Chevy 261 offset crank with 292 rods and caddy pistons

Sorry for all the Q's but here it goes:
1)What is the deal with 292 rods, Cadillac pistons, and an offset ground crank to stroke a 261 I6?
2)What will it cost?
3)What should be the compression ratio be?
4)What caddy engine do I order pistons for?
5)What will the new displacement be?
6)How "offset" should the crank be?
7)How much of a gain will this get me performance wise?

 

A call to Tom Langdon will answer all of that. He even sells a "kit" for doing it, IIRC...

There is also an article at webrodder.com about it.

 

The webrodder deal is for a 235 - uses 292 rods and Egge "custom" (sort of) 235 pistons.

Langdons kit is the caddy pistons. But, when we started the webrodder deal, the caddy stuff was obsolete, unless they are producing it again, maybe???

 

I emailed tom and am waiting for a reply...

 

I think you will find the cranks are interchangable between the 235 and 261. I am running an offset ground crank in my 261 but I'm using Howards aluminum rods that are based on flathead blanks and Arias pistons. It runs like stink and loves RPMs.

 

joe, with the offset crank what do you think your displacement is?

 

You need to call Tom on this, not email...


what makes a 235 a 261 is the pistons, rods & block. The cranks are the same.

 

I know that a few guys have done thos offset crank grinding and the different pistons and rod.It seems to work out ok I guess.I had talked to a good machine shop about the conversion.His opinion was it's quite a bit of machining on very old crankshafts that have seen millions of cycles.He was concerned it might lead to a crank failure especially when the engine is powered up and the driver uses the power.And being that a heatlhy 261 makes about .75 HP per cubic inch,it's maybe an extra 25 hp,is it worth it? His opinion not mine.I built a 302 GMC instead

 

There are several guys on Inliners that have done this and said they had no problems.

 

How about off setting the crank in a 292? Would it work? Is it worth it? What would the displacement be?

 

How much are you going to offset it?

Basically when you offset grind a crank you change the stroke. When you change the stroke you change the displacement as well as the swept volume. That can be a good thing or not.

Give you a real world example, the 307 Chevrolet is a dawg. In theory it sounds like a good deal, basically a 283 Chebby with a 327 crank. But aside from other things that GM did to it to make sure it was a dawg it has a poor bore to stroke ratio, or at the very least not as good as other bore to stroke ratios.

Anyway in order to tell you how much it will increase you CID one must know how much you intend to stroke or destroke it or how much offset you intend to put into your crank and which direction you intend to go. The assumption is that you want to make it bigger but there have been a lot of fellas over the years that have had good success makeing their engine smaller.

Anyway I know you are just trying to learn but before you ask questions you need to look into what you are asking about and give enough information to answer them.

Oh on the queston of worth, it is normally tallied by bang for your buck. Unless you are a died in the wool inline 6 guy than pretty basically nothing you can do to it is worth it. That said if you are a 6 cylinder guy born and bread than anything you do can be considered worth it. Sometimes .001 of a second is all it takes to win.

 

I've never offset a crank before, but Tom at stovebolt said the offset 261 crank would give two eighty something... Around 20 more cubic inches than stock~ a little less than a 350 stroker...
Can anyone recommend how much to offset it?
Si If I do about the same as the 261 I would get something in the area of a 320 straight six...

 

That sounds like a Tom question. Are you an Automotive machinist? Just curious.

 

for fun---

261 stock bore 3.75"
stock stroke 3.938"

formula for displacement-- bore x bore (bore squared) x stroke x .7854 x # of cyl

3.75 x 3.75 x 3.938 x .7854 x 6 = 260.96

261 stock rod journal- 2.3120"
292 stock rod journal- 2.1000"

offset grinding the 261 journal to fit the 292 rod/bearing : 2.3120 - 2.1 = .212" stroke increase.

New stroke : 3.938 + .212 = 4.15"

new displacement : 3.75 x 3.75 x 4.15 x .7854 x 6 = 275.01 cubic inches.

 

I did the same thing that moparsled did and increased the bore to 3.90 and came out with 311 ci... not as much as I had hoped for...

Maybe I'll just stick with a 292 and put a turbo or supercharger on it...

 

No, but I've got a bunch of friends that work in welding shops, machine shops, parts stores, and just about anything else that could help me in just about any way

 

Most people would call me a mechanic or operator, but I think of myself as a professional tinkerer...

 

On the offset ground crank your offset is figgured X2. The off set increases at TDC and BDC. The .212 differance in journal size in a perfect world gets you .424 increase. Isn't a perfect world though, and you would end up closer to .350 on alot of cranks. The crank may be undersized, stroke may vary, or if you index the journals......

 

you don't double the offset ground crank, because you didn't move the WHOLE JOURNAL .212, you only move "one side" of it. if you think of the rod throw at top dead center you've moved the "bottom" (or the "inside" of the journal) of it toward the top of the cylinder, but you haven't moved the "top". ("outside" of journal) with the rod throw at bottom dead center, the "bottom" of the rod throw ("outside" of journal) is still the same distance from BDC as it was before. So, you only gain the amount of stroke you offset ground, or, you only gain "up" stroke, no "down" stroke

 

A quick question
i have seen a built 250 or 292 with the good head and stuff and would flat run away on a 1/4 mile oval from the V8,s and they had about 3/4 in plate bolted to the bottom end of the block to hold it together

so is the bottom end of a 235 or 261 sturdy enough to handle some of the horsepower?? without reinforcements???

i guess I want to relive my childhood with a little more power

 

216/235/261 have 4 main bearings.
Later ones have 7 main bearings.

Some claim it doesn't matter and its really all about the harmonic balancer and precise machining.
But I remember reading from early rodders, that the early cranks with the 4 main bearings didn't last as long in a radical built.

 

Yep your right. It's been a while since I fiddled arond with stroke calculations. Your piston will travel further down the bore though. You change the centerline of the journal, so any extra travel you get at the top you will get at the bottom.

 

The question I guess is a retorical one. Obvious that you are not by the questions that you ask.

You can't offset a crank by much unless you weld it up. Welding one up isn't just a matter of fireing up the old miller and laying some wire to it. As a rule to get one offset is a rather pricey proposition. Not to say that it is impossible but it is something to think about.

A turbocharged 292 or even 261 could be a pretty slick setup. The turbo setup would be the neatest as you could more easity hide the turbo if you wanted. You will need to find a turbo sized properly to get it to spool up decently. I'll just about bet that texas hardcore on here could give you a hand with that, he's worked in the turbo engineering field in the past. He might even be able to steer you towards a turbo charger that you can cabbage off a junk yard car to help you keep the cost down.

So far off all your ideas huffing is your best as far as bang for the buck is concerned. Again a lot of it depends on how deep you are willing to dive. I'm a big fan of lots of boost, I was raised different than most and was always told that if you are boosting lees that 12 lb feet you are fooling yourself. But the problem lies in this simple truth, if you go much past 10 lb feet you are going to have to make some major adjustments in fuel supply, lower end strength etc. That said there are a ton of fellas on here that are boosting in the 3-6 lb range and are happy as a duck in mud.

Again that's not in the area of unobtainable its just something else to keep in mind.

No matter what you end up doing I'm sure that it'll make you happy. That is indeed what it is all about.

 

http://www.webrodder.com/article.php?AID=399&SID=4&CID=

http://www.webrodder.com/article.php?AID=401&SID=4&CID=

The whole series:

http://www.webrodder.com/index.php?page=showStories&search=stovebolt

 

Hey, I recognize that motor..............

 

Sorry. Was not trying to pour salt on a wound but I felt like the info would help this thread.

Hey, and while I've got you, did you get the latest Max pic?

 

No - no salt in the wound. Not to worry. Its sort of cools to see your own stuff posted. Its still good info, even considering the way it was obtained....

yeah, I got Max's pics. He must be teaching you a lot.

 

I've got a 261,guess I should figure out what I'm going to do with it.

 

Hey Tom,

I have to agree, it is very good information regardless of what happened. I can't tell you how many times I've referenced those articles during my 261 build up. I know you were involved early on, so thanks again!

 

Yeah - no problem. Live and learn, huh???