Johnson Service Bulletin #650 (1958) - Cylinder Head Gasket Differences

Johnson's June 15, 1958 service warning explains why cylinder head gasket 304720 for RD-19 series engines must not be interchanged with 305206 for RDS-20 models. Although the outline and bolt holes are similar, the different lower water passage openings can restrict coolant flow and cause overheating.

Johnson Service Bulletin letterhead
June 15, 1958SERVICE BULLETIN #650
REPLACING CYLINDER HEAD GASKETS - 35 H.P. MODELS

When comparing the cylinder head gasket, Part #304720, RD-19 thru 19C with the cylinder head gasket, Part #305206, RDS-20, you will find a difference in the cutout arrangement at the bottom.

It is possible, however, as far as general contour and bolt holes are concerned, to interchange the gaskets. This should not be done under any circumstances, since the water flow may be restricted and dangerous overheating may result.

Factory comparison of RD-19 series head gasket 304720 with one long lower passage and RDS-20 gasket 305206 with two lower openings
WM/aks
JOHNSON MOTORS
Walter McLeod
Walter McLeod
Service Department
Superior Service Spells Satisfaction

Gasket identification

304720: RD-19 through RD-19C, single elongated lower cutout. 305206: RDS-20, two separate lower cutouts. The factory specifically warns not to interchange these parts because of restricted cooling-water flow.

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