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Old 08-16-2012, 12:32 AM
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sbc deck clearance

So I checked the deck clearance on my 30 over sbc 350 tonight. It is a fresh rebuild I bought from a freind of a friend. The motor was buit by a "professional" machinist. I hope I am doing something wrong as the pistons are anywhere from .021 to .038 in the hole but they are close on both sides of the motor. One side all 4 cyls are .036 to .038 and on the other side they are .021 to .026. The way I'm measuring is with each piston almost at TDC with the dial indicator touching the piston, roll the motor over until the dial indicator stops sweeping and then check with a straight edge and feeler gauge. Not sure if this is an accurate way to do this or not?? Could I be out .017 cyl to cyl measuring like this??

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Old 08-16-2012, 09:00 AM
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Quote:
Originally Posted by prostcelica View Post
So I checked the deck clearance on my 30 over sbc 350 tonight. It is a fresh rebuild I bought from a freind of a friend. The motor was buit by a "professional" machinist. I hope I am doing something wrong as the pistons are anywhere from .021 to .038 in the hole but they are close on both sides of the motor. One side all 4 cyls are .036 to .038 and on the other side they are .021 to .026. The way I'm measuring is with each piston almost at TDC with the dial indicator touching the piston, roll the motor over until the dial indicator stops sweeping and then check with a straight edge and feeler gauge. Not sure if this is an accurate way to do this or not?? Could I be out .017 cyl to cyl measuring like this??
GM deck heights are all over the map as I am not surprised with your findings.
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Old 08-16-2012, 10:55 AM
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sbc deck clearance

I ran into the same problem with my BBC. Mine were .017-.021 on both sides. the bummer was I had the short block put together. I ended up disassembling the block and having the machine shop deck it to square it up. Ended up at .005 in the hole. Since I am building a 496 cid I also had the the three center mains drilled and installed Program 4 bolt main caps and align bored.
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Old 08-16-2012, 11:10 AM
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Yes this sounds about right. I had this same problem on my first shortblock build up recently. After doing investigation here is what I found: If you want perfection you have to pay for perfection (line hone block (or aftermarket block), deck the block, expensive aftermarket crank, quality aftermarket rods, quality pistons, quality measuring tools).

Here are some details...

There will be differences in deck height from each side of the block. GM made the blocks to be "close enough". If you want to make them exact, then you are going to have to get the block line honed. This is where they rebore the main bearing journals and caps for the crank. This puts the center of the crank exactly at the center of the block. If you do this, you also need to get oversized main bearings to accommodate for the removed material and you have to get a different sized timing gear set to accommodate as well.

Other factors that can contribute to this are:
-crank shaft throws are not indexed at exactly 90. This can be a problem with brand new factory cranks (not really that common though), or from reground cranks, or from cheap Chinese cranks (Eagle and Scat). Sometimes if they are not offset too much, you can get an offset regrind to bring them back on center.
-rods can stretch.
-rod bolts can stretch
-rod caps get elongated
-cheap pistons that do not have an precise compression height (wrist pin center to piston crown).

As an alternative to the line bore you can have the machine shop measure to the existing center of the crank to the deck. They will do this on all four corners (cylinder #1,2,7,8) and then deck the block so that all four corners are even. Generally it is noted that factory block corners are higher than the middle.

Also when you measure, try not to let the dial indicator go past its highest position. I usually try to make a slow pass, note where the high point is, then rotate back to the high point. Once at TDC I also use a straight edge and slip feeler gauges underneath to find the clearance. Also be sure to measure parallel to the crank (this is parallel to the wrist pin) and will eliminate any measurement errors due to the piston rocking in the bore.
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Old 08-16-2012, 01:09 PM
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Quote:
Originally Posted by prostcelica View Post
So I checked the deck clearance on my 30 over sbc 350 tonight. It is a fresh rebuild I bought from a freind of a friend. The motor was buit by a "professional" machinist. I hope I am doing something wrong as the pistons are anywhere from .021 to .038 in the hole but they are close on both sides of the motor. One side all 4 cyls are .036 to .038 and on the other side they are .021 to .026. The way I'm measuring is with each piston almost at TDC with the dial indicator touching the piston, roll the motor over until the dial indicator stops sweeping and then check with a straight edge and feeler gauge. Not sure if this is an accurate way to do this or not?? Could I be out .017 cyl to cyl measuring like this??
One side consistently tall than the other is probably telling you the crankshaft isn't centered; it would be offset to the lesser side. Another possibility is the decks left to right aren't the same height. Either of these things happens at the factory or by professional machinists.

You're probably not crazy! It only feels that way when you see this stuff.

Bogie
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Old 08-16-2012, 01:30 PM
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What do you guys think I should do?? Short block is already assembled. Should I run it or should I break it down and fix it???
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Old 08-16-2012, 01:39 PM
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Originally Posted by prostcelica View Post
What do you guys think I should do?? Short block is already assembled. Should I run it or should I break it down and fix it???
If this isn't a professional race engine, just run it. The cost and frustration of fixing is just too much to mess with, all you really have is a compression difference one side to the other cause by the about .010 inch difference. If the crank is off center there's a small angulaty difference going on with the rods. None of this is show stopper. If it really drives you crazy use two different thickness head gaskets to even it up the compression some.

Bogie
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Old 08-16-2012, 01:52 PM
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If you are racing then fix it. Otherwise slap it together and call it good.
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Old 08-16-2012, 02:39 PM
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sbc deck clearance

Prostcelica, I don't have any idea of the money you have spent so far on your rebuild. I was in the same situation as you with my block but not quite as far off. I kept thinking of what's going to happen if I put it together like this. As was suggested you could use 2 different size head gaskets. One would have to be a steel shim gasket to = .053 and the other needs to be .032 thick to = .053 thickness. If the crank center-line is off how is that going to affect how long your engine last. Like I said I had the block decked, the block drilled for (3) new 4 bolt main caps and align bored and honed. I also had him hone my block again while he had it. Then had it dipped for cleaning after the machine work was done. The machine shop charged me $500 which I thought was a really good deal. I would call and check the price to deck it and align bore it for peace of mind.
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Old 08-16-2012, 06:25 PM
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Thanks guys. Much appreciated. I only have $800 dollars invested in the Short block as I bought it from a freind of a freind. Aparently it was good to go Bored 30 over new flat tops, bearings and ARP rod bolts. I guess this is a classic case of you get what you pay for. I spoke to the machine shop today and he said my best bet is to right down all the deck heights for each bore, remove the internals and bring it to him. He said he will parallel deck it to get both sides close to the same. 185$ + tax. Im going to do it. I thought of using a steel shim on one side and a thicker mls on the other but it seems kind of hokey.
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Old 08-17-2012, 04:55 PM
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This will be my first time dissasembling a bottom end so I just had a few questions. Ive been doing some reading and it seems if I take everything apart and keep it all orginized and make sure it all goes back together the same way I should be good. I actually only have 1 question. Being that this motor has just been rebuilt can i re-use the carnk and rod bearings?? or are they no good to re-use after the crank and rods have been tourqed down??
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Old 08-17-2012, 05:38 PM
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Yes you can re-use the bearings, just keep them in order. When you put it back together plastagauge it for a peice of mind.

Last edited by theitalian; 08-17-2012 at 05:53 PM.
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Old 08-17-2012, 06:11 PM
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K cool. will do. thanks
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Old 08-17-2012, 08:32 PM
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I might be i little worried about leaving the cam bearings in. Talk to the guy doing the machine work get his opinion. I had my block decked and line honed and it ran me 170 to deck it 160 to line hone it.
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Old 08-17-2012, 08:47 PM
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What is the issue with leaving the cam bearings in when having it decked ??
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