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Piston Height Above Deck I know that the Ford blueprints show a defined dimension by which the pistons protrude above the block deck but has this been maintained by any of the various piston manufacturers over time? In other words, is there a variation in this aspect over the years or do they all tend to adhere to the original Ford design?
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Re: Piston Height Above Deck on ones that i have torn down i have had some that were level and some that protrude. i guess manufacturers do different things.
on the new ones i have installed they have all protruded a bit. |
Re: Piston Height Above Deck For many engines, the block has been decked during an overhaul and this process also changes how far above the deck the piston top protrudes.
John |
Re: Piston Height Above Deck So, what is the max?
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Re: Piston Height Above Deck Harry Ricardo wrote in his book in 1931 that the clearance between the top of the piston and the squish area pocket of the head should be 1/16 inch. He said that in his tests, more than that would not provide significant turbulence and less would not have any added turbulence and would cool the air/fuel there. So the test is to put a pad of modeling clay on the top of the piston and run it up to the top with the head installed. The head can be bolted on lightly but remember that the gasket compresses quite a lot depending on what gasket is used. Terry Burtz says in his instruction manual that "The Best 509C (clad) gasket compresses from 0.080 to 0.052 inches during the application of <head nut> torque, and the Best509G (graphite) compresses from 0.058 to 0.055 inches during the application of torque." I assume that those are the pre and post thicknesses and not the total amount of compression, but I could be wrong. If the clearance is too small you can use a thicker gasket or have the squish area pockets in the head machined. If too large you can have the head decked.
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The clearance between the combustion chamber an the top of the piston at TDC is called the quench.. ideal quench is .040 for performance, often an engine with piston protrusion is compensated by the head gasket thickness, but it’s not always the case, care should be taken and math should be done with performance heads that have no fly cut or piston relief cut in. Pictured is a head with piston relief or fly cuts.
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For example, a Mike K, who I would credit with considerable expertise, said: [quote] "Just for the record an original stock block with 11.500" deck height had the pistons 0.0315" above deck if all else was to spec. The blocks themselves were spec'ed at 11.500 -11.505" and usually came off the machining line nearer the high limit." |
Re: Piston Height Above Deck Jack, that is a nice combustion chamber design. Who made that?
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Re: Piston Height Above Deck That's an original Winfield, of course.
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Re: Piston Height Above Deck Ed Winfield designed it… I was scanning complete car ads and saw it in the ‘extra parts’ pictures.. a young man was selling his grandfathers Tudor, and all his parts..he agreed to sell the head and a dual carb manifold.. paid him a fair price, more than he asked, waited till I was down the street a block before dancing a jig..this was before Tod started making these new..
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What’s interesting is how close Ed was to perfection.. Briggs and Stratton and Jim Brierleys head are the latest in chamber design, shaped just like mine with a “step” in the chamber
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Re: Piston Height Above Deck I ran a milled cylinder head with 0.09" clearance and it was not enough. A slight carbon deposit caused a knock.
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Re: Piston Height Above Deck Milling a model A head,due to its combustion chamber design, doesn’t provide an effective compression increase and the resulting gain in efficiency needed for reducing the quench area.
Performance is the sum of all parts needed..air,fuel,compression, spark and exhaust working together..folks install a high compression stock appearing head and then run all stock components don’t get the full yield from the compression increase. |
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Thank you for responding. Why I was asking is I have many of the original prints and as I stated, I have never seen that written anywhere. I really wasn't asking for hearsay, but was more about just wanting to know where you had seen that was written. FWIW, I am not about to contradict others whom you may feel that "know their way around the A engine block", but my own factory Model-A prints disagree with what you are saying. That is why I was asking where you had seen this. To begin with, the engine A-6015 engine block print uses two numbers to designate the engine block overall height. They are 8.624" - 8.627" and 2.876" - 2.878". As you mentioned above, those two numbers total to be 11.500" - 11505". I'm not sure where you determined that the specified dimension came out to the higher number during the factory machining, but we'll use that figure for my question. All of my A-6110-* Piston factory prints call for the top of the piston down to the centerline of the pin to be 1.904" - 1.909". All of my A-6200 Connecting Rod prints call for the centerline of the rod pin to the center of the crankshaft pin to be 7.498" - 7.500". We know the stroke of the Model-A crankshaft was specified to be 4.1250" -so when you add 2.0625" (½ of the stroke) plus the specified length of the connecting rod, plus the specified height from the pin to the top of the piston, my math comes up to 11.4685", -or over 0.030" under the deck surface of the cylinder block. Therefore, if I am correct in my math it really does not indicate the piston protruded above the deck when it left the factory. So what am I missing?? |
Re: Piston Height Above Deck Whoa, Model A engine stroke not 4.125". Is 4.250"
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Re: Piston Height Above Deck Quote:
https://model-a-ford.org/top-page-1/...pecifications/ |
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Ford drawing A-6303 specifies the offset between main and connecting rod journals to be 2.125/2.127 inches which is a 4.250/4.254 inch stroke. |
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Good catch Bob & Terry! Thanks!! I have included an excerpt of the A-6303 drawing that Terry mentioned so that we are all clear that the average of 2.123 / 2.127 is 2.125. . . |
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