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#2427127 - 01/01/18 12:14 PM Offset grind 3.75 crank limit
PorkyPig Offline
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I have a new 440 3.75 crank that checked out good on journal size and taper. If I have it offset ground to use a 2.20 rod, what is the most stroke I should expect? Is there any option for an even smaller rod journal size that is still strong enough for a stroked 440?


Edited by PorkyPig (01/01/18 12:20 PM)

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#2427131 - 01/01/18 12:18 PM Re: Offset grind 3.75 crank limit [Re: PorkyPig]
sgcuda Offline
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Registered: 01/28/05
Posts: 4009
Loc: Charlotte, North Carolina
You can offset grind to a 3.90 stroke and use your favorite BBC rod. You could go a little more if you went with a sbc journal 2.100". I just sold a set of Manley aluminum rods exactly for that purpose.
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#2427134 - 01/01/18 12:23 PM Re: Offset grind 3.75 crank limit [Re: PorkyPig]
Al_Alguire Offline


Registered: 01/20/03
Posts: 16836
Loc: Las Vegas
3.90 is the most common way as stated. IMO makes a very good engine package for sure. I did have one done to a 4" stroke as well. Worked fine and that crank is still alive and working as far as I know in the old Duster.
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#2427187 - 01/01/18 01:03 PM Re: Offset grind 3.75 crank limit [Re: Al_Alguire]
Streetwize Offline
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Registered: 01/20/03
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There was an Engine Masters article where Buck Hinkle took an old 361 B and put a B based 3.74 crank offset ground from a 3.375" stroke B with a long 2.00" (327) SB Chevy rod and +.060 400 SBC 4.185" piston in it, so at the extreme with an RB 3.75" crank it looks like you could go easily to 4+ inches by going less than the 2.200 BBC journal, provided you can find a long enough and wide enough rod to work and a piston/pin combination as well.

Whether it's practical do do is another matter, buck actually also took the MAINS down to 2.45" SBC size and make steel bearing cap spacers to cut down on the bearing speed.
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#2427245 - 01/01/18 02:19 PM Re: Offset grind 3.75 crank limit [Re: PorkyPig]
polyspheric Offline
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Registered: 10/10/07
Posts: 2818
Loc: New York
The math: maximum offset = old journal - new journal.
Stroke change: offset (- .005" or so for the grinder to spark out unless the crank is new)
2.375" (B/RB) - 2.20" (BBC) = .175" max
3.75 + .170" = 3.92" max
You can have any stroke change between - the offset to + the offset.

[Please note: it's not the offset X 2]
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#2427249 - 01/01/18 02:22 PM Re: Offset grind 3.75 crank limit [Re: PorkyPig]
Al_Alguire Offline


Registered: 01/20/03
Posts: 16836
Loc: Las Vegas
2.100" SBC rod journals smile
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#2427253 - 01/01/18 02:29 PM Re: Offset grind 3.75 crank limit [Re: polyspheric]
BradH Offline
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Registered: 01/30/03
Posts: 14030
Loc: Valhalla... eventually
Originally Posted By polyspheric
The math: maximum offset = old journal - new journal.
Stroke change: offset (- .005" or so for the grinder to spark out unless the crank is new)
2.375" (B/RB) - 2.20" (BBC) = .175" max
3.75 + .170" = 3.92" max
You can have any stroke change between - the offset to + the offset.

[Please note: it's not the offset X 2]

That's a nice explanation.

At one point I made a simple Excel spreadsheet that allowed entering stroke, rode length & deck height to calc the piston compression height. It made it easier to figure out if there was any combination of stroke/rod/CH for an already-machined block that could be used with off-the-shelf parts, rather than having to get more expensive custom pistons made. Don't have it any more, but something like it helps figure out "real world" combinations quickly.
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#2427256 - 01/01/18 02:32 PM Re: Offset grind 3.75 crank limit [Re: Al_Alguire]
BradH Offline
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Originally Posted By Al_Alguire
2.100" SBC rod journals smile

I was wondering about that, but I don't know if they're made long enough to work with an RB block the way the OP described it. shruggy
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#2427266 - 01/01/18 02:46 PM Re: Offset grind 3.75 crank limit [Re: PorkyPig]
Al_Alguire Offline


Registered: 01/20/03
Posts: 16836
Loc: Las Vegas
We used a GRP aluminum rod..Makes for a world of possibilities smile

Motor didn't make huge power for sure. Ran 9.60's in a 3150lb back half Duster with iron heads. Not sure if I have any pics from the engine build it was a long time ago for sure. The engine was in the car when I purchased it and was assumed it was a 3.9" stroke deal. When I tore it down after making a few passes is when we found the 4" stroke and SBC journal, had Groden rods in it then. I replaced rods and pistons as well as new cam and rebuilt it. Was in this one from way back when...





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#2427268 - 01/01/18 02:52 PM Re: Offset grind 3.75 crank limit [Re: BradH]
sgcuda Offline
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Registered: 01/28/05
Posts: 4009
Loc: Charlotte, North Carolina
Originally Posted By BradH
Originally Posted By Al_Alguire
2.100" SBC rod journals smile

I was wondering about that, but I don't know if they're made long enough to work with an RB block the way the OP described it. shruggy


Like I said, I just sold a set of Manley aluminum rods. 2.100 big end. .990 small end. 6.72 length. They were custom cut at the time, but I'm sure I didn't have the only set.

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#2427671 - 01/02/18 09:14 AM Re: Offset grind 3.75 crank limit [Re: sgcuda]
PorkyPig Offline
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Registered: 08/31/06
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I don't like the idea of aluminum rods in a street strip car. Looks like 3.9 is what I should expect staying with a 2.2 steel rod.

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#2427708 - 01/02/18 10:45 AM Re: Offset grind 3.75 crank limit [Re: PorkyPig]
polyspheric Offline
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Registered: 10/10/07
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Loc: New York
Aluminum can last a long time, if you warm it up first and keep the RPM down.
Triumph 3 cylinder bikes: 70,000 miles.
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#2427779 - 01/02/18 12:45 PM Re: Offset grind 3.75 crank limit [Re: BradH]
toddinNH Offline
member

Registered: 04/21/16
Posts: 8
Loc: NH, USA
Originally Posted By BradH



At one point I made a simple Excel spreadsheet that allowed entering stroke, rode length & deck height to calc the piston compression height. It made it easier to figure out if there was any combination of stroke/rod/CH for an already-machined block that could be used with off-the-shelf parts, rather than having to get more expensive custom pistons made. Don't have it any more, but something like it helps figure out "real world" combinations quickly.


I only work on really odd stuff so everything is custom. Expensive yes, but I only do client's builds. Inline 4 valve stuff is a small market.

At any rate, here is a decent CH calculator:

Compression height calculator 1

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#2427875 - 01/02/18 03:22 PM Re: Offset grind 3.75 crank limit [Re: PorkyPig]
polyspheric Offline
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Registered: 10/10/07
Posts: 2818
Loc: New York
A calculator? A spreadsheet? How about a hand-crank adding machine circa 1880? This is far, far simpler than calculating chamber volume.

If you're still not sure:
deck height
- rod length
- stroke length ÷ 2
= piston CD
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#2427910 - 01/02/18 04:04 PM Re: Offset grind 3.75 crank limit [Re: PorkyPig]
gregsdart Offline
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I built a 4 inch x 4.350 440 (475cubes) offset grinding a stock crank to 2.100 using custom 7.10 rods several years ago. The rods were on 396 BME cores. For a street strip application with small port heads the stress on those rods would be very low. I talked to the guy i sold that shortblock to about fifteen years after the sale. He had been racing it , street driving it a lot, nitrous used. It was still running fine.
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#2427938 - 01/02/18 05:08 PM Re: Offset grind 3.75 crank limit [Re: polyspheric]
BradH Offline
I Live Here

Registered: 01/30/03
Posts: 14030
Loc: Valhalla... eventually
Originally Posted By polyspheric
A calculator? A spreadsheet? How about a hand-crank adding machine circa 1880? This is far, far simpler than calculating chamber volume.

If you're still not sure:
deck height
- rod length
- stroke length ÷ 2
= piston CD

Get over yourself. I threw the spreadsheet together for fun so that I could plug in the #s more quickly than repeating the same calculation with only one variable changing.
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#2428086 - 01/02/18 09:47 PM Re: Offset grind 3.75 crank limit [Re: PorkyPig]
TRENDZ Offline
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Time for a group hug😃
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#2428116 - 01/02/18 11:04 PM Re: Offset grind 3.75 crank limit [Re: PorkyPig]
polyspheric Offline
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Registered: 10/10/07
Posts: 2818
Loc: New York
In the event that this has escaped your notice:
My remark was about the previous posts.
His remark was about me.
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#2428295 - 01/03/18 11:34 AM Re: Offset grind 3.75 crank limit [Re: polyspheric]
pittsburghracer Online   work
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That’s why I stay out of these posts. I’m just a too-pid Millwright that likes to keep things simple. Lol.
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#2428306 - 01/03/18 11:49 AM Re: Offset grind 3.75 crank limit [Re: polyspheric]
WHITEDART Offline
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Originally Posted By polyspheric
Aluminum can last a long time, if you warm it up first and keep the RPM down.
Triumph 3 cylinder bikes: 70,000 miles.
the two things most Racers don't want to do
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