Two head gaskets real or myth?


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So guys I've always heard HP 409s had two shim head gaskets from the factory to drop CR. My first 64 was a 340 HP and only one.

Real or myth?
Yep it's real I was an apprentice helper on one(62?) and we didn't know one was better so replaced with two. But, heh Heh the car had a TRW Blueprint cam and ran pretty good anyway.

Fathead Racing

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Supporting Member 7
I have heard they did not come from the factory with two head gaskets but the second gasket was installed by the dealership after customer complaints about detonation . I can imagine some of the customers were either cheap skates who didn't want to pay for high test gasoline or were in a place where high test wasn't available.


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Sometimes the 2 headgasket deal was a tactic used to make for a streetable compression ratio for normal use but allowed the racers to to increase compression ratio by using 1 and still meeting factory specs.The factory specs. called for a headgasket of a certain thickness but rarely specified how many were used,so if they were torn down they were legal while maximizing their compression ratio.As skydogs post points out you had dig into the service bulletins to find that 2 headgaskets were used.Fords version of this for the 427 was that they had 2 steel shim gaskets with 2 part numbers,the .015 gasket and its part number was the one specified in the factory specs,and this was the one used in factory race cars like lightweight Galaxies and Thunderbolts,but the street version 427 cars had the .030 thick gasket installed from the factory.Its part number was the one specified for use in cars in for service when they were to be used for normal street duty to avoid complaints from use with lower grade fuels.


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Since the piston fitting procedure specifies all 409 engines does that mean that 340 hp engines also had extruded (forged) pistons?Or am I under an incorrect assumption that hi perf engines did.


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Got nothing in TSB's or CSN's for the 61 409. There must be some sort of official GM specs on the engines other than the AMA specs revised Feb. 61, but I haven't found it yet. Took long enough to find the 58-61 348 solid lifter carb specs. What is really needed are the official Parts Lists for all the RPO engines. They list all the components that make up each RPO engine part number. I'm surprised that Fran didn't have them.


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That bulletin was for 61 and 62 409 engines.The cast piston equipped 340 horse appeared with the advent of the 63 models John.
That makes sense I wasn't aware that the 340 motor was not available in 62,and the 61 360 horse motor was basically the hi perf short block with the 348HP cam and top end right?


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The 61 409 had special heads...3795586 part number.
But they were nothing but the 1147 heads with bigger holes for the push rods. Just my thoughts but I believe the 61 motor only had one head gasket, the motor was nothing more than an enlarged 348/350 horse motor. open a can of worms, it was nothing but Chevrolets quick answer to Ford's 390/375. Ford came out with the 375 horse motor and right behind that they put 3x2's on it and bumped it to 401 and here sat Chevrolet with nothing but a 348 rated at 350 horse. They had to do something and quick. The big changes came when they redesigned the 409 and that's when they needed to cut the compression to make the motor streetable. Just my thoughts


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I agree with your thoughts,when the 352/360 and 390/375 came out they were actually pretty realistically rated and were a big step forward in performance and Chevrolet had to quickly take drastic measures to stay competitive.The later 390/401 and 406/405 looked good on paper and would easily achieve their claimed power levels after a good round of power tuning but seldom achieved that off the showroom floor.It would appear that Ford's attention to detail on the production line wasn't up to level required for multiple Holley induction setups,it wasn't uncommon for the carb linkage to be set so that it failed to provide full throttle opening with the pedal to the floor.Having driven a well tuned and setup FE tripower I would have to say that it was as seamless and proportional as any car I've driven.I don't know if it was due to the simpler AFB or better attention to detail on Chevy's production line but their offerings seemed to more closely reach their rated power off the showroom floor.