Thread: BBC 402 looking for advice on head work, cam and stall

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Looks like the block has been cut a bit, see the picture, but best of my ability to read this thing looks like the bore is 4.150, so not stock. More likely it is .030 over and my skill at using the caliper are just lacking.

I've been trying to figure a way to measure the piston dome. No luck so far. Any pointers?

-unix

Originally Posted by unixoracle

I've been trying to figure a way to measure the piston dome. No luck so far. Any pointers? -unix

You will need a cc burette, 100cc, a holding fixture for the burette and a clear plexiglass or lexan plate with a 3/16" hole drilled in it. You will also need a bottle of Isopropyl alcohol from the drug store and a bottle of red, blue or green food coloring from the grocery store, your choice. You will not need 100 cc's for this operation, but if you are going to invest in a burette, get the 100cc unit.

With the short block on an engine stand, loosen the holding bolt on the engine stand and twist the block so that the cylinder you're going to check is almost vertical, but has one side of the bore a little higher than the other. Tighten the bolt. Span the edge of a steel rule across the block at the bore and bring the piston up until the pop-up is just slightly below the edge of the rule. (maybe 0.010" or 0.015"). Now, using the sliding function of your dial caliper, measure how far the flat part of the piston is down in the bore. That would be the crown of the piston that is flat and even with the edge of the piston just above the top ring. Write it down.

Let's say that the crown is 0.135" down from the deck surface of the cylinder block. Now, use Vaseline to seal the edge of the piston to the cylinder wall. Use as little Vaseline as possible. I used to wipe around the bore with a Q-Tip and wipe away the excess with my fingers and a shop rag. You want to make sure that the piston is sealed to the bore or you will have to do all this over again. Now, smear the block deck with Vaseline and place your cc sealing plate onto the block, with the filling hole at the high point of the bore so that air will automatically escape uphill.

Affix your burette and fill with colored alcohol. Pour the void and record your volume.

If the piston were absolutely flat on top, with no valve reliefs or domes, it would take a certain volume of liquid to fill the void atop the piston crown. We will determine that volume mathmatically.
We know that the bore of the block is 4.150" and the depth from the top of the ring land to the deck surface is 0.135. If we multiply 0.7845 times the 4.150 bore times the 4.150 bore times the 0.135 times 16.387, we find that if the piston were flat, the space we are pouring would be 29.9 cc's.

Now, we pour the volume and find that we actually used 14.3 cc's of alcohol to fill the void. If we deduct 14.3 from 29.9, we find that the pop-up is worth 15.6 cc's.

Simple, huh ?
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Thanks, but... The short block is staying in between the frame rails. It's not coming out.

This is a top end only refresh. Quick and dirty.

Would this be possible you think with the block sitting level?

-unix

Originally Posted by unixoracle

Thanks, but... The short block is staying in between the frame rails. It's not coming out.
This is a top end only refresh. Quick and dirty.
Would this be possible you think with the block sitting level? -unix

I've never done it with motor in frame, but yeah, I suppose you could if you're careful.
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