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401 pistons in an early 390

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brownspirit View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote brownspirit Quote  Post ReplyReply Direct Link To This Post Topic: 401 pistons in an early 390
    Posted: Nov/28/2020 at 9:40pm
I looking to put together a 1968-69 390 for my scrambler.  I’m thinking I have a 68-69 block and crank as well as a set of ported 291c heads I want to use.  This is a pump gas deal for sure.  From what I’ve seen I can use the off the shelf Weisco 401 pistons as long as I use 401 rods (planning on a set of molnars anyway).  I’m I better off with custom pistons, as I understand the quench ends up being an issue with this combo?  Just trying to save a few bucks, but want to make sure it’s not counter productive.  Thanks. Andy
69' BBB AMX 390/4-speed

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Post Options Post Options   Thanks (0) Thanks(0)   Quote 6PakBee Quote  Post ReplyReply Direct Link To This Post Posted: Nov/28/2020 at 11:11pm
I end up with the pistons being about 0.030 in the hole.

3.574/2 (390 stroke)
+ 5.858 (rod length)
+ 1.50 (pin height) =

9.145 (piston height)

9.175 (1969 390 deck height)


Roger Gazur
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Trader View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Nov/29/2020 at 8:05am
On the Wiesco site for the 401 pistons they offer the K1 H-beam rods that are advertised at 5.875". This would leave your pistons at 0.013" down the hole. With an off the shelf Cometic 0.027" head gasket, you could be well within a good quench height.
The problems of late with Felpro head gaskets, I avoid them now.

You will have to watch your CR. The above with the heads your using will put your CR at 10.5:1. A little high for pump gas. This could be resolved by choosing a cam that lowers the DCR to 7.5 to 8.5. A 993 head would put CR to 9.8:1 so if that is an option for you, it would be better.



Edited by Trader - Nov/29/2020 at 10:05am
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JGRANTAMX View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JGRANTAMX Quote  Post ReplyReply Direct Link To This Post Posted: Nov/29/2020 at 11:52am
I have a set of 75' 502-2 heads recently pulled from a 401 doing nothing, I believe they are 58cc and would have a CR of around 9.5:1.

J
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Post Options Post Options   Thanks (0) Thanks(0)   Quote garland performce Quote  Post ReplyReply Direct Link To This Post Posted: Nov/29/2020 at 4:19pm
.030 in the bore is not good for a pump gas engine it will be on the soft side and prone to detonation

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Post Options Post Options   Thanks (0) Thanks(0)   Quote garland performce Quote  Post ReplyReply Direct Link To This Post Posted: Nov/29/2020 at 4:23pm
You could mill around .017 off block and use a .027 cometic head gasket
and cut the chamder open a little to get comp ratio around 10 to 1
cut the chamber on the intake valve side it will also help low and mid range flow
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madmax View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote madmax Quote  Post ReplyReply Direct Link To This Post Posted: Nov/29/2020 at 5:00pm
wiesco makes 390 pistons, do your math on the comp ratios.


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Trader View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Nov/29/2020 at 5:40pm
Yes Wiesco does make 390 pistons, keep forgetting this and my computer insists on linking to their old catalogue. 
If you do some digging though, Wiesco and DDS 390 pistons base their posted compression ratios on a 54cc head. The 291's are 51cc. I don't know were they got the 54cc head number from.
If you work with the 51cc head, 25cc piston, 0.043" head gasket, 0.000" deck, I get 10.7:1 and not 9.8:1 advertised.
Even with the 54cc number I get 10.4:1.

I could be doing something wrong, please check. I also assume a 4.2" gasket diameter.


Edited by Trader - Nov/29/2020 at 6:34pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote JGRANTAMX Quote  Post ReplyReply Direct Link To This Post Posted: Nov/29/2020 at 7:00pm
 Edelbrock Performer 401 heads are listed as 54cc.

J
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Trader Quote  Post ReplyReply Direct Link To This Post Posted: Nov/29/2020 at 7:49pm
OH, the shiny ones with the $$$ in front of them. I never got past the $$$ to actually look at specs on those. The shiniest I get is the pistons. I once had a chance to buy an R4B for $200. That I'm still kicking myself for not picking up.
Good to know where the 54cc came from, Thanks J.
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