Jahn's Pistons ?!

[powerband]

The older Clifford Performance built 250 engine I've been investigating has Jahn's Racing Pistons which give a 10:1 CR as built. The engine has never been installed and run. I talked to the machinist I've worked with for years about this engine and he said he had a really bad experience with a set of Jahn's Pistons in a 351 VeeAte a while back.

After running into expansion seizing problems he said the Jahn's contact person told him they were not suitable for street use as they require excessive cylinder wall and wrist pin clearances due to the expansion characteristics of the alloy. He related that the Jahn's contact conceded that in a racing engine run after pre-warming and limited temp cycling they would be fine but would have trouble in street use especially in the cooler N.E. environment.

I trust my machinist that he investigated the source of the particular application problem extensively and I was intersted in any other comments. Although Jahn's was evidently merged into Grant Piston Rings and disappeared, the NOS boxes appear often on E-BAy and are popular with the traditional rod builders.

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[rbohm]

8) jahn's pistons are good pistons, but like they mentioned are designed for racing rather than the street. in fact most every forged piston on the market requires that you run more piston to cylinder wall clearance, as forged pistons have no expansion limiters built into the piston like cast pistons do. there are things that can be done to allow you to tighten up the piston/wall clearances with forged pistons, like cam grinding the piston so the piston is an oval shape rather than round, but even these tricks need to have the piston designed to expand in the proper direction, otherwise it is a lesson in frustration. best bet for a street engine is a set of cast pistons from someone like KB, or TRW. if you really want forged pistons, talk to mike at www.fordsixparts.com he has some that are designed for the street, but you will still need to have larger piston to wall clearances.

 

[addo]

Would there be a problem with buying it at the right price, and just running it?

 

[rbohm]

8) the only problem would be piston slap when cold. other than that none.

 

[xctasy]

Stick with a modern cast alloy piston.

If its a forged pistons for the street, your noramlly into a borderline engine with high compression, and if its being short run for less than 10 minutes, then your better off traliering it to the drags. Forged pistons have no place in a modern street and strip car.

Even state of the art modern Zollener coated cast alloy pistons in the late 90's 5.4 Romeo Fords and 5.7 LS1 Chevies have suffered fatal piston slap at 1500 rpm. It is endemic in the shallower pistons, and when its forged, it will be even more likely. As a race piston Jahns are as good as the best, but as a road piston, then its a risk.

The first 10 minutes of warm up of any forged piston (especially big thick items like the factory issue TRW 350 LT-1 specials used in Corvettes and some RPO Camaros and Novas) is crtical, and running to little clearance will kill an engine. Those old GM engines were real machines, with a good reputation, but they were real rattlers when cold

In 1988, my mate scraped his thick wall Aussie NASCAR 351C block just because he ran close clearances on his TRW forged pistons. Piston slap in a car still doing 14 second quarters and 22 miles to the gallon on 80 MPH inter province cruises was only mild, but on stripping, the bores were in need of 60 thou to clean up the score marks.

 

[Anonymous]

Forged pistons have no place in a modern street and strip car.

My Father built himself a 2.2L L20B Datsun stroker motor for his 1200 ute, and used forged pistons. It was running up around 12.5 Comp. and this motor lasted for YEARS. We pulled the motor down for inspection, just out of cuiriousity, after probably 40,000 klms. The bore still had the hone marks in it, and all the clearances were fine.

It ran consistantly low 13sec quarters, and copped a caining all day long. It was never noisy when first started, and is now still running in another 1200 in Townsville, at least 6 years later.

 

[wallaka]

There's always an exception to the rule ... Doesn't mean that in most cases there isn't a problem. It would probably be fine if he took the trouble to warm the engine completely every time before moving it, but that won't happen with a daily driver.

 

[powerband]

Further investigation into the "mystery" of the Clifford built 250 engine turned up a significant wrinkle, The Pistons turn out to be TRW cast / AMC 258cid which require a few simple mods to achieve the desired CR and is not well known:

Very intersting reading ...

http://fordsix.com/forum/viewtopic.php?t=34610

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[Anonymous]

There's always an exception to the rule ... Doesn't mean that in most cases there isn't a problem. It would probably be fine if he took the trouble to warm the engine completely every time before moving it, but that won't happen with a daily driver.

Hmmm ... my father is a professional performance engine reconditioner, and has been for over 20 years. He has built numerous (50+) engines with forged pistons, ranging from four cylinders to v8's and has never experienced problems specifically related to using forged pistons.

 

[Anonymous]

I know quite a few people running forged pistons on the street. The engines are far from stock, and it would be a real risk running cast pistons in those engines. They aren't having an issue. Granted, forged pistons are quite different than cast, but if you do it right, I can't really see an issue with them on the street. If you do it wrong, then you will have issues, but that is with everything.

 

[wallaka]

True. I wouldn't have a problem running forged myself, but I also got no problem running mechanical lifters. I don't mind adjusting things and taking a little more time if needed. With proper warmup, there are little to no problems running forged. While cold, they are prone to piston slap. It also depends on the application and how tall the piston is, how high the combustion temps are, the ambient temperature, etc, etc. The car could be fine in hot climates but not in colder temperatures with the same clearances.