1 3/8" between the boat bottom and the center line of the propeller shaft at the aft end. This measurement shall be made with the engine positioned as raced, with engine turned straight. The bottom of the boat will be the lowest planing surface at the bottom of the boat. Hydroplanes can not have a tunnel bottom. This rule does nor apply for straight away time trials.
The following is a list of restrictions, descriptions, and modifications which apply to this engine:
1. The engine shall maintain, as close as possible, its complete factory color and decals. Including power-head and gear case.
2. The engine shall be races with the production upper and lower motor covers installed as originally supplied by the manufacturer.
3.Mounting of the fuel tank to the engine or steering bar is not permitted.
4. Any type of spark plug is permitted.
5. Adjustable "high speed" carburetor jets are not permitted. However, larger or smaller fixed jets are permitted.
6. The addition or removal of material from any moving part, including the flywheel is not permitted.
7. The addition or removal of material from any internal engine component of surface is not permitted. Internal surfaces must maintain their original manufacturer finish. The exhaust passage in the lower-unit shall retain its original cast surface finish. Honing of the cylinder bores is permitted.
8. Broken parts may be repaired by the use of welding or other bonding compounds if all the contours and dimensions remain as original. The Lower-Unit may be profiled as long as minimum dimensions and templates are met.Engines with broken skegs and/or cavitation plates are permitted provided the edges of the break(s) have not been filed, smoothed, or otherwise altered., and provided reasonable time was not available for repair or replacement. One or two weeks is considered reasonable time for repairs. At Championship events the Inspector may not allow engines with broken skegs and/or cavitation plates to race.
9. Oversize tapping, or use of heli-coil or thread inserts to repair stripped threads is permitted.
10. Use of a thrust bracket of any manufacturer, provided the engine is not altered is permitted.
11. The steering bar brackets shall have their retaining bolts safely wired as supplied by the manufacturer.
12.Only genuine OMC service parts for this model of engine(1986-1992) will be considered legal replacement parts. This includes, steering bracket bolts, steering bar bolts, gear case bolts, connecting rod bolts, throttle plate screws, reed stop screws, and cylinder head gasket. Other than listed above, any fastener or gasket is permitted provided it is equivalent to the original.
13.only a minimal (thin film) of sealer may be used on engine gasket surfaces.
14. The intake and exhaust ports are drilled(round) holes. Only the center transfer port and the center exhaust port for each cylinder break through fully into the cylinder bore. All other ports do not break through fully into the cylinder bore.
15. The Lower-unit split line cavities shall be left as supplied from the manufacturer. (ie. unfilled)
16. OMC part number 396780 is the only permitted cylinder block. This part number can be found cast on the top exhaust side of the block. Service number 396010 includes block 396780
17. Boyesen reeds number 122 and 122R are permitted replacement parts. Except for reed thickness and removal of shim, all other specifications apply.
18. OMC part number 583077 or 583913 are the only permitted flywheels. The part number can be found cast into the bottom side of the flywheel.
19. OMC part number 397653 and service carburetor stamped with 388275 into the top of the carburetor body with full a butterfly, similar to manufacturers is permitted.
20. Removal of the transom cleats located on the transom surface of the stern bracket is permitted.
21. Engines must meet the requirements of the OMC inspection templates as listed below.
OMC part number 568050
OMC part number 568051 for the shape of the combustion chamber.
OMC part number 568052 for the height of the ports.
22. NOVICE class engines require the use of an intake restrictor plate. Either a National Boat Racing supplied restrictor plate, or an OMC part number 561277 or 568266 restrictor plate.Restrictor port and orfice edges shall remain as supplied by manufacturer, NO FILING, SMOOTHING, OR POLISHING.
23. The use of a single gasket, OMC part number 318932, on both sides of the intake restrictor plate is permitted.
24. OMC part number(s) 328820 and 333733 are the only transfer port covers permitted.
25. Any OEM piston ring manufactured for this year and model of engine is permissible for use in the OMC 15A engine. This includes OMC part number 386279, standard pistons and the OMC part number386288, 0.030" over sized. Use of the 0.030" over sized piston ring on 0.010" over sized pistons, by cutting the piston ring down to fit the cylinder bore is permitted.
26. The bore dimension shall be increased 0.010" with the use of 0.010" over sized pistons, and the bore diameter shall be increased 0.030" with the use of 0.030" over sized pistons.
27. Novice runabouts must meet a minimum length of 10 feet and a minimum beam of 44 inches.
28. The following parts are no longer available from OMC and may be produced by any manufacturer:
STEERING BR BRACKETS(MUST BE STEEL)
STEERING BARS
STATOR PLATE LOCKING BRACKETS
CARBURETOR LINKAGES AND BOWDEN WIRE MOUNTS
CARBURETOR BAFFLE PLATES
DRIVE SHAFTS
OMC 15 CLASS A SPECIFICATIONS
CC'S 13.9
VENTURI 0.875 +/- 0.005
THROAT 1.000 +/- 0.005
A 3.480 MAXIMUM
A1 0.625+/-0.010
A2 0.625+/-0.025
A3 3.490 MAXIMUM
G BORE 2.223 MAXIMUM
K 4.880+/-0.015
K1 4.520+/-0.025
K2 1.830+/-0.020
X 1.086+/-0.010
E 3.918+/-0.010
F 1.868+/-0.010
J 1.760+/- 0.008
L 3.000+/-0.006
M 0.642+/-0.014
H 0.256+/-0.020
H1 5.000+/-0.50
N 1.430+/-0.015
N1 1.010+/-0.030
N2 0.675+/-0.025
Y 1.260+/-0.030
U 10.633+/-0.050
U1 10.510+/-0.050
P 0.040+/-0.010
T 0.280+/-0.060
D 0.880+/-0.020
Q 11.550+/-0.200
Q1 9.100+/-0.200
V 18.000+/-0.060
R 2.050 MINIMUM
S 0.980 MINIMUM
W 4.765+/-0.020
W1 40340+/-0.100
GEAR RATIO 14:19
RESTRICTOR PLATE FOR USE IN NOVICE CLASSES
AA 0.482 MAXIMUM
BB 0.047 MAXIMUM
THICKNESS 0.037+/-0.005
STAINLESS STEEL REED THICKNESS 0.011+/-0.005
