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GAS FOR OLD FLATHEAD PLYKMOUTH SIX


37Plymouthguy
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Hi,

I've heard that you shouldn't use today's gasoline in old car engines. I've just rebuilt my 1937 Plymouth engine, and don't want to damage it. What kind of gas should I put in it? Or, alternatively, is there some way to modify today's gas with chemicals, to make it usable in my old flathead six? Thanks for your help. Willard Crawford

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If when you rebuilt the motor, you had hardened valve seat put in then todays gas is fine to use.

That's the key to be able to use unleaded gas is having the hardened valve seats. The lead in yesteryears gas had the lead so it helped cushion the valve.

I know guys with motors that not have been rebuilt that use todays gas as well. Your motor should be fine .

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the flathead valve seats have always been hardened from the factory..other than rubber issues with the fuel pump diaphram there is no problem running up to 12% ethanol as most folks report..I assume this is what you are referring to as today's gas...you nervous, go to the Pure station or check online for your area for station that sell non ethanold deluted fuel

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the flathead valve seats have always been hardened from the factory..other than rubber issues with the fuel pump diaphram there is no problem running up to 12% ethanol as most folks report..I assume this is what you are referring to as today's gas...you nervous, go to the Pure station or check online for your area for station that sell non ethanold deluted fuel

Concur with bigaadams: All those Plymouth L-6 engines from 1933 up came from the factory with hardened exhaust valve seat inserts from the factory. The compression ratio in '37 was still very low by today's standards so your octane requirement is easily met by the lowest grade you can buy today. Only issue is that modern fuel additives are not kind to old rubber, so replace the flex line between the frame and the motor, rebuild the fuel pump with a kit containing modern materials (Antique Auto Parts Cellar, a.k.a. Then And Now Automotive has them). If your carburetor has a rubber tip on the float valve needle, then it should be replaced too. Then just go and find the cheapest gas you can and your engine will be happy.

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Thank you very kindly, ply33, bigaadams, and chopped 50. Your experience and knowledge is much appreciated. Now I know what I need to do to keep my Plymouth happy. One more question, engine break in requirements. How long should I run the engine at idle to break it in properlly? Should I use special oil for breaking it in? Should I change engine rpm's while breaking in? Thanks again for your help guys. Willard Crawford (37plymouthguy)

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you do not break in at idle..you do so at a higher RPM for x period of time...I try to run the engine at 2250 minimum baseline for 30 minutes with occassional higher revs of the engine during the run in..do not go wide open or hold the higher rpm..I assume you have new cam and lifters? this is the only high pressure surfaces and will require good break in lubrication..I usuaolly will never use company supplied cam lube..I mix my own molybendylumdisufide paste for cam and lfiter break-in..the flattie is not high lift nor hign tension springs but if new surface you still wish to provide adequate protection on initial run..

insure if you can that the tiing is set for a good start, proper oil and coolant and inspect for leaks..try not to shut down in the midlle of the break in if possible..howver if you have lube coolant issues..then by all means correct and start again..

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I should have been a little more clear on my post..this is initial break-in of a rebuilt engine I was referring ..it is not a substitiute for proper driving habits of a new engine..this break-in is mainly for the new cam and lifter prior to seeing road service..

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Given that our Plymouth engines develop max torgue and HP at relatively low RPM's and low compression ratio the use of un leaded fuels should not be a problem.

The use of ethanol fuels is another thing and this is due more to the ethanol's effect on diaphragms in fuel pumps or any other elastomer in contact with the fuel.

In Texas we have low ethanol, max is 10%. It may be higher than that in other states.

Chris

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