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Old 11-13-2012, 12:13 PM
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Deck Clearance?

Many years ago I rebuilt a 1965 Chevrolet 327 engine. I used a kit I bought from a major hot rod supplier of the day. I put the heads back on the block but was always concerned that the pistons at the top of the stroke were at least 1/8" below the top of the block. Maybe even more. I never measured the distance. Is it possible that the pistons were actually made for a 350 cu.in.?? What is the deck height clearance for a 327? I have calculated the compression height for the 327 at 1.700 and the 350 at 1.740. I think that maybe the pistons in the kit were made for a 350 and not a 327. Everything else in the kit fit, including the rod and crank bearings. This engine is still on the engine stand!!! Still all painted nice orange with its AFB. It is a 300 hose motor. I remember I installed a 'blueprint' cam and lifters. I suppose I should take it apart this winter.

Kenny
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Old 11-13-2012, 12:22 PM
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Quote:
Originally Posted by bsakennyh View Post
Many years ago I rebuilt a 1965 Chevrolet 327 engine. I used a kit I bought from a major hot rod supplier of the day. I put the heads back on the block but was always concerned that the pistons at the top of the stroke were at least 1/8" below the top of the block. Maybe even more. I never measured the distance. Is it possible that the pistons were actually made for a 350 cu.in.?? What is the deck height clearance for a 327? I have calculated the compression height for the 327 at 1.700 and the 350 at 1.740. I think that maybe the pistons in the kit were made for a 350 and not a 327. Everything else in the kit fit, including the rod and crank bearings. This engine is still on the engine stand!!! Still all painted nice orange with its AFB. It is a 300 hose motor. I remember I installed a 'blueprint' cam and lifters. I suppose I should take it apart this winter.

Kenny
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rebuilder pistons have a low deck height so you can surface the block. so they are usally a little lower in the block but not by much. should be no big deal unless your going to max effort racing.
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Old 11-13-2012, 02:37 PM
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Quote:
Originally Posted by bsakennyh View Post
Many years ago I rebuilt a 1965 Chevrolet 327 engine. I used a kit I bought from a major hot rod supplier of the day. I put the heads back on the block but was always concerned that the pistons at the top of the stroke were at least 1/8" below the top of the block. Maybe even more. I never measured the distance. Is it possible that the pistons were actually made for a 350 cu.in.?? What is the deck height clearance for a 327? I have calculated the compression height for the 327 at 1.700 and the 350 at 1.740. I think that maybe the pistons in the kit were made for a 350 and not a 327. Everything else in the kit fit, including the rod and crank bearings. This engine is still on the engine stand!!! Still all painted nice orange with its AFB. It is a 300 hose motor. I remember I installed a 'blueprint' cam and lifters. I suppose I should take it apart this winter.

Kenny
khargens1024@msn.com
An uncut block will be right at 9.025" from crank c/l to deck. The compression height (pin centerline to piston deck) of a stock 350 piston is 1.56". The difference in compression height between a 327 and a 350 piston would be about 1/8".

Chances are (hopefully, at least) you used "rebuilder" pistons that are made about 0.020" shorter than stock pistons to account for milling the block and heads. That would put the pistons down the hole about 0.045", not counting the head gasket thickness.

If you DID use 350 pistons, with the piston deck clearance so wide the compression would be very low (unless you have domed pistons) and the quench effect non existent. But it should run on Coleman fuel!
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Old 11-13-2012, 03:12 PM
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Deck Clearance??

Quote:
Originally Posted by hcompton View Post
rebuilder pistons have a low deck height so you can surface the block. so they are usally a little lower in the block but not by much. should be no big deal unless your going to max effort racing.
Hello Cobalt327,
You know, It has been well over 20 years since I rebuilt that 327 and hardly a day has gone by that I have not thought about that piston/deck clearance. I guess I was just in a hurry to get all the parts together in one lump. And then it has just sat on the engine stand since. I WILL remove on of the cylinder heads and do some accurate measuring. Probably take a piston and rod out. I should be able to do that this winter among putting the '57 Corvette engine into my '41 Ford Coupe, the V12 Lincoln into the '30 Model A Coupe, and the restoration of the two vintage BSA's!! Kenny
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Old 11-13-2012, 03:33 PM
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First things first. There is block deck height, which is the measurement from the centerline of the main bearing bore to the block deck where the heads bolt on. Then there is piston deck height, which is the measurement from the crown of the piston to the block deck where the heads bolt on, with the piston at top dead center.
Block deck height, piston deck height. Get used to using the correct terminology and everyone will understand where you're at.

As Mark stated, a stock Chevy block is ~9.025" block deck height. The factory used a "stack" of parts to reach 9.000", leaving a piston deck height of 0.025". The "stack" would be the piston compression height, the rod length and half the crankshaft stroke. For instance, on a 350, the piston would be 1.560", the rod would be 5.700" and half the stroke would be 1.740". Adding up these 3 values would equal 9.000". Adding a steel shim gasket at the factory (roughly 0.020" thick), would yield a squish of 0.045".

A 327 piston would measure 1.675", 327 crank radius of 1.625 and rod length of 5.700", for a "stack" of 9.000", so using a 327 piston with a 5.7 rod and 350 crank would push the piston out of the hole by 0.090" (0.115" less 0.025" piston deck height).

When machine shops take a cut on the block decks to equalize the height of the block decks from the centerline of the crank or to correct a flaw of some kind on the block decks, they can, for instance, take a 0.020" cut on the block decks and then install "rebuilder" pistons, which are 0.020" shorter on the compression height. The problem arises when the engine builder uses these pistons in a block that has not been cut. It puts the piston down in the bore by 0.020" deeper than the stock arrangement and generally negates any useable "squish" that the builder had planned for. For instance, let's say that a builder used a stock block deck height of 9.025" and a rebuilder piston along with a ~0.040" gasket (mandatory with aluminum heads due to brinelling with a thinner gasket). Now, instead of having a squish of 0.045", he ends up with a squish of 0.085". I should say "lack" of squish. Now, the motor has a much better chance of detonating, depending on the DCR and fuel quality.
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Old 11-13-2012, 04:25 PM
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Quote:
Originally Posted by bsakennyh View Post
Hello Cobalt327,
You know, It has been well over 20 years since I rebuilt that 327 and hardly a day has gone by that I have not thought about that piston/deck clearance. I guess I was just in a hurry to get all the parts together in one lump. And then it has just sat on the engine stand since. I WILL remove on of the cylinder heads and do some accurate measuring. Probably take a piston and rod out. I should be able to do that this winter among putting the '57 Corvette engine into my '41 Ford Coupe, the V12 Lincoln into the '30 Model A Coupe, and the restoration of the two vintage BSA's!! Kenny
I guess your hoping for a long winter lol. Unless you feel there is something else wrong i wold just run it. It will be fine. Happens all the time even if. Its not right it will run just fine. For an old hotord it will be very nice. Now if your going to some big power then maybe decking the block is in order or installing different pistons. But not only will decking cost less its also easier to get the correct measurement. Getting measurements and calling a piston maker to get the correct piston is going to be hard for an engine builder to pull off and even with the right measurements they will still send the wrong ones out. You can have the block decked with ease and be back together in the same day.

You havent messed with it in a long time. Its a good bet this winter is not going to have an open scheduale either so best to just quiet thinking about it and let it fly. If its slow sell it and start over with a 383.
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Old 11-13-2012, 04:52 PM
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If its slow sell it and start over with a 383.
....or chrome it....
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Old 11-13-2012, 05:47 PM
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....or chrome it....
At some point we have all had a stalled project and you just got to slap it together and get it going. Either to find some pashion for the project or drive it to the junk yard so its does not cost you any more cash to tow away.

Once you had an engine on the stand complete for 20 years its time to do something with it. Even if that means giving it to the kid down the block and moving on. Because you know an empty engine stand will find a way to fill its self.

Hope this helps.. Also hoping I live down the block from you...
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Old 11-13-2012, 06:35 PM
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Quote:
Originally Posted by bsakennyh View Post
Hello Cobalt327,
You know, It has been well over 20 years since I rebuilt that 327 and hardly a day has gone by that I have not thought about that piston/deck clearance. I guess I was just in a hurry to get all the parts together in one lump. And then it has just sat on the engine stand since. I WILL remove on of the cylinder heads and do some accurate measuring. Probably take a piston and rod out. I should be able to do that this winter among putting the '57 Corvette engine into my '41 Ford Coupe, the V12 Lincoln into the '30 Model A Coupe, and the restoration of the two vintage BSA's!! Kenny
I LOVE BSA's! I had a 441 way back, my second bike ever, and wish I had it back on a regular basis.

When you get around to the 327, pull a head and bring a cylinder up to TDC. Then use a straight edge and feelers or a depth micrometer to measure from the deck of the block down to the piston deck. That way you don't need to remove the pan to take a rod and piston out unless you just want to look the bottom end over, too, while you're at it.

This would be a great opportunity to verify the timing marks are accurately indicating TDC, too. There were three commonly used damper line/timing tab combos used through the years (more now, if you count the plastic timing covers on the late model Gen 1 SBC engines). The three older styles can get mixed and matched up all wrong and this can skew things when it comes time to set the timing on it.

With those cool projects, sounds like you're in for a fun winter! If you get a change, some photos would be nice.
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Old 11-14-2012, 06:35 AM
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My 4.3L V6 has Speed Pro Sealed L2256F pistons (350 CI V8). As-built deck height specifications:

5.700" rod length
+1.563 piston comp. height
+.022" std. piston to deck
- .010" resurfaced deck
+ 1.740" half 3.48" stroke

9.015" Total deck height

10:1 CR with 64 CC heads. Borderline with 93 octane pump gas.

Engines run cooler with high initial timing advance (12 deg.) and a minimum allowable (zero) deck height (quench) and permits higher compression ratio. I was concerned about being able to use 93 octane pump gas with 10:1 CR but it turned out fine even with my camshaft which only has 39 deg. valve overlap.

Last edited by MouseFink; 11-14-2012 at 06:58 AM.
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