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Old 06-01-2005, 11:00 AM
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Please help 781 cylinder heads..ASAP

Ok guys I have this problem.. I wanted to build up my 454 and everyone said the -781 heads are great! Get them who hoo...Well now I have some I found out there are "bad" heads out there with short intake valve springs! Whats the deal with that? How can you tell if you have this undesireable head? I also found out there are 2 different casting numbers? 336781 and 386781? Is my information incorrect or if it is correct what should I do? How do I fix this problem if I have one? Thank you all so much!!! any and all information welcome

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Old 06-01-2005, 11:28 AM
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This is all the info I found on them:
336781.....73-85...oval...OPEN...454, 118cc chamber, 256/114cc ports
I didn`t find a record of a 386781, I didn`t find any big block castings with the prefix of 386. It`s possible that the 3 was mistaken for a 8 as is the case on many castings. It seems to me this would be a decent performance head, it don`t have the peanut ports, and the large chamber is the only draw back I see.
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Old 06-01-2005, 11:36 AM
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Have you heard about the valve spring before?
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Old 06-01-2005, 12:09 PM
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heads

The reason the springs are short on the intakes is because the 781 used rotaters on the exhaust side. There is nothing wrong with these heads. You will need a .300 thick insert to go on the exhaust side so that both valves use the same height springs.

For any typ of street, street/ strip engine these heads are the real deal. Just ad a set of 2.19/1.88 valves and they will support 600+ horse power and not loose any low end....

Keith
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Old 06-01-2005, 12:16 PM
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Yea what Keith said...
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Old 06-01-2005, 12:17 PM
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Is there years that seperate the 2 models? Im really not sure if I even have the rotator heads..how can I tell for sure?
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Old 06-01-2005, 12:24 PM
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They all had the rotators... (for 781`s & 049`s) Just contact your local machine shop for the skinny on these... Also you can find them on fleabay (the eliminators that is...)


Oh and yes BTW that 8 is a 3 if you look real close... But their are some different 781 casting heads and they have the X on them also... Not sure if its true but they are supposed to be like SBC heads with the X and flow about 10cc more than a reg 781 non X head...? Anyone else with info this please share it...?
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Old 06-01-2005, 12:59 PM
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Quote:
Originally Posted by Bumpstick
They all had the rotators... (for 781`s & 049`s) Just contact your local machine shop for the skinny on these... Also you can find them on fleabay (the eliminators that is...)


Oh and yes BTW that 8 is a 3 if you look real close... But their are some different 781 casting heads and they have the X on them also... Not sure if its true but they are supposed to be like SBC heads with the X and flow about 10cc more than a reg 781 non X head...? Anyone else with info this please share it...?
You made my day thanks for making me feel a little better
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Old 06-01-2005, 01:08 PM
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these??
http://cgi.ebay.com/ebaymotors/ws/eB...sPageName=WDVW

or these:
http://cgi.ebay.com/ebaymotors/ws/eB...sPageName=WDVW

sorry guys very new to heads.. I do appreciate the help

Last edited by badazz81z28; 06-01-2005 at 01:28 PM. Reason: add info
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Old 06-01-2005, 05:17 PM
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Thats the ones... BTW I had just bought a set from the guy in the first link you posted... Have a nice set of semi-closed chamber virgin untouched big ovals that they will be going in...
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Old 06-01-2005, 05:48 PM
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I have a set of those! The installed height of the springs is shorter for some reason on both the intake and exhaust (with rotators or spacers)! I did find a short sentence in "a how to build a bbc book" about some rare 781's having this shorter than normal spring height.

Yes the rotators need to be replacement with spacers. But the installed height of the springs is about 0.100" less than a normal head. Something like 1.750" versus 1.860".

These heads with the short intake installed height are a little rare. I had to machine the exhaust spacers down a little to corrected it but that only works on the exhaust valves. Then I used 10 degree locks and retainers to gain more installed height. FYI, Performace retainers have higher spring installed heights as compared to stock retainer. Kind of a pain in the butt.

I ended up buying a spring pressure tool to set these heads up correctly. You might be able to have a machine shop cut the intake spring pockets down a little. Hp retainers and 0.050" off the spring pocket in the head should fix it.
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