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Old 10-09-2008, 07:47 PM
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Truth or Myth

I have heard from a few of my buddies that when they have tore down rebuilt
sbc 350's in the past, that they ALMOST NEVER encounter the well known stock .025" deck-piston clearance.

My questions are, IF that is a truth, WHY is that

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Old 10-09-2008, 07:56 PM
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there is a LOT of truth to that- its because of core shift during the casting process- also one reason why you have a block decked befor e a rebuild- the other is due to twist.
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Old 10-09-2008, 08:11 PM
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So, then it follows that "many/all/majority" of the rebuilders just slap the motors back together without any concern for deck/piston height???
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Old 10-09-2008, 08:13 PM
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I dunno if your saying more or less. Since it`s true with core shift and etc.
Deal is sometimes you dunno what you`ll find, the decks are often tipped and must be machined to set them straight. Thats why when I do a mock up assembly I only install the number 1, 2, 7 and 8 pistons, this way, I can check the tip. Sometimes the number 1 will be .026 or more in the hole while the number 7 on the same cylinder bank will be .010 or less. I`ve heard of cases where the number 1 was .040 in the hole and number 7 was dead zero.
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Old 10-09-2008, 08:31 PM
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Oh boy NOW that was interesting .... and I will probably be going in and get clearances soon on my motor, as I have to change head gasket anyway (Long story - but followed a recommendation that I shouldn't have and have a WAY TOO THICK ONE, I think it was made by SERTA Mattress company .041"), on my unknown remanufactured corvette motor. It even has a "REMANUFACTURED" tag on outside of block, which I am hoping is a good thing .....but who knows until you get it opened up

So if they are all different clearances, would you have to get precise volume of each cylinder to find your CR??
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Old 10-09-2008, 08:59 PM
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Quote:
if they are all different clearances, would you have to get precise volume of each cylinder to find your CR
Even if you averaged them out I think your CR would be 'close enough'. This is why there are machine shops that specialize in this type work. I don't think it's going to make so big a difference that you will notice unless on an all out race application. Suppose your CR is equal in all cylinders but the fuel distribution is not equal, what then?
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Old 10-09-2008, 11:28 PM
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I think it's very important to cut the decks to square and set the squish with whatever piston you're going to use. Take the example given by DV above. If you had #1 0.040" in the hole and #7 at zero and you used a 0.040" gasket, you'd have the proper squish on 7 and no squish at all on 1 (0.080"). So #1 could be detonating and number 7 could be makin' good power and be ping-free.
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Old 10-10-2008, 06:16 AM
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Quote:
Originally Posted by Stroke
Even if you averaged them out I think your CR would be 'close enough'. This is why there are machine shops that specialize in this type work. I don't think it's going to make so big a difference that you will notice unless on an all out race application. Suppose your CR is equal in all cylinders but the fuel distribution is not equal, what then?
air and fuel distribution is almost never equal on a street engine- especially on a carbed one.

And as stated squaring the decks is VERY important any time you are doing a hopped up rebuild. I supposes on a stock rebuild you could get away without doing it, but I would never even think to when you plan on hitting 6,000 RPM and having compression ratios close to 10:1.
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Old 10-10-2008, 09:30 AM
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One day, I got a call from the engine re-builder across the parking lot from me. He asked me to come over, he wanted to 'pick my brain'.

During our discussion, he was assembling a HP 460 Ford. His customer had supplied, all new, stroked crank, rods, pistons, etc. The only used parts, was the block, and oil pan.

He was slapping in the pistons, and it was 'obvious' #4 was sticking up out of the hole. I mentioned this to him, and he said "the customer supplied the parts, with thick head gaskets, it will be OK".

3 weeks later, it was back. The 'customer' had pulled the cylinder head, and you could see where #4 was hitting the head. He told his customer "you supplied the parts, we just used your block".

Do you have any doubts, why I refuse to use them, for any 'machining' I need done? Or, do any work for him?
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Old 10-10-2008, 04:27 PM
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Quote:
Originally Posted by Tomard
I have heard from a few of my buddies that when they have tore down rebuilt
sbc 350's in the past, that they ALMOST NEVER encounter the well known stock .025" deck-piston clearance.

My questions are, IF that is a truth, WHY is that
.025 is a manufacturing spec at GM which is determined by the deck height to the crank centerline minus the installed height of the piston at TDC. So under the best of circumstances the actual clearance is subject to a great number of part tolerances in addition to block machining tolerances.

Plus there are a great many engines that are built off the production line at a rework station where components salvaged from machining errors (reworked) are used to build up engines. A common production error is damage to the block while being machined. Typical of this are nicks and gouges on the decks and or bearing saddles usually caused by excessively dull tools being worked at too high a speed. These are taken off line and are hand re-machined where the dimensions of the damage is recoverable or fixable by welding and machining. Where errors or damage are to great to be fixed it the part goes back into the pot to be melted and recast. It was once said of the slaughter house industry that nothing escaped the process but the final squeal. This is the way industry operates, nothing goes to waste. The result of repair and rework can be a deck machined closer to the crank center line thus the distance to the piston is closer, or to the main saddles which through the process of re-align boring will raise the crank in the block making the distance from the piston to the deck shorter. The factory, also, will do weld repair on castings, while not too successful in the outside world, they have the resources to control the situation to where welding of cast iron is highly successful. If you rebuild a lot of engines and look carefully, you'll be able to identify many of these reworked and salvaged parts.

When you get to rebuilt engines, the big re-builders run these things on a production line not too unlike the OEM. All the blocks get align boring and decking. Since the crank is now closer to the deck and the deck is closer to the crank the aftermarket piston manufacturers reduce the crown height of the typical piston to restore the proper compression height. When you rebuild an engine at the corner shop it's a whole nuther world, the deck and or the main bore might not need any or perhaps not as much align boring or decking, but they use the same pistons as the big guy re-builders. The result is that it's very common to see a rebuilt engine with too much compression distance between the TDC piston crown and the deck.

This is all junk you need to know going in as the guy at the corner shop may not even know this stuff. Measure, measure, measure and ask questions; go in dumb come out smart. I data record everything to varying extents depending on how hard the engine will be used. On a street rebuild, which I don't do anymore, I used to produce a couple three pages of measurement data and construction notes per engine. For a Hot Rod or working truck engine build maybe 5-6 pages of info goes into my files and a copy to the purchaser. For a race customer, they get a 3 ring bound dossier discussing everything found, corrections made, part numbers used, and dimensions on every component and every clearance. These commonly run 15-20 pages or more with a copy for me and copy for the customer. This is the only way to insure you produced what is required and will help when you need to go back in or if the customer has to take it to someone else for repair when they're out on the road somewhere.

Bogie
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Old 10-10-2008, 04:31 PM
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Quote:
Originally Posted by carsavvycook
One day, I got a call from the engine re-builder across the parking lot from me. He asked me to come over, he wanted to 'pick my brain'.

During our discussion, he was assembling a HP 460 Ford. His customer had supplied, all new, stroked crank, rods, pistons, etc. The only used parts, was the block, and oil pan.

He was slapping in the pistons, and it was 'obvious' #4 was sticking up out of the hole. I mentioned this to him, and he said "the customer supplied the parts, with thick head gaskets, it will be OK".

3 weeks later, it was back. The 'customer' had pulled the cylinder head, and you could see where #4 was hitting the head. He told his customer "you supplied the parts, we just used your block".

Do you have any doubts, why I refuse to use them, for any 'machining' I need done? Or, do any work for him?
Although his bad kharma will eventually catch up with him, this guy needs to be tied to a pole and caned....repeatedly.
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Old 10-11-2008, 02:16 PM
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Anyone who would assemble an engine without checking,
(regardless of the customer desire)
needs a vigorous introduction to Woolverine.
I would never do any business with an assembler like that.

I have turned down lots of business.
I told somebody last week that

"I was going to do it my way, or take it somewhere else.... because when someone else sees this... they won't see that you didn't want to spend the extra money, all they will see is that this job came out of MY shop."

I promise you= all other people know is he assembled an engine that hit the head. The customer is NOT telling everyone how HE himself screwed it up.

"The bitterness of poor quality is longer remembered than the temporary excitement of cheap price."

Last edited by ScoTFrenzel; 10-11-2008 at 02:28 PM.
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Old 10-12-2008, 07:24 AM
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Old Boogie

Thanks for that explanation, it was actually very cool, as your synopsis was close to what my little brain thought was the world .....

And the documentation that you do, I was a little overwhelmed, as it was my first attempt at a major motor, transmission & rear end rebuild, so I was learning as I went along, being careful and methodically keeping parts straight

Now, if I would have checked more (that I will forever on), I wouldn't even be changing this head gasket, but for someone that didn't know JACK, I am pretty pleased I have my 1980 smogger 350 sbc up and running well, with new Alum heads, cam, intake, carb, exhaust, removed all the smog stuff, rebuilt a 200r4 tranny & a working 3.73 rear end (was a 3.08).

I don't even know my quench, which really sucks .... as now I want to get more technical and squeeze every bit out HP/TQ out of it, but kind of hard when you are just guestimating CR!! Which by the way was 8.2:1 stock .... so I think I am up around 8.9:1 or there abouts, but don't know deck or exact bore size, YET!
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Old 10-12-2008, 06:18 PM
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In most cases on Dyno tests a tight quench only improved power by about 5 horses, tops. But don`t let that be misleading into thinking it`s not important. As Tech explained, no quench with higher compression ratios is detonation looking for a place to happen. Also since these tests were performed on a old style dyno and not tested in a vehicle the real world figures will be different as a vehicle has to deal with weight and constantly changing driving conditions that a dyno doesn`t have to. Since I learned about Quench years ago every engine I`ve built since I shoot for around .045 quench. Plus I do other tricks of the trade to create a even better mix. Quench forces the incoming air/fuel mixture to mix really well thus improving combustion with less timing lead and helps reduce detonation.
Take for example my 4.3 liter V6 in my chevy truck. It has about 10:1 compression, a .043 quench distance, a real mild cam, horrible swirl port heads and TBI still in use. Used to I could only run 93 octane fuel, but after I replaced the stock injectors for Venom injectors and raised the fuel pressure it runs fine on 87 octane.
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Old 10-13-2008, 06:05 AM
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Quench

I am guestimating (at BEST), my quench is around .066" ..... possibly more ...

Been playing around with timing, and had it up over 38* (didn't want to keep going) with 87octane gas, and I have NOT YET, ever heard the dreaded ping on this motor!!!!

Then again, this is an 80's smogger that came from factory with 8:2 compression, 190hp/280tq, 3.08 rear .... a little look/feel difference nowdays

Come to think of it, I should put this stuff in my profile so you expert guys can see it when I make my best attempt to explain things!!
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