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401 pistons in an early 390

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Forum Name: AMC V8 Engine Repair and Modifications
Forum Description: AMC-made V8 engine mechanical, ignition and fuel from basic repair to high-perf modifications
URL: https://theamcforum.com/forum/forum_posts.asp?TID=108426
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Topic: 401 pistons in an early 390
Posted By: brownspirit
Subject: 401 pistons in an early 390
Date 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

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69' BBB AMX 390/4-speed

69' s/crambler project

69’ International F210d Cummins/5+4



Replies:
Posted By: 6PakBee
Date 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)




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Roger Gazur
1969 'B' Scheme SC/Rambler
1970 RWB 4-spd Machine
1970 Sonic Silver auto AMX

All project cars.

Forum Cockroach


Posted By: Trader
Date 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.



Posted By: JGRANTAMX
Date 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


Posted By: garland performce
Date 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



Posted By: garland performce
Date 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


Posted By: madmax
Date Posted: Nov/29/2020 at 5:00pm
wiesco makes 390 pistons, do your math on the comp ratios.


https://theamcforum.com/forum/wiseco-390-pistons_topic105108.html" rel="nofollow - https://theamcforum.com/forum/wiseco-390-pistons_topic105108.html


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American Heavy Metal

restoration parts and mechanical restoration services

70 amx black
1968 kaplan amx ss/racecar
1969 amx racecar
1970 trans am javelin
1969 amx white/platinum
1972 gremlin


Posted By: Trader
Date 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.


Posted By: JGRANTAMX
Date Posted: Nov/29/2020 at 7:00pm
 Edelbrock Performer 401 heads are listed as 54cc.

J


Posted By: Trader
Date 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.


Posted By: brownspirit
Date Posted: Nov/29/2020 at 8:49pm
I didn’t know they were offering 390 pistons these days, they must be relatively new.  I could do a set of larger chamber heads, which I should have in stock.  I’d rather not though because 291s I have were done by Adkins and they have decent parts in them.  I’d rather pop for custom positions to get exactly what I need.  I don’t want to end up with a pinger, that’s no fun.  There is a good chance the block only need to go .20 over, if rings are easy enough to find that may be worth the aggravation of going custom. Andy  

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69' BBB AMX 390/4-speed

69' s/crambler project

69’ International F210d Cummins/5+4


Posted By: 6PakBee
Date Posted: Nov/30/2020 at 9:22am
Andy, I'm glad to see you reconsidered selling your SC/R and are getting back into it.  It'll be a great winter project.


-------------
Roger Gazur
1969 'B' Scheme SC/Rambler
1970 RWB 4-spd Machine
1970 Sonic Silver auto AMX

All project cars.

Forum Cockroach


Posted By: BassBoat
Date Posted: Nov/30/2020 at 10:29am
the Wiseco "390 pistons" are for a 70.  1.553 compression height if memory serves.  
Nothing wrong with your original plan.  Yes, pistons down in the hole are not optimum but a 390 starts out farther down in the hole than your 401 pistons will be.  
Stick to your plan.



Posted By: Trader
Date Posted: Nov/30/2020 at 11:59am
These people make a proper 390 piston. I'm sure you could custom order the CR you desire. I've only had one set of their pistons in hand, but they are equal if not better then Wiesco's.
https://dssracing.com/AMC-pistons-360-390-401.htm" rel="nofollow - https://dssracing.com/AMC-pistons-360-390-401.htm


Posted By: wrebelmachine
Date Posted: Nov/30/2020 at 12:08pm
x2 on the DSS Pistons


Posted By: SC397
Date Posted: Nov/30/2020 at 3:13pm
RaceTech and Diamond will make you exactly what you want as well.
I buy a lot of RaceTec pistons. It usually takes about 3 weeks to get them.


Posted By: brownspirit
Date Posted: Nov/30/2020 at 8:18pm
Originally posted by 6PakBee 6PakBee wrote:

Andy, I'm glad to see you reconsidered selling your SC/R and are getting back into it.  It'll be a great winter project.

Thanks Roger, I don’t think I will be able to dig into the welding until the spring.  But the more small projects I can knock off the list the better.  Andy


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69' BBB AMX 390/4-speed

69' s/crambler project

69’ International F210d Cummins/5+4


Posted By: brownspirit
Date Posted: Nov/30/2020 at 8:23pm
Thanks for all the info guys, looks like I got some thinking to do.  Just looked at the order form on Diamonds website, there was a lot of questions I couldn’t answer.  Lol.  Andy

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69' BBB AMX 390/4-speed

69' s/crambler project

69’ International F210d Cummins/5+4


Posted By: garland performce
Date Posted: Nov/30/2020 at 8:57pm
Yes .020 rings are .060 400 sbc can get several ring packs 
I`m a race tech dealer if I can help you give me a call at the shop 
Monday -Friday 7 to 4 816-547-5695
          John Garland 


Posted By: PHAT69AMX
Date Posted: Dec/01/2020 at 5:24am
On the Wiseco website the last time I looked recently the catalog has a typo error for the AMC 390 Piston Compression Height that is incorrect, even called them and they confirmed.  Maybe some day the online information will get updated.

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Posted By: klvn8r
Date Posted: Dec/02/2020 at 8:55pm
I'm currently helping my son build his first 401.....probably settling on the Wiseco + .045   Is anyone a dealer with IN STOCK?  I'm unfamiliar with DSS and Racetech.....but will gladly listen!  Thanks!

klvn8r


Posted By: klvn8r
Date Posted: Dec/02/2020 at 9:16pm
I'm currently helping my son build his first 401.....probably settling on the Wiseco + .045   Is anyone a dealer with IN STOCK?  I'm unfamiliar with DSS and Racetech.....but will gladly listen!  Thanks!

Also, SC397....I posted a reply to you on the topic in the very top, regarding balancers.  Thx.


klvn8r


Posted By: PHAT69AMX
Date Posted: Dec/03/2020 at 12:30am
Have ya called Wiseco direct in Ohio to see if they have any in stock?

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Link to a http://www.youtube.com/watch?v=MySiKQsmWxU" rel="nofollow - Short YouTube Burnout Video



Posted By: AMXFSTBK390
Date Posted: Dec/03/2020 at 12:13pm
Was I ever astonished when I tore down the 390 in my 68 AMX. It had 401 rods and pistons at .040. I was told to make that work, customs pistons are required with a different wrist pin location. At that juncture, I acquired a std bore 68-69 390 bare block from Galvin's and went a different route. 

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Questions are powerful tools...what's in your toolbox?


Posted By: brownspirit
Date Posted: Jul/31/2021 at 12:20pm
Okay, 8 months later I got a block to the machine shop, it needs to go larger than .030 over.  The bores are true at .030 but there is a small amount of rust. The block had previously had .020 taken off the deck.  Anyway, I was going to order the Wiesco PTS540A45 and a set of Molnar 5.858” 401 rods.  According to my calculations this should come out to the pistons being .010 in the hole using the 68/9 390 block decked to 9.155, 390 crank 3.574 stroke, Wiseco pistons at 1.500 compression height.  Also it should get me about 10.2 compression with the -27 cc reverse dome pistons, felpro head gaskets, and my 291c heads.  I actually bet those chambers are bigger as Adkins ported them for me.  I will get them cc’d to find out for sure, but I would imagine the compression ratio will be a little lower, which I perfer.  Does this all make sense, just asking before I order.  Thanks, Andy

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69' BBB AMX 390/4-speed

69' s/crambler project

69’ International F210d Cummins/5+4


Posted By: 6PakBee
Date Posted: Jul/31/2021 at 12:32pm
Andy, before you break out the boring bar, how bad is the surface rust?  If you run a ball hone through it will it disappear?  I put a 2.2 Mopar together with a bit of rust in the bores (against the recommendation of my machinist who wanted to overbore) and five years and another 100,000 miles it was still going strong when I sold it.


-------------
Roger Gazur
1969 'B' Scheme SC/Rambler
1970 RWB 4-spd Machine
1970 Sonic Silver auto AMX

All project cars.

Forum Cockroach


Posted By: brownspirit
Date Posted: Jul/31/2021 at 1:13pm
I went over and saw it in his honing machine, there was still visible rust in 2 of the four bores he had done, and the 2 are at the outer limits of having acceptable wall clearance for .030 pistons.  He said if I had the pistons he’d recommend I run it, where I still have to buy them he said just go .040 or .045.  He also said the bores are so straight the he can just hone to fit the new pistons, he said it wasn’t gonna cost me any more on his end.  He is in a holding pattern until I bring him some pistons.  It made sense to me, but I don’t know. Andy

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69' BBB AMX 390/4-speed

69' s/crambler project

69’ International F210d Cummins/5+4


Posted By: 6PakBee
Date Posted: Jul/31/2021 at 2:48pm
If you have to buy new pistons anyway, and your machinist says that he can fit 0.040 oversize for the same cost as 0.030 oversize, I can see why you are leaning towards going 0.040 over. It's cost neutral and the concern over rust will be gone, or at least minimized to the point where it isn't a factor.

Tickled that you kept your SC/.  That's a decision you won't regret.


-------------
Roger Gazur
1969 'B' Scheme SC/Rambler
1970 RWB 4-spd Machine
1970 Sonic Silver auto AMX

All project cars.

Forum Cockroach



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