at al grenning seminar yesterday he cautioned about "decking" a block on a rebuild,because you loose the stamp pad marks, but if your machine shop uses a "deck mounted" boring bar and you do not "deck" the top of the block parallel with the crank center line the bores will not be perpendicular to the crank center line. find a machine shop that uses a table mounted boring bar that does not use the block deck for mounting of the boring bar.
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Re: decking and boring a block
I will be tearing down my motor and am concerned about this also. I'm not a machinist (and don't play one on tv), so I am wondering what the tolerance on a deck is, before it is considered needing a decking. From the "if it ain't broke don't fix it" angle - if it's not blowing a head gasket or doing anything else weird, and you use the same heads that were on it to begin with, why would it need to be decked anyway? -
Re: decking and boring a block
I've been building engines for 40+yrs.
In all that time, as far as a block NEEDING to be surfaced, I've maybe seen 2-3. One of the most common reasons for surfacing a block is to lower the surface of the block deck so that the tops of the pistons will be closer to the top of the block (to increase compression, this is called "deck clearance"). This is perfectly fine for an engine that is not original and additional performance is the goal. NORMALLY, for an engine that has had decent care and not been severely overheated, you just DO NOT see blocks that are warped enough (if at all) to justify having the decks surfaced.
Surfacing a block definitly does not hurt a thing and will certainly true it up, no question about that. And unless you are building a hardcore, all out race motor where EVERYTHING needs to be square with the world, then it is just added $$$$$ in the pocket of the machinist that you could have better spent elsewhere.
SOMETIMES, SOME machine shops surface the block without telling you and you find out when you get the bill. This is what happens when a customer takes an engine to a machine shop and says "Do whatever is needed"! BIG, BIG, BIG mistake!!!! But remember, you "TOLD" him to do whatever is needed!
SOOOOOOOOOOOOOOOOOO, when you take your engine to the machine shop-----------MAKE SURE IT IS ABUNDANTLY CLEAR TO THE MACHINIST THAT HE NOT SURFACE THE BLOCK!!!!Comment
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Re: decking and boring a block
Tom,
I agree 100%. Decking a block is generally a waste of money for any mild street application. It falls into the same category as the "double undercoat" or the "lifetime wax job" that the dealer tries to sell every new car customer. Unles there is a problem with the deck, it's just not necessary.
Same deal with align boring/honing and rod reconditioning, especially if not done properly. More bad than good is usually the result.
As far as I'm concerned, the most important block machining operations for street are the cylinders bore dimensions and proper honing with an automatic feed precision hone. (don't believe your engine builder when he tells you he can properly hone cylinders without it. If he could afford one, he'd have one)
Unless a block deck has gouges or areas that have pits from rust, I wouldn't ever have one decked.Comment
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Re: decking and boring a block
agree with tom and mike. of all the engines i've done over the years, only one, a 55 265, required decking. i usually lay a straight edge across the deck and if i can't get the thinnest feeler gauge blade between the straight edge and the block, i consider the blcok not in need of decking. the 55 265 was warped to the extent a 015" feeler gauge slipped under the straight edge.have no idea why the block warped. mikeComment
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Re: decking and boring a block
As I've said what seems to be a zillion times, before you disassemble the block, measure the piston crown to deck clearance on all eight cylinders. Do it multiple times to be sure you get it right by having consistent data sets, then analyze the data.
It will tell you if the deck is parallel to the crank axis or "tilted". Rarely will you find any significant out of parallel condition, so using a deck indexed boring bar is usually okay, but one that indexes of the crankshaft axis is always best.
Your deck height measurements will also tell you if the decks are equal height side to side. Usually they are off five to ten thou. This can effect compression ratio, and of course, knowing deck clearance so you can COMPUTE the actual CR is critical if you want the best performance by assembling the engine with the highest acceptable CR that will operate detonation free on pump gas for the particular engine configuration.
Unequal deck heights can be compensated by using different thickness head gaskets side to side, or if the left deck it high, it can be machined because no one is ever going to look at it and give it a second thought.
It's all in the details, as usual. Deck clearance is a critical and is usually overlooked, but mere .015" alters the CR by about half a point.
Most machine shops cut the deck and head surface "because we do 'em all that way". You have to discuss machining operations with the shop and then specify, in writing, exactly what operations you want accomplished. If the guy doesn't accept that you are the customer and he is to do it YOUR way, you better find another shop.
Get the deck clearance measurements and post them here if you want help with the analysis. Also state the engine configuration and what your overall object is for the rebuild.
DukeComment
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Re: decking and boring a block
I agree with the above and will reiterate; put your instructions in writing, including emphasis on NOT decking the block. I know of a recent case where the original block for an old Corvette was decked even though the machine shop was "told" not too. They "didn't remember" that instruction.
VerleComment
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Re: decking and boring a block
Verle,
Yup, very common. I used to write "DO NOT DECK" across the deck with a great big felt tip when I brought blocks in to machine shops. Often times, the guy that does the machine work is not in touch with the work order that's in the office and he just does his job, which includes decking all blocks.
Better still.... be there when your block is going into the process.Comment
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Re: decking and boring a block
When I had my engine rebuilt, one of the places I visited was a highly recomended speed shop here in Indy. I was walked through the extremly clean, modern shop and was quite impressed with the set up, I was not impressed with the manager who brushed my concerns with not decking the block. He seemed not to care if I wanted it done or not, only that he would do what he thought was necessary. I chose someone else who totally understood my concerns. By the way, the shop had three very nice restored corvettes in there show room that the owner owned.
So go with someone who understands you.
ChasmanComment
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Re: decking and boring a block
Tom-----
Yes, I'd say that finding 2 to 3 engine blocks in 40+ years that really needed to be decked sounds just about right. I THINK that I MIGHT have one of the extremely rare exceptions, though. This is a block that is part of a SERVICE 350/300 short block assembly that I purchased from GM in 1975. Since day one I had serious oil burning problems with this engine as I've described previously on this board. I did EVERYTHING possible to correct this problem. I won't go into everything here again since it would make this post extremely long.
After what I've been through with it, I am absolutely convinced that the problem stems from oil being drawn into the intake ports through the lifter valley. I tried NUMEROUS fixes directed at this theory, too. Changed intake manifolds 3 times, heads twice, gaskets about 10 times.
I think that the problem relates to the fact that the block deck surfaces were never properly machined at Flint and are not square to the crank centerline. They're close enough that everything bolts up ok and the engine runs ok, but one just cannot get a proper seal at the lower edge of the intake manifold-to-cylinder head mating surface.
When I finally get around to performing an "autopsy" on this engine, I'm going to take the block to a good machine shop I know of and have this fully checked out and corrected (so I can save the block for possible future use). If they say that they can't find anything wrong with it, then I'm going to tell them to junk the block. I just KNOW that the problem's in the block somewhere, whether it can be found, or not, and if it can't be found and corrected, then this block needs to DIE.In Appreciation of John HinckleyComment
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Re: decking and boring a block
One more thing in the block prep work is not to let them shot blast the bare block to clean it. Some shops what to use a small steel shot to clean the blocks and you can lose some of the broaching from doing this. Don't ask who I know this.
Page CampbellComment
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