I have some NOS Jahn's pistons and pins due to arrive Monday. They are supposedly 3.810 over 292 bore. When they arrive I'm going to have my mechanic immediately mic them for oversize and compression height, as well as general inspection.
Dome volume is unknown. Couldnt I get some clay, press them in and fill up the domed impression with a syringe for volume?

Also, they do not include rings. Are the rings the same as flat tops, and if not are they going to be available?

The pistons are returnable within 14 days, so the assessments have to be done quickly.

Mike

 

1) Yep. Dust the top with talcum powder a little first. Makes life much easier when the clay is gooey.

2) Depends on the ring groove style as to what will fit...but I'd bet $$ that they will use conventional 5/64 top & second rings, or possibly 1/16. I'd be totally shocked if it was anything "weird" like Dykes, or .043s. Your mech should be able to confirm.

2b) Sealed Power makes a couple of sets for 3.810. There is a cheaper set, part of the E series, for $70 or so, and this: http://store.summitracing.com/partde...5&autoview=sku

Note that it is for a 4 cylinder application, so you have to buy two sets....it is from the R series though, which are GOOD rings.

There are also a couple of TS & SP sets in 1/16.

2c) Have your mech check the ring back & side clearance; older pistons aren't always machined so carefully, & proper back clearance can be an issue.

Jahns pistons are usually cast, but don't necessarily have the steel expansion struts cast in to them, so you may want to use a slightly larger wall clearance than what a normal cast piston calls for in that bore size.

Pretty neat find!

 

Thanks Homespun. Great tip on the talcum powder. What if they ARE .043s?, and/or if there is too little back clearance, then what?
By increased wall clearance, would that be something like .001 inch more in diameter for .0005 inch more in clearance?

 

Hi, I picked up the hughly oversized 312 +.112 jahns pistons on ebay. I hope one of my blocks can be overbored that big? I'm going to have the machine shop check out the blocks see if I can get this to work.

 

Tom, do you know the thickness of your compression rings, and the displacement of the domes? How about posting a pic of one?

 

If they are .043s (which came into vogue in the early '70s), truthfully, I'd send them on their eBay way & look for another set. I couldn't find any .043 rings for that bore size, but I didn't have much time to look in depth. They are strictly drag racing stuff, usually (though not always) combined with Dykes-type back faces (looks like an L profile) & gas porting, & have a very short life expectancy. They also don't transfer heat to the walls as effectively as a wider ring.

Like I said, though, I'll be totally shocked if they are. All the Jahns pistons I've seen were from the '60s, possibly early '70s. .043 rings are fairly exotic & normally only found for the most common engines/bore sizes, like 4.030 SBC/SBF & 4.280 BBC. I'm not wild about them even for track-only stuff, except for serious combos; it's just overkill for the average 7500- 8000 rpm bracket racer, money poorly spent.

I suppose a good machinist might be able to re-cut .043s to a std. groove...I've been told so...but I suspect the odds of failing are fairly high, & it would cost some $$.

It is usually possible to increase back clearance if needed. If it's excessive, a special shim can be used, but there's mixed feelings about using it.

Just as a ballpark figure for that bore size, if the pistons don't have struts, .001 extra would probably work. Hopefully the pistons will still have the original guidelines packed with them. The steel struts can usually be seen on the underside, where the head transitions to the skirts, perpendicular to the pin.

Tom, only your sonic block test will say for sure. Worth the money, with that kind of overbore.

Boy, I sure do love to spend your guys' money!

 

Homespun, thanks for all your help. I'll post the good or bad findings. If you've said before, I've forgotton, but what is your bio? It sounds like you have a hell of a lot of experience with motors.

 

Should have thought to try and post this pic link much earlier. Hope it works.
http://i5.ebayimg.com/02/i/07/ab/02/2d_1.JPG

 

A lot of experience?...Why, no, but I did stay at a Holiday Inn Express last night! (I always wanted to say that!)

Either 1/16" or 5/64", so it's all good. Looks like a nice little domed street piston, should be just about perfect for a pump gas Y streeter. It's gonna be interesting to see what the actual ratio works out to be with your chambers.

My dad died when I was a kid & we wuz poor, so I learned how to fix things early & squeeze a buck 'til it screamed. Putting a car on a diet is a lot cheaper than buying speed parts. I bracket raced a '70 Chevelle with a SBC, later a baby BBC 402, & got it down to the high 11's, no bottle. This was around '82-'83, when an 11 second street car was really something. Nowadays you can buy 'em at Wal-Mart. It was a rusty piece of crap & I made a lot of $$$ from people who figured anything that ugly was slow, too. I got it down waaay under 3000 lb. Eventually a friend & I started racing a '71 Hemi Challenger in A/SA (NHRA) divisional stuff. It was a good education as you have to work within strict parameters, and it taught us how to THINK as opposed to spending $$$ (since a lot of aftermarket parts are illegal, you learn to make the best of what you have from the factory). I was doing some circle track claimer engines for local guys as well. In the late '80s we started tractor pulling with a Modified running (at first) twin Rodeck 482 BBC blown alcohol engines. Apparently we had gobs of cash that needed to be thrown away. Later we switched to an Allison.

I was lucky enough to meet a lot of guys who are (or were...) a lot sharper than I'll ever be, like the late Buddy Morrison & late Lee Shepherd of NHRA Pro Stock legend. Between them & the A/SA experience I learned a lot about combustion processes & resonance/inertia tuning. The last thing I learned is to never assume I know everything; guys like Morrison & his former partner, David Reher, are ALWAYS thinking, questioning themselves. If they are trying to make, say, 2.3 hp/CI, & they finally reach it after a year, they immediately start shooting for 2.4. The guys who think they know everything may do ok, may win some races, but they'll never be as good as they think they are.

For the past 3-4 years I haven't had much opportunity to do much with my own junk, so I like to spend y'alls' money instead! And be long-winded.

 

duplicate...FTE's servers are bogged down again...

 

Thanks for the history! It was interesting! Pistons are in hand. They look good, with some minor surface rust on pins. Maybe that can just be glass blasted off. All 16 clips were there too, along with most of the instructions. A mouse ate the rest. Inside the piston is 1979, so I guess that was when they were cast. A trip to Bill the Dyno Man (my retired mechanic) comes tomorrow.
The instruction sheet says Jahns used .007 clearance on solid skirt racing pistons, and lesser clearance for T-slot pistons. An addition to the sheet said .009 clearance for forged units.
The Jahns part number is 8-979 H RP .060. The Jahns label with part number had been taped over another label years ago. That label said forged pistons, with a part number of F8 1164 H2 RP Std. It probably is meaningless.
It looks like struts are present. They go around the pin and to the bottom of the head, but they look like an integral aluminum part of the piston. There is aluminum waffle pattern on underside of head.
They seem heavy to the hand. Weight of one with pin and without clips is 758 grams. I'm going to weigh and old .030 over flat top for comparison tomorrow.

 

if your going to drive this engine daily i would think.005 clearance would be lots.

 

Good point 312. The instructions say "In general we grind .007 clearance on solid skirt racing type pistons and lesser clearance for T-Slot pistons (for street use only) as follows: up to 3" - .0025, 3 plus .010 to 3 7/8-.0035, 3 7/8 plus .10 to 4 1/4-.0045, and 4 1/4 plus .10 to 4 1/2-.0055."
So which are these, solid skirt racing type or T-Slots? I wouldnt want the pistons banging around with .007 clearance.

 

They are a solid skirt type- the "T-Slot", if it was there, could be seen at the back of the oil ring groove. (The slot itself is straight, but the piston head looks vaguely like a T in side view, with the slots underneath the crossbar of the T. If that makes sense.)

Older pistons & alloys often call for larger clearances than we're used to nowadays. .007 is typical. If you're a good boy & are careful during break-in, you can probably tighten it a bit, but .005 would be as tight as I would go- the expansion rate of some of the old stuff is astounding. These aren't TRWs & .0035-.004 is not likely to work for long. It'll rattle a bit till it warms up; sounds good with those solid lifters, kinda like it's on the ragged edge of destruction. In that '46, with the right stance & finish, & those vibes, you'll strike fear into the hearts of mothers everywhere & make the young ladies' knees wobble. Or something like that.

I was kind of hoping that the spots on the pin were just Cosmoline....

Those are fairly heavy, in big block territory. I'm wondering if they are forged. They're certainly heavy enough. Does it say "cast" on the top label anywhere?

 

The pistons are keepers. They range from 3.803-3.804, and theyre definitely cast. Pins cleaned up and pin bores are good. I calculated the dome displacement based upon measurements and it is 16-17 cc, from that little wedge shaped piece of metal! I was hoping for 10 cc. It definitely means no more will be surfaced from the heads, so at least that expense is spared.
Now to save up for the rod redo.