3.875" crank in a 350 or 400 sbc? - Chevelle Tech
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post #1 of 3 (permalink) Old Jan 27th, 04, 12:42 AM Thread Starter
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Location: skiatook, ok
Posts: 441

Has anybody ever done either of these? I have built a few 383's, and they didn't need much block clearancing for the bigger stroke...how much worse would a 3.875" stroke be? I was dreaming up a new mega torque truck engine combo the other day and after running some numbers, it looks like this would work (on paper anyway).

400sbc, cheap 5140 3.875" crank with 400 mains, 5.94" GM PM rods, and any piston designed for a 4.125" bore, 3.75" stroke, and 6" rod.


350 sbc, 3.875" crank with 350 mains, 5.94" rods, pistons for a 4" bore, 3.75" stroke, and 6" rods.

I am thinking mid 9's compression, small base circle dual pattern cam, and a set of vortec heads. Looking for brutal torque under 5K, 6K rpms max. Shopping around, the crank, rods, and pistons would run about $1200 or so. Do you think it would be worth the effort? Are there any long term durability problems with piston buttons (required for such a short compression height)? The 5.94" rods would keep the r/s ratio within reason. Any thoughts?

'86 chevy 3/4 ton...9.4-1 355, vortec heads, vortec performer intake, 600 edelbrock, xe268, headers, duals w/turbo mufflers, BTO 700R4, 3.23 gears

'77 nova, 388, 200cc pro-toplines, holley 300-36, 750 edelbrock, comp 294s, 10" ATI, 3.73's

'83 ford F-150 XLT...351w, C-6 and 3.55's.
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post #2 of 3 (permalink) Old Jan 27th, 04, 7:49 AM
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With a .030 bore (SB400), this gives you about 420ci.
Now, as I understand, you're asking for opinions, right.
Based on your description, here is what I think would be a better option. First, stay with 5.7 rods with the 3.875 stroke. You will be able to stay with a readily available SB400 dished piston for a 5.7 rod, which will require milling some metal from the top of the piston and this will reduce the dish of the piston. Any good machine shop can do this. DO NOT have the block surfaced if it is not ABSOLUTELY necessary. You are going to need all the deck height you can get!!!!
If you go with the longer rod, with the 3.875 stroke, your pin hole is going to be extremely high up in the piston, WELL INTO THE OIL RING GROOVE. This combination would be just fine for a race ONLY engine, BUT NOT FOR A DAILY STREET ENGINE. Yes, absolutaly, without question, the longer rod would reduce side loading, BUT, your piston is going to be so short, that piston rocking will most probably be much worse than any side loading. With the 3.875 stroke and 5.7 rods, this will put the r/s ratio back close to what it would be in a SB400 with the shorter 400 rods (5.565) and for what you describe as you needs, this combination will be just fine. There are LOTS AND LOTS of SB400s out there which are running the 400 rod and doing just fine. Also, in its original factory configuration, MANY SB400s gave good, trouble free service during their lifetime. And those 400s that did have a lot of miles on them when they got rebuilt, were easily cleaned up with a .020 or .030 bore, which indicates their cylinders weren't worn too bad.

For bottom end rod clearance (in a SB400), it is likely that you may only have to use a die grinder/rotary file at 2-4 locations, but in a 350, it will require LOTS of clearancing for the rods (and you may get dangerously close to water). I had my machinest turn down a SB wrist pin so that it was just a slip fit in a rod. This allows me to test fit and rotate each piston/rod in its hole to check for any clearance needs. After test fitting and any needed clearancing, I then take everything back to the machine shop for balancing and pressing pistons onto the rods.

Tom Parsons
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post #3 of 3 (permalink) Old Jan 27th, 04, 9:15 AM
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take a look at this post.(grumpyvettes)
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