91304-HP5-601 Honda O-RING (20.6X1.9)

Description of Change
The new block is not directly interchangeable with the former block without ordering additional parts. The changes to the new block and its related parts include the following.* A new cam bore diameter. The bore diameter has been increased in size from 92.00 mm (3.622 in) to 98.00 mm (3.858 in). Replacing the former block with the new block will now require the installation of thick-walled camshaft bearings.* The hole pattern and shaft diameter of the cam idler gear stub has changed. The new stub shaft has a three hole bolt pattern which will not allow the use of the former "Series A" stub shaft. The diameter of the stub shaft has also been increased from 75.00 mm (2.953 in) to 81.00 mm (3.189 in).* The rear gear assembly now uses an 8 pitch gear. The former 10 pitch gear cannot be used with the new block because of the larger stub shaft diameter. A new 10 pitch gear with a larger bore is available for replacing the gear in the former 10 pitch gear train. A new 8 pitch gear is also available for the newer 8 pitch gear trains.Ordering Information
Depending on the configuration of the engine, the parts required to upgrade the block to a "Series B" can vary. Refer to Charts B, C, D, E, F, and G for specific part numbers."Series A" Engine Blocks (Prior to serial numbers listed in Chart E)
To upgrade a "Series A" block with a 10 pitch rear gear assembly to a "Series B", order the parts listed in Chart B. "Series A" Engine Blocks (For Serial Numbers 4MJ150 - 161)
To upgrade a "Series A" block with an 8 pitch rear gear assembly to a "Series B", order the parts listed in Chart C. "Series B" Engine Blocks (For all serial numbers within this range 6PN, 7SM, 2BM, 3DM, 4GM, 8RM, 4TN, 7HM, 8EM, 6WN, 7RN, 8KN, 8CN, 9AN, and 6HN)
To upgrade a "Series B" block with an 8 pitch rear gear assembly, order the parts listed in Chart D. "Series B" Engine Blocks (For the serial numbers listed in Charts E, F, and G)
The following serial numbers and later serial numbers only require the replacement of the cylinder block to upgrade the engine. All other parts are directly replaceable. Rear Housing Rework Procedure
The new camshaft bearings cannot be removed through the front or rear engine housings. The current radius of the housing is 48 mm (1.9 in) and will not allow the bearings to be removed. If the camshaft bearing must be removed through the front or rear of the engine, the housings must be reworked. If the camshaft bearings can be removed by pressing them inward (toward the center of the engine), no rework is necessary.To rework the housings, grind a 51 mm (2.0 in) radius (A) into the wall of the housing. To remove the excess material use a grinder and the appropriate grinding bit. Since the center of the 51 mm (2.0 in) radius is extremely difficult to locate, carefully grind the wall of the housing until the bearing can be removed from the rear of the housing. Do not remove more material than necessary; only enough to remove the camshaft bearing. The thickness of material between the radius and the hole should be equal to twice the diameter of the hole.After completing the rework, thoroughly clean all metal shavings out of the housing.
Remove Excess Stock If Bearing Must Be Removed Through The Front Or Rear Engine Housings
(A) 51 mm (2.0 in) radius.
Failure to remove all foreign debris, resulting from reworking the housing, can cause damage to the engine and its related components. Make sure the block is clean and all metal shavings have been removed before assembling the engine.
Installing New Camshaft Bearings
The new service tools listed in Chart H are required for installation of the new camshaft bearings. All other removal and installation tooling has not changed.