383SB or 400SB retro into a L31 1998 Suburban

[L31MaxExpress]

I'm on my third TH700, the second two were rebuilt by an age-old and well-regarded local trans shop.

Both failed transmissions had broken planetaries.

Engine was essentially-stock 5.7L.

The only 700r4 derived transmission I have ever had that lasted was the one that should not have. The 10/82 built unit with what the experts regard as the small weak input shaft, 27 spline converter, weak pump, poorly designed valve body, etc, etc. That thing lived 180K behind a stock 305 in the G20 van and towed trailers everywhere with the shifter in Overdrive. Then I hopped up the 305 a good bit and put 50K on it. Then I put a 350 making about 400 hp into it and it lived another 50K. When it finally went, it took a planet set out of it. Its replacement built off a 1992 unit, by a very reputable performance 700r4 builder lasted 20K and died again after about 15K just outside of its warranty period. I threw the 700r4 in the scrap bin and put a nicely built 60E in it from another well regarded performance builder which lasted about the same. Then there is the 97 that ate the OE GM unit in 38K and again in 40K after being built by a different builder. Both shreded hard parts. My buddy with the TPI Firebird went through 3 different 700r4 builds from 2 different builders in about 25K miles as well. We ended up putting a TH400 in that car. I had another buddy early in my OBD2 tuning stage that wanted all the torque management removed. He finally convinced me to do it because he had a "500 hp" built 4L60E in his 97 behind a 350 with efans, shorty headers, exhaust and tuning. Lets just say it did not make it 10K before it became all neutrals. My brothers stock 4.3L S10 nuked one. The 305 in the Tahoe smoked 1st gear of all things in it. There is a big reason I have so much of a sour taste for them, I have R&R'd more of those garbage transmissions than I have ever wanted to in my life. I will never put a penny into one again. If I ever have a vehicle with one again and it is due for a filter and fluid change it will be driven until it quits and it is getting a 4L80E swap and I am 100% not kidding. With the luck I have had with them I could throw together a Powerglide, TH350 or TH400 in my garage, swap it in and save money overall for the life of the vehicle losing overdrive for a more reliable transmission. I think I would rather have a Turboglide, SlimJim or Rotohydramatic or even a Dodge 42LE to anything 700r4 derived. Literally a Metric 250 would hold up better than most of them.

 

[Schurkey]

The '60E in my '03 Trailblazer has ~280K on it. Slow to engage reverse in the winter, perfect spring/summer/fall.

Changing fluid and filter fixed it for awhile. Gonna do that again in January and see what happens.

I've heard from several sources that if the converter clutch isn't engaged in OD, the planetaries get no lube. It makes no sense to me--they'd quit moving fluid through the cooler when the converter is likely to get HOT??? My truck kicks off the converter clutch at ~75 mph. Pysses me off.

Photos from the second trans failure in 2020.

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[L31MaxExpress]

The '60E in my '03 Trailblazer has ~280K on it. Slow to engage reverse in the winter, perfect spring/summer/fall.

Changing fluid and filter fixed it for awhile. Gonna do that again in January and see what happens.

I've heard from several sources that if the converter clutch isn't engaged in OD, the planetaries get no lube. It makes no sense to me--they'd quit moving fluid through the cooler when the converter is likely to get HOT??? My truck kicks off the converter clutch at ~75 mph. Pysses me off.

Photos from the second trans failure in 2020.

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My 83 exploded the smaller planet. The 97 took the case lugs out of the back of the unit, believe they are the ones that hold the Low/Reverse. It sheared every last on off and it was not even a WOT start when it went all neutrals with a sound similar to the jolly green giant zipping up his jacket. That unit was literally trashed and it had an almost new 4L65E from an Escalade built with the correct bellhousing, input shaft, stator support and torque converter to go behind the SBC. It got a beast sunshell, billet servos, and numerous other goodies. I never even opened that one to find out what carnage was in it. It failed on a 3/4 throttle 3-1 downshift, started sounding like a bicycle with a card in the spokes in 1st gear, slipping in 1st. It would take off in 2nd gear start and would shift to 3rd. 4th was gone. I should have opened it up just to see what had let go, but LKQ accepted it as a core for my 4L85E, so it got slapped on the pallet and picked up to go right back to them as soon as it came out saving me the $250 core charge. IMO the best use of that junk. My LKQ rep I used for work at the time knew he was getting a 65E instead of the 4L85E core and said send it. Probably because we spent thousands of dollars a month buying low mileage Versa engines and Nissan CVT transmissions.

 

[L31MaxExpress]

I've heard from several sources that if the converter clutch isn't engaged in OD, the planetaries get no lube. It makes no sense to me--they'd quit moving fluid through the cooler when the converter is likely to get HOT??? My truck kicks off the converter clutch at ~75 mph. Pysses me off.

Nevermind, fixed.

I kind of have to wonder if that is why that 10/82 built unit lasted the way it did. The lockup wiring was unique in the old non computerized trucks and vans. A vacuum switch through a TVS in the thermostat housing connected to ported vacuum to control lockup in 2nd and 3rd and a 4th gear pressure switch that forced it into lockup regardless of engine load in 4th. You could literally cruise that in 4th with Zero vacuum and it would stay locked up without the lockup cycling everytime you were on and off the throttle. That early 83 unit would also hold OD until it was near WOT, like 85% throttle before it would force a 4-3 downshift. The later units downshift at much less throttle. It was actually kind of nice, driving for mileage. At 65-70 mph I could pin the Q-Jet primaries almost all the way open and it would stay locked up in overdrive. The Q-Jet secondary return springs put added resistance on the last 1" or so of pedal travel and it took a bit firmer shove to get them to open and the 700r4 to kickdown. That last inch of pedal travel took about double the pressure on the pedal to pin it on the floor. Made it easy to stay out of the secondaries trying to stretch the fuel in the tank as much as possible. By comparison a TBI or Vortec pedal just effortlessly flops to the floor.

 

[PlayingWithTBI]

unless you perform some inconvenient modification, you'd use an aftermarket roller cam plus the stupid aftermarket thrust-button instead of the simple and wonderful thrust-plate and step-nose cam core.

Ta add to this, you need a different timing chain set since the step nose has a smaller diameter. I used this one on top. The flat tappet cam gear is below.

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This cam gear has a thrust bearing so, when the cam is trying to slide forward, the thrust bearing keeps it from rubbing on the thrust plate. Keep in mind this is on my TBI block, not one with a crank sensor. YMMV

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[Schurkey]

Ta add to this, you need a different timing chain set since the step nose has a smaller diameter.

Yes. Found that out on the 454 boat engine, when I also needed a special marine-style water pump drive tang that uses the cam sprocket bolts--I had to get one with a smaller bolt circle to match the smaller circle on the cam and cam timing gear.

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Marine water pump, attached to marine timing cover.

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This cam gear has a thrust bearing so, when the cam is trying to slide forward rearward, the thrust bearing keeps it from rubbing on the thrust plate.

FIFY. When (if) the cam slides forward, the step of the step-nose cam rides directly on the back-side of the thrust plate. When the cam goes rearward (from the distributor and oil pump drag acting through the helical gear of the distributor) the thrust bearing on the rear of the cam timing gear would contact the front side of the thrust plate.

For the record, I'd rather have the cam timing gear rubbing directly on the block (old style) or thrust plate (new style) than introduce the potential for a zillion teeny-tiny roller bearings to come loose from the thrust bearing and end up in the oil pan.

There have been issues with the cam timing gear rubbing directly on the iron of the block--wears a groove into the block around the cam bearing. This is overall really rare, but it does happen, especially when there's preload from those crappy thrust buttons loaded against the timing cover. The "fix" is to cut the worn block for an appropriately-sized hardened valve seat insert, fitted into the block around the OD of the cam bearing, which will END any further wear.

Another way to eliminate any wear to the block face from a non-rollerized timing gear is to poke a small oiling hole into the thrust surface of the block in the front cam bearing area--which is what I did on the boat 454 before I upgraded to the step-nose roller cam.

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[1998_K1500_Sub]

Thanks, everyone, for your comments.

I'll have to decide what's of value to me and chose a path.

The Dart option is $exy, and could be built to 412cid if I wished.

An L31-rebuilt-as-a-383 could easily be made 390cid by using a 3.80" crank instead of the 3.75".

How much is enough, when too much is never enough?

 

[JeremyNH]

390 is what I have newly installed this Spring. You need to go 0.040 over to get there though. Standard 383 overbore with a 3.8" stroke gets you to 388. In any event although the Dart block is more expensive than an L31 stroker the spread is likely less than you think. Reputable machine shops, at least here in New England, are expensive and have very long lead times. My motor ran $2600 and took 4 months on a wait list then 10 months in the shop to complete and that was with me supplying all the parts. But I did get the motor back as a short block by a 40+ years in the business shop owner internally balanced and guaranteed to work. One note on the 3.8" Scat crank. It's listed as internal/external balanced and I provided an external balanced flex plate to the shop but they returned it uninstalled saying there was less work needed to add heavy metal to the crank than there was to remove it from the flex plate so I have a neutral balanced flex plate on final build. FYI.

If I had to do it again I would still go my route though. I was mostly doing it for fun and education (never built a motor before) and felt good that I made no mistake on my build parts that the shop had to fix. But if I were going for max power the Dart block would only add a very few hundred dollars to final cost on an expensive build anyway. I do think Vortec heads would starve a 406+ though. Probably want to go Etec 200s or such. With regards to your original question though the one piece rear main and roller provisions of the L31 are worth the few cubes that a stock 400 has to offer so I would go L31 stroker no question.

 

[1998_K1500_Sub]

With regards to your original question though the one piece rear main and roller provisions of the L31 are worth the few cubes that a stock 400 has to offer so I would go L31 stroker no question.

This echos my perspective. I'm not so keen on the idea of trying to use the used 400 block I spoke-of in my initial post; I'm quite certain it's been rebuilt once already.

As a side note, we've got a 1978 K20 Chevy (reg cab long bed) with an OE 400SB in it, and the engine's virtually untouched; it had heads done a few years ago ("few" is maybe 20) and that's it. The truck's still in fine shape, it's hasn't seen salt in at least 40 years, maybe more. Some time ago it was repainted (original color white) and it looks pretty nice, although the interior could use some love. For the longest time it's been a farm truck dedicated to spraying; there's a tank in the bed and booms in the back. When it's not spraying, it's sitting. Unfortunately I don't have a picture to share.

 

[L31MaxExpress]

There is a new Dart block on one of the FB groups I follow not far from you.

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