What is Engine 'Break-in'??

[BRENT in 10-uh-C]

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Originally Posted by Bud

Do modern builders bore and re-bush the tappet bores? I would think most do not, but I cannot be sure.

Former MAFCA Tech Director Jim Cannon and I were discussing this a week or so ago regarding someone he was helping on finding a noise. I have a special Go/No Go gauge made to check Tappet Bores diameters that is 0.5945 & 0.5960. The print calls for the bore size to be 0.5940" - 0.5945" and that was meant for about a 0.001 total clearance(-½ thou per side). As long as the .960 end does not fit into the bore, then I usually will let it go.

I would venture a guess that maybe less than 50% of the blocks I check are worn enough where the .596 will go in, -but many will have a lot of clearance with the .5945 portion of the gauge inserted.

Several of the Model-A vendors offer a 0.005" oversized tappet to use however set-up to accurately machine the block for these is a little labor intensive. Sure, it is not that hard to run a hand reamer thru the bore to "open it up", but if that is done without a fixture, then the bores can be out of alignment from each other and/or the camshaft. (You really need a fixture built to index off of the main bolt datums.) Like with most things, a good machinist can correct many of these problems if he has the budget to do so.