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Old 09-12-2012, 02:24 PM
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crosswinds intake/ procomp heads sbc

First time on this forum but I was wondering if anyone is running procomp heads with a professional products crosswinds manifold. I have heard that you need to run a thicker gasket to help runner alinement. Is this true ? thanks for any input.

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Old 09-13-2012, 12:05 PM
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Quote:
Originally Posted by charlie3030 View Post
First time on this forum but I was wondering if anyone is running procomp heads with a professional products crosswinds manifold. I have heard that you need to run a thicker gasket to help runner alinement. Is this true ? thanks for any input.
Fel Pro makes intake gaskets for many bolt arrangements and port sizes for the SBChevy head that come usually in .060 inch or .120 inch thicknesses. Sometimes you can find choices in .030/.035 and .040/.045. You have to also check for the head angle, production is 23 degrees but there over the counter GMPP and aftermarket heads at 18 and 15 degrees so you have to watch this as well when ordering.

There are differences from engine to engine depending upon operations done, or not, to the block for normal production dimension tolerance for both the block and anybody's heads and intakes. So once you have the main castings ready to go, you must, or at least should, mock assemble the engine to determine what the assembly dimensions will be. Building an engine, especially one with parts from multiple sources requires 1 or more mock up assemblies to find out if and how it all goes together. Seldom is a custom built engine's assembly as simple as taking parts from the bench and bolting them together. This also precludes telling you to go and buy this or such gasket thickness.

- You will need to start with a head gasket purchase that provides both the proper material to sandwich between and iron block and aluminum heads. Then you have to select a thickness that when combined with the clearance between the top of the block and that of the piston provides the needed squish/quench clearance.

- Then you bolt it up with a small lump of clay on a piston and pull it around to get an impression of the valves in the clay. Then take it apart and check the amount of valve to piston clearance and adjust the relief cuts in the top of the piston as needed to get something on the order of .050 inch or more for the intake and .080 or more for the exhaust. Of course you have to be careful here when cutting as you don't want to put a hole through the top of the piston, nor make a thin area that becomes a hole in the piston a few miles down the road.

- While the heads are on in the above mock up you can also check for intake alignment and while youíre there and the pan isn't on you can flip this thing over on the engine stand and check the distributor to oil pump installation to see the tangs and grooves engage with about a minimum .050 bottom clearance when the distributor is bolted in with its gasket.

- While it's upside down if this is a stroker and if you haven't checked clearance of the connecting rods to the pan rail and cam lobes you might as well catch that will your here.

- Since the Playdough is out put a lump on the oil pickup screen, drop the pan gasket on followed by the pan to check where the pick is from the bottom of the pan, I like 1/4 to 3/8ths inch. Some folks will go to 1/2 on the long side. Rotate the crank to be sure nothing catches on the windage tray, crank scrapers and trapdoors. Be sure that none of the Playdough got squished into the oil pump intake tube before you fire the engine.

This is just a high level cruise of the mock up stage. You will need several thicknesses of intake gaskets and maybe heads as well but given some of these can be quite expensive it's way easier on the ol' Visa card to have a dimension in mind going in for the head gasket then adjust the problem gaps at the intake to head.

Since youíre not going to start the engine at this point these gaskets can be reused for final assembly so be careful not to damage them.

- Next up will be tracking the rocker to valve stem contact patch. These aftermarket heads are often dimensionally different than GM production so to move the intake port to a better flowing position but needing to keep the combustion chamber in the OEM position it is common to see longer than OEM valve stems .1 inch seems to be a common practice. Since this will change where the rocker tip rides on the stem, the rocker may have to be raised to put this interface into correct alignment. This means extra length push rods, but due to where the valve stem is at angle and distance from the rocker stud, where the rocker stud is from the push rod, etc. the actual pushrod length adjustment may not be a simple increase of .1 inch. Here you need an adjustable push rod to use for determining the needed length that puts the rocker in the proper location on the valve stem.

So by the time you do all these checks, the engine is usually mocked up and disassembled certainly once and often a couple times before final assembly. These steps can be skipped for an OEM rebuild as the factory hasnít time to do this stuff so the original dimensions are such that the parts can just be bolted together having sufficient clearances. When you start putting aftermarket parts into the equation of assembly, then a lot of checking has to take place. Since dimensions and tolerances change between suppliers and also within a supplierís different production runs, you the engine builder will always have to check the assembly for proper alignment and clearances even when doing common builds with same/similar components.

Bogie
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Old 09-13-2012, 12:23 PM
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Thanks for your input. I know now that it is not just throwing together a bunch of parts to go fast. Thanks again, Charlie3030.
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