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By OlWeldinrig - 7 Years Ago
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These old Jahas pistons I have came in this old 312.These rings have what appears to be a bevel top and bottom on upper ring.They also appear to have several lines around curcumferance.The engine only had 2000 or less miles on it and I planned on reusing these unless there is a better option.
How does this style ring work?I do not understand function of this,due to no sharp edges on top side of ring.
Thanks Dave
 http://forums.y-blocksforever.com/uploads/images/db3d86ec-7bd4-4cd0-aeb3-42c7.jpg">ahn's
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By charliemccraney - 7 Years Ago
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I don't know about the rings, but the piston looks pretty bad for only 2000 miles. Make sure that the clearance is good.
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By OlWeldinrig - 7 Years Ago
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charliemccraney (2/6/2017)
I don't know about the rings, but the piston looks pretty bad for only 2000 miles. Make sure that the clearance is good.
I was told it was noisy till it warmed up.Has a 3.875 bore,haven't measured a piston yet.
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By NoShortcuts - 7 Years Ago
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OlWeldinrig. Notice the thickness of the skirts on your Jahns pistons. They're much thicker than what many of us are used to working with. My recollection is that in fitting Johns pistons after having the cylinders bored oversize, Jahns called for something like a .009 piston skirt to cylinder wall clearance. This was due to the amount the piston skirts expanded as they came up to engine operating temperature.
Pistons fit with that amount of clearance, that expand that much in reaching operating temperature, will provide an audible piston slap when the engine is started up from being 'cold'.
I think that Tim McMaster put a y-block together not too long ago for someone on this Forum using a set of Jahns pistons that the engine owner had supplied. Consider contacting Tim @ yblockguy.com to verify my recollection of Jahns piston fitting practices.
Hope this helps. 
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By miker - 7 Years Ago
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IIRC, The Jahns pistons (pop up, 11:1) I used back in the late 60's were forged, and had a higher expansion rate. The piston slap cold was noticeable. Can't help any more, that's the end of my recollection.
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By slumlord444 - 7 Years Ago
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I also seem to remember that forged pistons back in the day tended to be noisy until they warmed up. Seemed to be the nature of the beast.
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By NoShortcuts - 7 Years Ago
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My recollection is that Jahns offered both cast and forged pistons in some engine series.
The Jahns pistons I ran into were cast Ford y-block design with a relatively small dome.
As I mentioned the piston skirts were quite thick. While most piston manufacturers use a cam ground piston skirt, I wouldn't be shocked if Jahns did not. This would be another reason why the skirt to wall fitting clearance would need to be soooo great.
Much has changed since the '60s in piston aluminum alloys used, too.
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By PF Arcand - 7 Years Ago
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As Charlie points out, those pistons don't appear to be 2000 milers. And for what it's worth, years ago a buddies Y-blk had Jahn's domed cast pistons in it. When the heads were pulled on two occassions, we noted that there seemed to be evidence of poor flame spread on the piston tops. And if you look where the upper edge of the dome fits in relation to the spark plug, that may be the reason. Many later race pistons have the front dome either sloped back or there is a tapered groove where the spark plug end is.. Oh! and don't use long reach plugs in a Jahn's equipped Y-Blk, they will probably contact the dome!
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By OlWeldinrig - 7 Years Ago
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Thanks for the response guys!Yes they are fairly thick skirts,I imagine they would make noise with the wide clearance.I am just going to cleaning everything up and reassembling so whatever I have for clearance and wear will be what is. Thanks for tip on the deep reach plugs.I will try to get a better picture of the rings if its possible.
Dave
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By OlWeldinrig - 7 Years Ago
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Best shot I could get of top ring.
Dave
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