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6.2 Diesel to 350 swap "Trans" Questions?

44K views 25 replies 8 participants last post by  frodin1 
#1 ·
I have an 83 Chevy Diesel with a TH400 that I'm building a "2 piece rear main" 350 for. My question is, what about the flex plate? I was told I need to change out the Torque converter and governor. However, I'm wondering about the flex plate, because the diesel flex plate is a 138 tooth unit, and from what I've read on here, it's heavily weight balanced for the diesel engine, which means it can't be used. Also, does changing the flex plate mean that I need to install the original 350 starter? Basically, I need to know what flex plate I should purchase that will work on my setup? If anybody can help me out with this I'd appreciate it. Thanks
 
#2 ·
You have to change flexplates because the one bolted to the diesel won't bolt on to your 350 crank.

You match the flexplate to the engine/starter. Then the torque converter only has three options for bolt pattern: small 3-bolt, large 3-bolt, and 6-bolt. A 6-bolt converter can be used on a large 3-bolt flexplate, you'll just only use 3 bolts.
 
#4 ·
Either the 153 or 168 tooth flywheel could be used, but I would get the larger one since it will almost definitely have the large pattern. Many aftermarket flexplates are drilled for both patterns.

Make sure your 350 block has the offset hole drilled for the starter. Large flexplates (168 tooth) use starters with an offset mount pattern. Small flexplates (153 tooth) use starters with an inline bolt pattern.

As long as your engine is mostly stock (low rpm torque) the diesel converter will work fine and give you the most MPG and the least heat. If you have upped the engine's power, I would consider a stock gasoline torque converter since it will stall a touch higher. There was a time that those diesel torque converters were highly prized for swaps into gas trucks, RVs, and heavy cars for their low stall and heat. Put it this way... I paid over $450 for a custom built diesel converter just like yours about 10 years ago.

I'm not sure about the 6.2 TH400s... I don't think they have a vacuum regulator. That will need to be addressed. Try the governor as-is first. If it shifts too early for you, its pretty easy to swap that out later with the transmission still in the truck.
 
#8 ·
Your the first person that's said that. Why wouldn't it? TH400 is the same either way isn't it? The only difference I heard was the torque converter and the governor. The 6.2 liter diesel block is the same as a 350 block? I'm just curious , why?
 
#10 ·
as I recall the 6.2 and 6.5 diesel engine share the chevy bell housing bolt pattern and the location of the side mounts on the block are chevy location too.

it has been a few years since I worked on those old diesel engines... correct me if I am wrong
 
#12 ·
In 1981 I pulled a sick diesel from a 1980 Chevy P/U and tried to replace it with a 350. It would NOT bolt to the tranny by bolt pattern. I had to rebuild the diesel. Beyond those details, haven't got a clue as to why 30 years later beyond the fact it cost the owner a lot more bucks. B-O-P bolt pattern vs Chevy, perhaps??

Dave W
 
#13 ·
the 5.7 Olds engine block based diesel was another animal and had a few problems. Seems like it was required to pull at least 1 cylinder head for repair on each engine with in 20k miles from new. That was a BOP design bell pattern
 
#14 ·
What transmission was in the 81 truck ? From what I understand, the older trucks had a different transmission than what I'm dealing with. I am dealing with a TH400, and trying to bolt up to a small block. Not only that, but I'm pretty darn sure if, I do have any problems, someone probably makes an adapter. I also have a TH350 that was in the back of the truck when I bought it, but I was told this would not bolt up.
 
#16 ·
30 years later - not a clue :pain: .

My thinking and opinion only - you had best use a gas engine tranny. Modulators as mentioned previously and torque converters are probably different as well as possibly the internal valving.

B-O-P = Buick-Oldsmobile-Pontiac. Many of us here tend to use acronyms that confuse others. Sorry

Dave W
 
#17 ·
There's about a million threads out there, including on here that claim the TH400 will bolt directly to a 350 and or a big block since they share the same bolt pattern. I was told the only swapping necessary is the converter and the gov. The gov.only was mentioned because it controls the shift points and since the diesel shifts at a lower RPM, that's the reasoning for the swap on that, which may not be necessary. Other than those 2 items, the transmission is identical. The majority of information I've found on this swap came from this site. Here's just one of the many. http://www.hotrodders.com/forum/6-2-diesel-swap-350-gas-38161.html
 
#18 ·
I should mention as well, that the 6.2 liter is not in anyway related to the Olds motor. This is from all the information I've found pertaining to it. The 6.2 liter in the 83 Chevy's is a Detroit diesel. The same as used in the military Humvee. A lot of people get them confused. The 6.2 is basically a 350 base, with diesel heads and a heavy duty rotating assembly in it. I find about 50 50 on these motors. Half like them, half don't. I weighed my options and I believe this is the best route for me to go. My motor was garbage, Heads garbage, & Diesel currently $4.39 a gallon. If you compare the mileage vs. cost of fuel, I think it's about the same either way. We have a Cummins diesel and hardly ever drive it because of the fuel cost.
 
#19 ·
The 5.7L diesel was a BOP. The 6.2/6.5 is GM, and uses a GM bell housing.

A 6.2/6.5 uses a weighted fly wheel, where as a 350 does not. So that kills the use of a 6.2 wheel, on a 350 right there. I swear I remember the crank pattern being the same though... Not that it matters. Like said though, a 350 6-bolt flex plate is what you need with a tooth count to match the starter setup.

The 6.2/6.5 uses a 1500 stall converter. I believe the lowest gasser 400 stall was like 1800?

Frodin- "The 6.2 is basically a 350 base," That is not true at all. Nor is the 5.7 based on a 350.
 
#20 ·
I meant as far as the bolt patterns go, like I had said before. The 350 and 6.2 are very different motors when you look at them taken apart. I just sold a bunch of stuff out of the 6.2 and they are VERY different inside. For 1, I've never seen a 2 piece 350 come with roller lifters from the factory? They are way different as far as strength goes. Of course, I've never seen a set of heads so beaten down until I took this diesel apart either. Just a note to those who do this! Running straight water in an engine is evil, and unless you want a whole handful of issues in the future never do it. Antifreeze is for more than just cooling. Cracked heads and valves on both sides, heavy rust though out the motor, freeze plugs leaking, corrosion eating it's way through to the internal trans cooler and filling the transmission with water. These are all things that WILL happen if you do this. Spend the $10 bucks and put the right stuff in your radiator. Please! Lol
 
#21 ·
Chevy diesels have the same bolt pattern as Chevy gas engines. You can even bolt your TH400 behind a duramax if you don't mind missing the extra bolt holes that the duramax has on the oil pan.

The early chevy truck diesels (before the 6.2/6.5) were the Olds 5.7 diesel, which has the BOP housing pattern.

But, regardless of diesel or gas, every GM passenger vehicle since about 1963 came off the line with either a Chevy or BOP bellhousing pattern.
 
#23 ·
I've been driving this stuff for years and still do. The 6.2 block will mate with chev turbo 350, 400, 700R4. My 81 5.7 diesel , these trucks came with a dual patterm 350C tranny meaning it will bolt to the olds block or the chevy block, which was neat cause if you blew up the olds diesel then you could throw in a olds gas or a chevy gas if you wanted. It will work ok if you just bolt your gas 350 to that diesel tranny just use a gas flywheel. That 6.2 is fantastic on fuel especially with the o/d tranny and is far cheaper to run than any gas motor, too bad yours is done.
 
#24 ·
Yea, I weighed the options, and I seriously tried to save her. But after I did a compression test, and pulled the head and found a busted valve I figured it's time to retire it. I ever looked into getting a military 6.2 off ebay, to put into it, but they want an arm and a leg "or two" for those diesels. Kind of a bummer, but oh well, I gotta go with what's cheapest and available to me. Since I already had most the stuff for a gasser, I figured that would be the way to go. Thanks for the input!
 
#25 ·
I presume where you are there ain't many diesels in the salvage yards. Up here there are many so a guy can pick them up fairly cheap yet. Rare to break a valve but I bought one once with only 30K 6.2 that the guy said was just idling and bang ! , took it apart and broken valve . You can still use your hydraulic brake booster if you have the brackets to bolt the pump to the gas, remember the diesel ps pump puts out more cause of the brake booster than just a reg gas ps pump. Unless you go to a vacuum booster then it doesn't matter on the ps pump. Otherwise not a big job to swap.
 
#26 ·
Yea, A lot of the yards have started disappearing out here. They crush everything and move on. It's sad actually, they're in a way eliminating a hobby for many people. Its all about the mighty buck. I still do have everything for the diesel, so I do plan on using the PS pump. If anybody knows this, it would help me out. What's the difference between the Diesel fuel pump and regular gas? I purchased a brand new electric pump, ran it for about 5 minutes and shut it down before I took the motor out. I'm just wondering if I can still use it for the regular fuel without anything catching fire or blowing up? The pressure is rated 14psi max. I was planing on using a 600CFM 4 barrel? I figured I could just use a regulator if it's to high for it? What do you think?
 
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