Jahns pistons
Printed From: TheAMCForum.com
Category: The Garage
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=82583
Printed Date: Apr/24/2024 at 1:30am
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Topic: Jahns pistons
Posted By: Woosterkevin
Subject: Jahns pistons
Date Posted: Sep/13/2016 at 4:14pm
Can anyone identity these Jahns pistons? I know they are .30 over by measuring and the faint Jahns name on the underside.I removed them from my 360 that I believe was rebuilt in the late 80's. I want to reuse them in my new 360 build but would like to find out a little more about them if possible. 
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Replies:
Posted By: 401jim
Date Posted: Sep/13/2016 at 4:21pm
Need more pictures of the bottom. Also the top please......  ------------- 
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Posted By: Woosterkevin
Date Posted: Sep/13/2016 at 4:45pm
| I am trying to get more uploaded thanks. |
Posted By: shootist
Date Posted: Sep/13/2016 at 5:49pm
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The look to be forged but hard to tell without seeing the underside. If they are not damaged you can reuse and you can use a syringe and flat piece of plexi glass to get a read on the dish CC. On the surface from the one pic they look like a pretty nice piston.
Even if they are cast you could still reuse them without much issue provided they are not damaged. Look at the top ring lands and make sure they are not lifting (very uncommon in an AMC motor common to a Pontiac).
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Posted By: shootist
Date Posted: Sep/13/2016 at 5:56pm
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Oh, I would like to add that if you are not supercharging or spraying with NOS cast is more than adequate. Forged is nice for performance just to be more resistant to detonation but aside from that you still have a cast crank in the 360 which is the weakest link and limits any build to 500-600 HP before the life expectancy will drastically begin to fall off.
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Posted By: Woosterkevin
Date Posted: Sep/13/2016 at 6:04pm
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Posted By: Woosterkevin
Date Posted: Sep/13/2016 at 6:06pm
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Posted By: Woosterkevin
Date Posted: Sep/13/2016 at 6:31pm
| Not planning on supercharging or spraying, I just want a nice performance driver. Something that i can drive everyday if i want, but also get up and go when some tuner thinks he is king i can knock off his crown lol. |
Posted By: shootist
Date Posted: Sep/13/2016 at 7:37pm
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Well that is a dome piston. Still need to see the underside where the connecting rod goes in to figure out if it is forged but if you are not going huge power I would not be worried. If you look up in where the wrist pin is and it has cross hatching then it is definitely cast. This is usually the only place to tell if it is cast vs. forged. A forged will never have the cross hatching there internally but even a cast piston could have them removed. However, there would very few people (if any) that would want to perform custom machining on a cast piston
because it is still just a cast piston. If anything you will need to watch your valve clearances if using that piston since it will go up into the combustion chamber as it is. Do you have the heads that were used with the previous build? I would ensure that you look them over closely and ensure whatever heads you decide to use have the same clearances. Edited for literacy issues. I cannot think, I has the beer.
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Posted By: Woosterkevin
Date Posted: Sep/13/2016 at 7:42pm
| I will stop by the machine shop and get a pic of the underside. I do have the heads but they have pitting on the valves and seats so they would need totally reworked to use them. |
Posted By: shootist
Date Posted: Sep/13/2016 at 7:45pm
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That is ok, just look at them closely to make sure that they don't have material removed for clearance. If they do then when you do assembly you probably will want to run the piston up to TDC and check for clearance issue with the head itself to ensure no interference.
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Posted By: SKeown
Date Posted: Sep/14/2016 at 12:19pm
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To the best of my knowledge Jahns never made forged pistons, only cast. If they are forged then they were made to Jahns spec by Ross. SKeown
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Posted By: 6PakBee
Date Posted: Sep/14/2016 at 1:06pm
Unless my memory is failing me, I was buying Jahns forged pistons for Mopars in the early '70's prior to Ross even being in business. ------------- Roger Gazur 1969 'B' Scheme SC/Rambler 1970 RWB 4-spd Machine 1970 Sonic Silver auto AMX All project cars. Forum Cockroach |
SKeown wrote:Posted By: shootist
Date Posted: Sep/14/2016 at 2:14pm
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Jahns are a bit before my time of performance builds. Dad was installing them when I was still handing him wrenches and asking a bunch of questions but no direct experience myself. Either way if they are Ross or Ross bought them they would still be nice pistons.
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Posted By: Woosterkevin
Date Posted: Sep/14/2016 at 2:25pm
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Posted By: Woosterkevin
Date Posted: Sep/14/2016 at 2:27pm
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Posted By: Woosterkevin
Date Posted: Sep/14/2016 at 2:29pm
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Posted By: shootist
Date Posted: Sep/14/2016 at 3:48pm
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They are definitely cast but they also appear to be very nice cast piston. -------------
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Posted By: Greyhounds_AMX
Date Posted: Sep/15/2016 at 8:32pm
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I would say those look forged, not cast. Cast pistons have a shiny look to the metal surface when you look at the underside, and forged pistons have a more satin finish like these do. Also forged pistons have a more gradual radius to corners on the underside than cast (other than the areas that have been machined of course). Here's a cast Subaru piston:  And a forged one:  ------------- 1968 AMX 390 w/T5 |
Posted By: TX390AMX
Date Posted: Sep/16/2016 at 6:32am
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I'd also watch your cam lift spec's. Even with the valve reliefs, you wouldn't want to risk those nice pistons with a big lift cam.
------------- Terry 68 AMX 390 4 Sp Rally Green 68 AMX 390 4 Sp Matador Red 68 AMX 390 4 Sp Calcutta Russet 56 Chevy Sedan Delivery 2016 Can AM Spyder 55 Chev pickup |
Posted By: pfordamx
Date Posted: Sep/16/2016 at 9:24am
| with that dome i wonder what compression ratio will end up at. pump gas could be tough to run those pistons look like they should provide atleast 12 to 1 |
Posted By: Woosterkevin
Date Posted: Sep/16/2016 at 5:58pm
| For what I can find on other forums Jahns made both cast and forged pistons, with both being heavy but of good quality. I was hoping someone in our AMC world had used these before and could give a little more information on compression ratio and cc. I do understand the pump gas and clearance issues. Onice I get the block bank from the machine shop and the pistons in I will measure clearance to be sure, but these were used in my old 360 with stock heads so I am pretty sure they will be fine and although it was late 80's early 90s when it was ruining, he used the cheapest gad he could find and claimed to have no issues. |
Posted By: Hurst390
Date Posted: Sep/18/2016 at 8:36am
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Jahns forged pistons were made by what is now JE pistons> Yours look forged to me?
------------- SC/Hurst Rambler 11.62 120 100% Street Legal |
Posted By: SKeown
Date Posted: Sep/18/2016 at 12:33pm
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"Jahns was a cast piston company (pretty rough too). Jahn Jr. wanted to get into the forging piston scene so they set up Jahns Engineering (later shortened to JE). This is were JE Pistons came from." "Most Jahns pistons were cast, and long have been. If the underside of the piston is rough with crosshatch markings, then it is cast. If it is smooth to the feel, with some striations visible, then it is forged. But I have never seen a forged Jahns piston. Jahns did once tell me that Ross will make forged pistons to Jahns specs, because they had an agreement." SKeown
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Posted By: shootist
Date Posted: Sep/18/2016 at 2:56pm
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Perhaps I am looking too critically, I could swear there appear to be very light parting lines present.
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Posted By: Greyhounds_AMX
Date Posted: Sep/18/2016 at 4:17pm
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It definitely looks like Jahns offered forged pistons for sale, but whether they actually forged them in their own basement or not is hard to say. 1970 Jahns Catalog:  1972 Jahns Catalog:  ------------- 1968 AMX 390 w/T5 |
Posted By: goops
Date Posted: Sep/18/2016 at 6:45pm
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I definitely had forged Jahns pistons in a 301 chevy small block in early 70,s . they were forged for sure as I ruined 1 and had to get a replacement. Goops |
Posted By: Woosterkevin
Date Posted: Sep/18/2016 at 7:47pm
| I wish I had them catalogs to look thru. |
Posted By: rlumber
Date Posted: Jul/12/2019 at 7:59am
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Hello, We have a set of new 942B Jahns Pistons. Can you tell me what these were for? Thank you, Ross
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