6.2 conversion checklist

Is there. General checklist for a diesel conversion? After a lot of time debating the subject I decided I'm going to convert my 1965 C20 to a 6.2 gm Diesel engine. I currently have a SM420 4 speed so that is not an issue (plus local highways are only 55MPH anyways, so who cares?). Obviously I will be replacing the gas tank (and relocating it to the rear). At this point I've found some 6.2's on craigslist from $250-$750 ($750 had a turbo) so the cost seems fairly low to me. I would prefer to keep everything possible mechanical as there's not a computer on this truck anyways. I've found a nice one for $500 or make an offer on what's left of the truck, though in thinking about waiting until I find one that's all there, just beat up or cosmetically in poor condition, I feel like this would save some headaches.

So what is the simplest way of going about this? The wiring on the truck can't reall get any more straight forward, what does this engine truly require to be bolted up to a SM420 then started? I have very limited diesel experience on passenger vehicles, and more experience on heavy equipment, so I'm not really sure where that places me.

Pretty straightforward. The 6.2s and early 6.5s were mechanical. It wasn’t until later 6.5s that they were computer controlled. Bellhousing configuration and bolt patterns and motor mount configuration are the same gas to diesel. You’ll need the diesel accessories including starter and larger radiator.

6.2s were mechanical lift pumps, so I’m thinking you’re basically talking wiring the starter, glow plugs, fuel shutoff, and charging system.

Best situation is always to buy a donor vehicle so that you have everything you need. If you just buy the block, you’ll likely end up nickel and diming yourself getting all the little things you need.

Awesome, thanks! I'll probab wait and look for an old farm truck or a a wrecked truck then, I'm in no hurry. I have a running 250 I6 hooked up right now.

It's more simple, and less simple, than just that.

Fitment-wise, what you want to do is probably the simplest conversion. Everything will fit, and mounting parts will be universal. Electrical is also very simple. The current Ign. circuit is all that's needed to power the injection pump. The HPCA and high idle (to coolant temp switch) will tee from that lead. The starting/charging circuits will be the same as original, with the simple change to an internal voltage regulator. The starter, obviously, is different, but connects the same. You'll need to add another battery tray, or accommodate a much larger single battery. The glow plugs can be either original, with the engine-mounted controller system, or manually switched (or both). These are all simple accommodations, and should be cheap or free, depending on what you get with the engine, or chassis you buy. I don't think you need a whole truck, as you'll be re-plumbing the fuel lines, anyway. You'll need to add a fuel return line from the engine to the tank. If you are relocating the tank to the rear/center (spare tire space), you can use a late model Suburban or Blazer tank and sender (meaning, if you buy a donor vehicle, a Burb/Blazer/Jimmy would be best).

Where I think you'll have a problem is the tranny. Your current 420 won't hold up to the Diesel, especially if there's a turbo in your future. That is, if you can even source a clutch/plate that will match. An SM465 would be a much better choice.

Good luck! This should be a fun project.

The 420 trans will work fine. It's not much weaker than the 465 and a 6.2 goes nowhere close to the max power it can handle.

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Originally Posted by DmaxMaverick

It's more simple, and less simple, than just that.

Fitment-wise, what you want to do is probably the simplest conversion. Everything will fit, and mounting parts will be universal. Electrical is also very simple. The current Ign. circuit is all that's needed to power the injection pump. The HPCA and high idle (to coolant temp switch) will tee from that lead. The starting/charging circuits will be the same as original, with the simple change to an internal voltage regulator. The starter, obviously, is different, but connects the same. You'll need to add another battery tray, or accommodate a much larger single battery. The glow plugs can be either original, with the engine-mounted controller system, or manually switched (or both). These are all simple accommodations, and should be cheap or free, depending on what you get with the engine, or chassis you buy. I don't think you need a whole truck, as you'll be re-plumbing the fuel lines, anyway. You'll need to add a fuel return line from the engine to the tank. If you are relocating the tank to the rear/center (spare tire space), you can use a late model Suburban or Blazer tank and sender (meaning, if you buy a donor vehicle, a Burb/Blazer/Jimmy would be best).

Where I think you'll have a problem is the tranny. Your current 420 won't hold up to the Diesel, especially if there's a turbo in your future. That is, if you can even source a clutch/plate that will match. An SM465 would be a much better choice.

Good luck! This should be a fun project.

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Awesome, thanks! I'm not planning a turbo (no point if I keep a 4 speed, if I go and swap out the tranny I may as well do the 4x4 conversion while while I'm at it. for now I just want a good clean running work truck for puttzing around and short cruises (we have a 55 mph speed limit on the highways here). I know people hook up 4bt diesels to the sm420 all the time so I figured since I'm not hot rodding it or anything it would be okay. I'll figure it out when I have the cash and parts accumulated. Thank you again!

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Originally Posted by AKMark

The 420 trans will work fine. It's not much weaker than the 465 and a 6.2 goes nowhere close to the max power it can handle.

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Cool, I figured a N/A 6.2 wouldn't be a huge deal, long term I either was a short bed stepside that I'll put a good solid tranny in and a turbo diesel or I'll just keep upgrading this. I used to not want a long bed, but I've fallen in love with mine.

I was going to go with a 420 in my K-5 until we found out that the transmission in my buddy's yard was a 465, not a 420.

I agree the 6.2 will not be a problem for the 420. I am planning on building 6.2 with all my parts for my 65 GMC 4x4 and it has a 420.
I did some investigating and found that the 6.2 and my 292 6cyl have almost the same HP and Torque rating.
I don't push my truck hard anyway so I am not at all worried about it. The 420 is also highly sought after in four wheeling circles because of how tough they are for their small size so that boosts my confidence even further for doing this.

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Originally Posted by tomtaylor

......I did some investigating and found that the 6.2 and my 292 6cyl have almost the same HP and Torque rating.......

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This is irrelevant. HP is completely irrelevant. Torque @ RPM IS relevant. The 6.2L will shred the 420, eventually (seen it more than once). Quicker if used near capacity. Applying the same torque on a tranny at less than 2/3 the RPM will compound the applied torque exponentially. Even with the correct bell housing, I don't think you can fit a large enough clutch/PP, which is also a weak link when factoring torque/RPM. Also, the 420 is not dampened and will require a dual-mass flywheel, if it has any chance at survival, at all.

I don't know why you keep saying the 420 cannot handle the power.

I've seen 420's used behind high horsepower V8's and they've worked fine. I've seen them in medium to heavy duty farm equipment pulling 40 tons of potatoes for over 40 years with nothing but oil changes behind the old 5.0 V6 or even 454 V8's.

Dual mass flywheels? Really?

Man I guess we've been lucky all these years on the farms and other places I've seen them used.

If the 465 more modern? Yes, doesn't mean the 420 is crap. I still run the Rockwell T-221 T-case in my K-5. Is a NP205 better? Yes, but the Rockwell does everything I ask of it so it's not worth upgrading because a guy on a forum says so. If he has a 420 and all the shafts to make it work, run it until it quits. Then upgrade.

That's my opinion, for what it's worth.