small pockmark in 400 cylinder wall

by fishhead Tue 9 Dec 2014 - 19:48

I bought a used sbc 400 oem bare block that was magnafluxed and checked out crack-free (powder residue still in cylinders),but there is a small pockmark in one cylinder wall that I need to have checked before I dump my "selfish Santa" money into the machine shop.Is sonic testing the only way to know how deep that pockmark goes into the cylinder wall surface or is there some way to get a tiny depth gauge on that pockmark?

by chevellelaguna Wed 10 Dec 2014 - 4:13

Is it a standard bore block? Aren't you going to re-bore the cylinders. That'd be the only way to get rid of it, if it was mag'd the crack would show up...if there was one. Tell the shop to pay closer attention to that cylinder. Makes me second guess why the block was for sale in the first place....especially after being mag'd.

by 77mali Wed 10 Dec 2014 - 4:45

A 400 is a "Siamese" bore. The wall in the block between all the cylinders is closer together than say a 305/350 & leaves less room for over-sized bores. If it's small they may be able to flame weld a patch & then just a good honing might fix it. IDK if you can sleeve a 400 but that might be another option.

by bracketchev1221 Wed 10 Dec 2014 - 5:09

Where in the ring travel is it?

by fishhead Wed 10 Dec 2014 - 12:30

Luckily it's a standard bore block. The bb-sized pockmark is about 1/2" from the bottom of the bore toward the crankshaft side.I eyeballed it closer and it isn't as deep as I thought.I knew when I bought it there was a risk because there is evidence of a possible rod bolt hitting the bottom of the "pockmarked" cylinder but all the damage is in the safe zone at the very bottom on the crank side.If all checks out ok I'll throw some money at it to beef up the bottom end.

by bracketchev1221 Wed 10 Dec 2014 - 13:34

If it didn't show up as cracked, bore it and hone it, and you'll be fine. That will take off the high edge around the pock mark if indeed it did get hit with something.

by pila Wed 10 Dec 2014 - 19:42

If it's that close to the bottom I wouldn't get excited about it, if it's that small. Mine is bored .040 over, from .030, to get rid of a groove from a broken ring, but I would not want to do .060 over.....core shift etc to think about etc.

by fishhead Wed 10 Dec 2014 - 20:02

Thanks guys,I tend to get all "Charlie Browns Christmas Tree" over scratch-n-dent car parts.I can't help but bring them home and show 'em some love.If the block doesn't pass muster I'll paint it and put a glass tabletop on it.(or donate it to the Whitehouse for use as a paperweight).

by 77mali Wed 10 Dec 2014 - 20:38

fishhead wrote:Thanks guys,I tend to get all "Charlie Browns Christmas Tree" over scratch-n-dent car parts.I can't help but bring them home and show 'em some love.If the block doesn't pass muster I'll paint it and put a glass tabletop on it.(or donate it to the Whitehouse for use as a paperweight).

How about getting a whole bunch of bad blocks & making donations all over capitol hill as "Boat" anchors? Suspect

by Dinomyte Wed 10 Dec 2014 - 20:58

fishhead - there was a tiny mark in my block too. Boring it .030 over left little more than a shadow of it. The machine shop here said it would be just fine and it has been. Mine was likely a little bit of rust as the engine I bought to rebuild had been out and sitting for a few years.

I'm with Ray, you'll be OK.

by fishhead Wed 10 Dec 2014 - 22:44

Thanks fellas,I feel better now.While I'm mooching info..Years ago my old machinist wanted the pistons with him when he bored/honed the cylinders.If I don't know how much overbore is required do I let him bore it first and then order the pistons to be used before he hones the final finish?

by bracketchev1221 Thu 11 Dec 2014 - 7:45

If it was me, I would wait until you have the pistons, they can measure the bore right now and tell you how far it would have to go over so you can order them. Unless there is a huge defect in the cylinder wall, more than likely it will need to go .030" over. The shop I worked at kind of liked to start on stuff that they could finish. They didn't want to bore it and then bag it and store it until they could proceed again. It was a job for the day, measure the pistons, bore it, wash it, then put the deck plate on and hone it to final size and wash it again.

by pila Thu 11 Dec 2014 - 20:03

In the old days, many small machine shops used a boring bar that sat on the deck of the block, which didn't account for squareness etc. These days the block is bored while it sits on a mandrel in the main bearing webs. And back then they wanted all the over sized pistons to do an over-bore. Seems like they didn't trust the pistons to be all the same, which was a good idea. So they numbered each piston to match it's bore, with the proper clearance. Even now we can't trust everything to be perfect when we buy it.

by bracketchev1221 Thu 11 Dec 2014 - 20:48

The shop I worked at still measures every piston and marks them 1-8. That way we could keep track of clearances and weights.

by fishhead Thu 11 Dec 2014 - 22:32

Got it.Thanks much,guys.

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small pockmark in 400 cylinder wall

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