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Discussion Starter · #1 ·
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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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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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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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 90º. 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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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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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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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.:thumbup:
 

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Discussion Starter · #10 ·
Thanks guys. Much appreciated. I only have $800 dollars invested in the Short block as I bought it from a freind of a freind. Aparently it was good to go Bored 30 over new flat tops, bearings and ARP rod bolts. I guess this is a classic case of you get what you pay for:D. I spoke to the machine shop today and he said my best bet is to right down all the deck heights for each bore, remove the internals and bring it to him. He said he will parallel deck it to get both sides close to the same. 185$ + 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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Discussion Starter · #11 ·
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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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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All the junk that will get in the oil journals. In order to clean the block out properly I would remove them. You can buy a cam bearing tool from summit for 60 or 70 bucks, and a good set of cam bearings will run ya 50 bucks. They are not hard to install, just take your time.
 

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No problem just trying to save you some stress. Everytime i have taken a short cut it has bit me in the ***. The last thing you need is a chunk of crap to plug up a lifter and wipe out your cam or score up your bearings. When i get a block back from the machine shop i spend a few hours cleaning and measuring. I had a block done last year and the machine shop said it was spotless ready to go. When i got it home the thing was full of sand from when they bead blasted and baked it. When i went back to tell them about it he said he's so confident in his work that he doesnt let stuff go like that, check everything yourself. Spend a little more time and money the first time cause doing thing twice sucks.
 

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Before you take it to the machine shop, tear it apart and use one piston and rod assembly to measure cylinders 1,7,2,8. This will eliminate any differance in the rods and pistons of the other assemblies. I pull the rings and use a lil masking tape to keep the piston snug in the bore. I feel for you, my 400 is nice and square but -.032:(
 

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Discussion Starter · #20 ·
Thanks for all the replys guys much appreciated. I am ging to get the deck cut down so the pistons are around 5 thou in the holeand use the 38 thou head gaskets that are recomended with the heads I have. By doing this they will have to take 16 thou off one side and 32 thou off the other. Do you guys think I will also have to have some material taken off the intake so it will line up properly??
 
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