3/8x3/8 jahns pistons?

3 3/8" bore and a 4 1/8" stroke (3/8 over stock Ford). The famous 296" flathead.

This was before Merc cranks.
 
My experience reminds me that those numbers mean those pistons were designed to be used to build a 296 cubic in. flatty yielding a 3/8 " increase in piston dia. and a 3/8 inch increase in stroke over the stock 3/3/4 in. crank. It was known as a stroker kit and you had an engine that was "bored and stroked".

The first "3/8" denotes the dia. of the piston. The second "3/8" denotes the length of the extra stroke needed to yield 296 cubic inches.

If you mathematically add 3/8 in. to the stoke stroke of an 8BA engine (3 3/4 in) you get 4 1/8 in. That is the new stroke needed to give you 296 in. cubic inches of displacement. A 4 in. Mercury crank was then welded and ground to produce the needed stroke along with new pistons and rods.

So, a 3/38 x 4 1/8 stroke engine gives you 296 cu. in. Back in the day, I bought a complete 296 strokerkit from Crankshaft Co. in CA. for about $500. It included Jahns pistons, a newly ground crank and 21A rods with full floating bearings. It's been over 50 years but, I hope my explanation makes sense.

Here's the math.

Area of piston equals Pi times Radius Squared. 3.1416 x (1.687 x 1.687) = 8.94 square inches at top of piston.

8.94 times 4 1/8 in. stroke equals 36.892 cubic in. of displacement of one cylinder.

36.892 times 8 cylinders equals 296 cubic inches.
 

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Merc 4inch stroke crank does NOT need welded to get 4 1/8 inch stroke. It gets offset ground to the small 2 inch pin size. Hence the use of the 21A or 91A rods.
 
The reason I added "before Merc cranks" is because if a Merc crank is used, the stroke increase is not 3/8", but 1/8". It's still a classic 3/8 X 3/8 engine. It's more about the terminology involved than anything else.
 
KiWinUS said:
Merc 4inch stroke crank does NOT need welded to get 4 1/8 inch stroke. It gets offset ground to the small 2 inch pin size. Hence the use of the 21A or 91A rods.
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Or the aftermarket H-Beam rods with insert bearings. In today's world, it is actually about the same $$$ to buy a new set of H-beam rods and run the small Buick bearing . . . as it is to find a good set of 21A rods, have them rebuilt and then find a set of full-floater bearings to go with them. And, you have to hand-fit the full-floater bearings (with a rubber mallet and usually some ScotchBrite polishing to get them to correctly float - not that many shops/folks know what this is all about in todays world).
 
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If you happen to have the pistons mentioned, would like to see the tops of them and the sides (where the pins are located). On most Jahn's cast pistons that I've seen, the stroke is marked "3/16 LC" . . . for lowering the crown or compression height of the piston by 3/16 of an inch (which is 1/2 the increased stroke of 3/8").
 
28fordroadster said:
thanks for your help! I was looking for a set of 3 3/8 x 3 3/4
How did they get 4 1/4 stroke before the 4" crank came out?
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They welded up the crankshaft and then reground the journals with an offset. In order to get to 4.250 stroke, this would have to be done on the Merc crank as well . . . as offset grinding only gets you to 4.125.

The other option has always been a custom billet crankshaft.
 
This bring back allot of early memories. I only built one 296 and it's still running, doesn't burn a drop of oil, because it has a set of auto thermotic pistons, or stock type steel strut pistons. I never found out who made them but they were 4 ring 3/8 by 4 1/8 stroke . i built the engine back in the early 90 and sold it to Ron Harrell, L-100 cam and all. Compression was pretty high at 8.7. I had a ride it and was very impressed with the performance I installed the top 3 rings and fit the pistons with about .002' Clearance. Pistons were quite heavy and I should have cut off the bottom ring land to lighten it some. Ron recently replaced it with a 294 with a Magnusson blower ASSY This makes 235 hP at 4500
Actually, a stock style steel strut piston can be fit with .001" piston to wall clearance and is why they never burn any oil, when properly cared for.
Gramps
 
Bored&Stroked said:
Or the aftermarket H-Beam rods with insert bearings. In today's world, it is actually about the same $$$ to buy a new set of H-beam rods and run the small Buick bearing . . . as it is to find a good set of 21A rods, have them rebuilt and then find a set of full-floater bearings to go with them. And, you have to hand-fit the full-floater bearings (with a rubber mallet and usually some ScotchBrite polishing to get them to correctly float - not that many shops/folks know what this is all about in todays world).
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All very correct Dale. Fortunately I have a large stash of NOS 21A rods along with many nice used sets. Also a large stash of std. floater bearings to suit. I’m a “opportunity buyer” so over the years have accumulated these and much more as “the price was right “.
 
Full-Floater bearings are available - though most that you'll find are the harder copper colored ones. I prefer to use the Cadmium-Silver "For Heavy Duty Truck and Bus" versions - are a lot softer and easier on the crank . . . but also much harder to find. If you're looking at Federal-Mogul bearings, the Cad-Silver ones have a part number with 'CS' in them.

Using full-floater bearings is really a good system - as long as you know how to tune/fit the bearing shells. Usually I have about 6 sets to start with - then find the ones that fit the best and then tune/polish them from there. The 91A/21A rods are just fine for naturally aspirated engines, but are a bit weak when you start thinking about blowers/boost, Ardun conversions, etc..

I'm running 21A rods in my 32 Cabriolet (street engine) - but will not run them in the next engine I'm working on (big stroker, injected engine) or the Ardun that I'm finally getting into.
 
Bored and Stroked:

CONGRATS and all the best with your engine projects.
Pretty exciting. Just like the good old days.
 
Concerning Jahns pistons, they had a reputation of disintegrating when you put rpm and horsepower on them.
 
Pete said:
Concerning Jahns pistons, they had a reputation of disintegrating when you put rpm and horsepower on them.
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When you look inside of one . . . not terribly surprising . . . pretty crude little castings. I'm fortunate that I didn't have this happen when I was a teen-ager as I flogged my first engine pretty hard and then put a blower on it and flogged it some more! LOL
 
Bored&Stroked said:
When you look inside of one . . . not terribly surprising . . . pretty crude little castings. I'm fortunate that I didn't have this happen when I was a teen-ager as I flogged my first engine pretty hard and then put a blower on it and flogged it some more! LOL
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They look pretty Dale. I have 2 sets of them 1 semi finished. The finished set is polished on the outside. Mine are actually Edelbrock but look identical to the Jahns as I have some of them used. Definitely ****** looking on inside. Actually they have Edelbrock on one side & JE on the other below pin bass inside
 

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I bought a stroker kit from C&T automotive, and they user the29a floating bearings, Got 342CI, Clay smith super matchless cam and Hilbrn. Kinda wish I had it dynoed because it could lite the tires at 60 in hi, by just putting ot to the floor. First time I ever had that much power.
Gramps
 
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