I performed the manifold seal pressure test, as per Tom Cotton (I love this test!) on my '64 XLCH and I noted that there is minor leakage at about 15 psi. Now I know that when performing the test on big twin manifold seals (Peek or brass) there should be no leakage at all and I did achieve this on my '47 FL. But the later XL's and FL's the O-Ring sealing mechanism works a little differently.
Unlike on the big twins, as the negitive pressure in the manifold increases and approaches -14.7 psi, the O-Ring is drawn tighter into it's groove, presumably affording a tighter seal. However when performing Cotton's pressure test the O-Ring is increasingly pushed away from the groove as internal manifold pressure increases, thus indicating possible leakage when that might not be the case under operating conditions.
The above is not intended to discount the importance and validity of Cotton's test - I believe its an essential step which I include every time I build a motor or install a top end. In this case it tells me that the assembly is fairly close to leak-free and therefore a constructive use of time. But I do wonder if a minor amount of leakage in O-Ring setups can, or should be tolerated.
Has anyone had similar results?
Unlike on the big twins, as the negitive pressure in the manifold increases and approaches -14.7 psi, the O-Ring is drawn tighter into it's groove, presumably affording a tighter seal. However when performing Cotton's pressure test the O-Ring is increasingly pushed away from the groove as internal manifold pressure increases, thus indicating possible leakage when that might not be the case under operating conditions.
The above is not intended to discount the importance and validity of Cotton's test - I believe its an essential step which I include every time I build a motor or install a top end. In this case it tells me that the assembly is fairly close to leak-free and therefore a constructive use of time. But I do wonder if a minor amount of leakage in O-Ring setups can, or should be tolerated.
Has anyone had similar results?
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