Reply To: What do you use to seal block halvles? permatex aviation sealant ok to use?

[Pete]

I have seen some use the Permatex No3 on the split cases you note but i wouldn’t use it – too thick.

On the split crank case 2 strokes I use Permatex anaerobic 51813 (Locktit 518 equivalent), it is a red gel. Important to note the blocks on split Crank Case engines are machined with the halves bolted together without any sealant. Therefore any sealant that is too thick will upset the dimension of the machined halves and can effect the crank centering and bearing fit. If done right the anaerobic will be much less than 0.0005″ ( 1/2 part in 10,000). In fact when cases are separated for service I hardly see any sealant but cases surfaces must be true.

I always check the flatness of the 2 mating surfaces with a machinists steel rule in all directions using a back light on the rule to see highs and lows (they happen especially at bolt and locator pin areas) and lightly dress if needed. If not perfectly flat and parallel there will be seal problems. Clean the surfaces with alcohol before applying sealer, then apply a ‘very small amount of the sealer as a bunch of small dots on both surfaces and spread around with clean finger so you get a uniform very slightly red coating so when bolted there is no squeeze out. All you are doing is filling the factory rotary surfacing machine marks, small scratches and other small imperfections on the surfaces. I have never had a leak at the case joint on the Mercury race engines. I torque in 3 stages in proper sequence from center out.

Add – I keep the cranks case halves slightly apart while inserting the bolts and nuts in the case to prevent the
anaerobic sealer from starting to set too quick and it will, prevents extra thickness of sealer from forming.

I have never tried the Permatex Motoseal, the anaerobic does the job so well I see no reason to change.