The parts manual for my "68" T-120 shows two 'O' rings listed for the tappet guide blocks, one for the intake and one for the exhaust. The guide blocks that I took out of the cylinder have a groove cut deep enough for an 'o'ring in the intake but not in the exhaust block. There is a very shallow groove but if an 'o'ring was fitted I'm sure it would be cut when installing the guide block in the cylinder. I'm thinking the exhaust must be from an earlier year that wasn't machined for the 'o'ring. If I use it, is it going to seep oil? Any feed back/recommendations greatly appreciated.