Jahns Pistons Questions

I bought an engine that has been sitting for a long time, looks like a 20 year old build. Very low mileage and looks pretty good inside. It has these Jahns pistons in it, they appear to be forged and are a very unique design. I've never seen pistons with bracing under the top. There are no markings or numbers on them, not even a size and no information about these pistons online, so I thought I'd ask who has used them and their experience with them.
Thanks
Brent

 

Yes, they look like Jahns and they are a forging .

good for a stock rebuild, but those thick skirts would be quite heavy for racing above 5500 rpm not to mention the 5/64” compression rings!

 

I expected Jahns pistons to be forged.

Those sure look cast to me. How would they forge the "Jahns" name into the inside of the skirt?

I guess the answer is to hope Jahns is still in business, and contact them.

 

Great point, I did not notice that!
Now that I think about it again those thick skirts make much more sense on a cast piston,

 

The insides even looks cast. Forged pistons are normally very very smooth

 

After some online research comparing photos, from your pics I agree these Jahns pistons are cast. Apparently cast was by far their biggest seller over the years. Seems the good forged stuff came about in the '70s under the new name Jahns Engineering (JE Pistons today).

Devon

 

I had a set of Jahns Pistons, the superlight forged versions bought thru Kenne Bell for a .060 over 430 in the late 70's. Those pistons were light, on a stock rod, if you picked it up, all you could feel was the rod. I don't recall what they weighed, but it must have been down around 500g or less. 495 rings a bell for some reason..

I raced with them for many years in two different cars... until a stock rod broke just off the starting line at BIR, in the spring of 98.

That 430 would free rev like no 455 I have ever dealt with. Would have been insane with alum rods, wish I would have used them, I would still have that shortblock today.

JW

 

What cam was in the 430 jim?

 

Nothing personal, but some of you guys who try getting away with running the factory rods for more then One race season are nuts!

I would sooner run cast pistons and pay 1200 bucks for good aftermarket rods in a motor making 425 to 475 hp.
If your motor is making more then 475 and you don’t yet have forged pistons and aftermarket rods in the motor then your crusing for a Bruising sooner then later!

 

The old KB Mark 4. Loved that cam so much that when it wore out, I sent it to a cam company and had them make me a new one.

JW

 

Ya but back then, there was no such thing as a forged Buick rod. like we have now. It was either stocker Buick, modified pontiac rods, or big dollar billet Crowers.

Around the end of the Century, the first TA sportsman Crower rods came out, and then in recent years, offerings from Molnar and Eagle (TA).

No excuse these days to not have a good 7/16 cap screw rod in your hot BBB.. the only motors I build with stock rods are dead stock rebuilds, and even then I mention the availability of a better rod.

The Buick stock rod is better than most, many guys back in the day ran some serious power thru them for a long time. But they break eventually.. My rods probably had 2000 runs on them, and were due to be replaced with a motor freshen up that winter.. one season too late as it turned out.

JW

 

I ran stk rods in my 1st 464 combo....low 11s and high 10s for over 2000 1/4mile passes no refresh no issue....maybe I was lucky........I can say my molnar didn't live 80 passes most 1/8th mile

 

Yes but it’s highly unlikely that the molnar rods actually went first. More likely something else failed and the rods went along for the ride/parts collided....

 

I'm almost certain it was a bolt that didn't do ots job, allowed the clearance to fet wacky there.......the rest is bye bye down hill fast.

They use some special arp bolt built to their specs........I just will never know was that one over or under torqued......all I know is every other bearing looks reusable even after it knocked a rod through the block.....its just racing, but sure leaves a sour taste for that product in my mouth

 

Ouch that’s frustrating. Having spoken to Tom Molnar on the phone I’m sure he would like to see the rod to help diagnose the failure. He’s a solid guy and if he could develop a better product based on a failure I know he would do that.

 

Don’t be so fast to blame the rod!

Especially in racing usage you don’t want to reuse rod bolts more then 4 times, and 5 times is really asking for a bad day at the track or on the dyno.

Think about it , when the rod was made and machined the bolts where torqued up the first time and then when you assembled your motor that made for the second time the bolts where stretched, so that’s 2 out of 4 already!

This is why if your going to use the bolts a even a 4th time you had better be using the stretch and micrometer method.

 

And it you had the rods reconditioned (with original bolts) they torqued them then too.... now you're up to 3 times with final assembly. Did you check clearance with Plasti-gauge? Now your at 4 times.

 

Hi Ben,

You mentioned that rod yesterday, so I recalled something about it, so I found the thread and re-read it. I know I was pounding on the detonation stuff, but I just noticed that the cylinder walls of number 5 were split.. and then saw the block was bored to 4.385. Was the block completely filled? Do you have a sonic sheet for that block? I am not sure that I have ever seen a 455 that you could bore that far without it being completely full of hard block.

I understand your feeling, but Molar's rep in the industry is unblemished. They are the go-to rod and crank supplier for nearly every engine builder in the private facebook engine builders group that I am on.. and that's a lot of very smart, very experienced engine machinists/builders.

The custom arp bolts that Tom has made for his rods are serious overkill for engines like ours. They are designed to hold up to mountain motors spinning 8500 rpm. That's why there is no bolt upgrade option.. there is nothing to upgrade to.

What did you stretch the bolts to when installing? This is critical for the well being of any high performance connecting rod. Did you use the supplied CMD3 lube on the bolts?

So, you said then that you were going to send the rod back to Molnar for inspection.. did you, and if so, what did Tom have to say about it?

JW

 

675hp stock rods in since 2003....lost track of number of passes....motor never apart (knock on wood!).....9.96 134 best in 3710 GS

Key is internal balance and light pistons.

 

Light pistons... yes... That, along with keeping the rpm down, are the main factors in rod durability. Any rod..

How you balance it is a more complicated question. Without the center counterweights, putting the extra weight in the existing counterweights, vs on the ends of the crankshaft has questionable benefits. And plenty of pitfalls. Most engineers will tell you that the benefit is more for the block, than rotating parts. Especially number 4 main bearing and that housing.

Accurate balancing, regardless of how you get there, goes a long way to avoiding the stresses that kill race engines.

JW