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Old 07-24-2007, 08:58 PM   #1 (permalink)
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Convert a 350 diesel back to gas, How to?

Found a Diesel 5.7L GM crate motor in a Cadillac at the junkyard. I have read that with the right application of parts, you can make a stout GASSER 350 bored and stroked to over 430ci? I like the idea because its different, and kinda retro. It is the later updated DX It has the roller cam, and updated bottom end. I have been searching my fingers off, and have found no recipe on the conversion, just racers used to do it.

What kind of cost am I looking at if I decide to persue this hair brained idea? I'm wanting to get an idea of the cost, parts requirement, and what is the recipe for building such a monster.

As a diesel, I know they were a piece of JUNK, underpowered, were based off an olds 350, made the top 10 worst engines list, and make excellent boat anchors.
Any other irrelevant diesel trivia I don't care about. I am trying to focus on the positives and negatives of converting a diesel 350 back to a gasoline motor, not how bad of a POS it was as a diesel. That does not matter for what I am looking to do with it.

All I know so far thanks to google.
the block can accept a .100 greater overbore than a normal 350
bore/stroke to 430-440 c.i.
have to somehow plug the boss for the injection pump
distributor will bolt into place of the vacuum pump
Off the shelf parts wont work
stock bottom end is too heavy for HP use

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Old 07-24-2007, 09:33 PM   #2 (permalink)
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IMO it's not even worth it

350's are a dime a dozen. why even mess with it? why dont you reinvent the wheel while your at it
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Old 07-24-2007, 09:49 PM   #3 (permalink)
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i have heard of it being done, replace glow plugs with spark plugs make a distributor work, get some dished pistons to start
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Old 07-24-2007, 09:51 PM   #4 (permalink)
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i had a friend about 10 yrs ago that built one at a tech school in arizona. made tons of torque, like 600 #s but i never heared what exactly they did to it. bored and stroked the piss out of it.
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Old 07-24-2007, 09:56 PM   #5 (permalink)
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I believe the block is the good part. Heavy everything and 4 bolt mains. Those suckers sure would blow head gaskets in diesel trim. So look for everything that goes along with a blown head gasket, ie cracked heads, block ect.
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Old 07-24-2007, 11:03 PM   #6 (permalink)
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I have a friend at work that did the exact same thing your talking about. He acts like it's the best kept secret in GM small blocks. He's talking more than 800 H. P. so they can't be too bad. That and he's a cheap mother fucker so it can't be too bad in cost. I'll post up what I find out tomorrow.

It's not exactly re-inventing the wheel. This is old school tech from what I've been told. If we all did what everybody else was doing we would all be rollin' in Jeeps on 31's.

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Old 07-25-2007, 12:54 AM   #7 (permalink)
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Many years ago they used to build Pro Stock motors out of the 5.7 diesel blocks. I think that the only thing that they used was the block.

Gus
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Old 07-25-2007, 01:19 AM   #8 (permalink)
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1. buy newspaper
2. read classifieds
3. buy good running 350 for $500
4. swap them
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Old 07-25-2007, 09:15 AM   #9 (permalink)
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If you are looking to make a gazillion HP, the DX block is the way to go. If all you want is a moderately powerful engine, look somewhere else.

For DX builds, look www.realoldspower.com
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Last edited by Mikel; 07-25-2007 at 09:16 AM.
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Old 07-25-2007, 11:48 AM   #10 (permalink)
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Try "olds dx block" on google... turns up a lot of hits and good info.
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Old 07-25-2007, 01:30 PM   #11 (permalink)
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Ok this said, there are some olds guys who are kookoo cachoo fanatical about these old diesels.

My understanding is that they are not "based" on a gas engine any more than they happen to use the same cylinder spacing on a similar block. Anyone who thinks that you simply convert a gas engine to "run on diesel" either confuses the basic design similarities between all diesels and all cylinder-based gas engines with miscegenation, and/or doesn't understand the diesel engine design in the first place.

I'm just saying - I've been hearing that "converted 350" talk forever and people make it sound like it's as easy as converting carb to propane. It's not just a different fuel; it's also a different ignition/combustion system, fuel delivery, timing, intake, and in general just a whole different ethos of engine entirely. It's like saying, "I converted a standard transmission to an automatic." Um, yes, they both have gears and clutches and power goes in and power comes out, but they are pretty different beasts.

And yes, there is a design flaw, which, it's my understanding, was that the headbolts were undersized and undercounted for the application. Said fanatics now use high strength ARP studs for the heads (and they may oversize these as well), which solves the headgasket issues.

Anyway, hell of a day here, and I needed to think about engines for a while. Hopefully this post isn't too egregious.
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Old 07-25-2007, 05:32 PM   #12 (permalink)
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I worked for "the Olds" during the diesel days, and the issue was the pump and its (in)tolerance for bad fuel (i.e. Water) and heat soaking. (The customer service notion that all dealers/customers with warranty claims were thieves notwithstanding.) The blocks were superior, but, then again, so is today's LSX block.

Bottom line: There are easier ways to make big power.
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Old 07-25-2007, 05:55 PM   #13 (permalink)
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Bolt in a stock 403 with a mild cam and call it good
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Old 07-25-2007, 10:29 PM   #14 (permalink)
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Quote:
Originally Posted by KWTMECH View Post
Bolt in a stock 403 with a mild cam and call it good
I said I wanted something different.

Quote:
Originally Posted by hightechredneck View Post
Try "olds dx block" on google... turns up a lot of hits and good info.
Lots of info, still trying to weed thru the negative hype, and trying to find the real tech.

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Originally Posted by Mikel View Post
If you are looking to make a gazillion HP, the DX block is the way to go. If all you want is a moderately powerful engine, look somewhere else.

For DX builds, look www.realoldspower.com
Not looking for a gazillion HP, just something I can make that is rare, and not seen very often anymore

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Originally Posted by nissancrawler View Post
1. buy newspaper
2. read classifieds
3. buy good running 350 for $500
4. swap them
I would have never figured that out, thank you for your input. BTW $500 is way too much for a good running 350, especially when I have 2 of them already in my garage.

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Originally Posted by mc5cent View Post
I have a friend at work that did the exact same thing your talking about. He acts like it's the best kept secret in GM small blocks. He's talking more than 800 H. P. so they can't be too bad. That and he's a cheap mother fucker so it can't be too bad in cost. I'll post up what I find out tomorrow.

It's not exactly re-inventing the wheel. This is old school tech from what I've been told. If we all did what everybody else was doing we would all be rollin' in Jeeps on 31's.
Can you find out what he did to build it, or where he got his parts from?

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Off the shelf parts wont work


so what make you think this is a reasonably feasble plan?
I am researching what parts will work, so I dont spend a gazillion dollars on custom shit, when its already been figured out.

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IMO it's not even worth it

350's are a dime a dozen. why even mess with it? why dont you reinvent the wheel while your at it
I want something different. I was going to repower with a 5.3 vortec, but this seems more like something I would like to have. My intention is to not re-invent the wheel. Doing that takes more smarts/motivation than what I have. I think this motor would make an interesting conversation piece.



I went ahead and gave the guy at the yard $50 for the complete motor. I know I paid too much, but he had it lined up to go to Scnitzers to crush it tomorrow, had to do something. I might be able to make my $50 back by selling the unused parts to somebody still running this motor. They are still out there. Worst case, I found a place in Paso Robles that buys these blocks...
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Old 07-25-2007, 10:35 PM   #15 (permalink)
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I have heard rumors of a simple head and intake swap for a low rpm powerhouse. Compression can be a problem as well as internal balance.
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Old 07-26-2007, 05:00 AM   #16 (permalink)
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I have heard rumors of a simple head and intake swap for a low rpm powerhouse. Compression can be a problem as well as internal balance.
what would you do for spark. I think this would be a big concern considering there is no whole or mounting for a dist. (atleast thats what it sounds like. i haven't done any research)
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Old 07-26-2007, 07:36 AM   #17 (permalink)
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What about keeping it diesel, and turbo'ing it with like 10 psi or something? That would be cool. And, if the IP is problematic, run a serious water seperator and a good particulate filter (care and feeding for any IP - looks like Olds may have skipped that step to make diesel fuel system care as easy as gasoline). Also, ULSD may be problematic to run in that old unit as well, which probably would need more lubrication in there.

As for the heat soaking issue, get a big ass 1 ton rad and an oil cooler, and run an intercooler as well.

Studs in the heads would be a definite, at that point.

Just saying. A twin or dual turbo'd (maybe in series or in parallel) diesel olds, if the bottom end will hold up to the power, would be fairly unique in and of itself.
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Old 07-26-2007, 05:27 PM   #18 (permalink)
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I just have to ask why anyone would something "weird" out on the trail?
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Old 07-26-2007, 07:51 PM   #19 (permalink)
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Quote:
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what would you do for spark. I think this would be a big concern considering there is no whole or mounting for a dist. (atleast thats what it sounds like. i haven't done any research)
The diesel has a vacuum pump where the distributor would go. You yank the pump, put a dizzy in its place, done.

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What about keeping it diesel, and turbo'ing it with like 10 psi or something? That would be cool. And, if the IP is problematic, run a serious water seperator and a good particulate filter (care and feeding for any IP - looks like Olds may have skipped that step to make diesel fuel system care as easy as gasoline). Also, ULSD may be problematic to run in that old unit as well, which probably would need more lubrication in there.

As for the heat soaking issue, get a big ass 1 ton rad and an oil cooler, and run an intercooler as well.

Studs in the heads would be a definite, at that point.

Just saying. A twin or dual turbo'd (maybe in series or in parallel) diesel olds, if the bottom end will hold up to the power, would be fairly unique in and of itself.
Heh, yeah right. This thing was barely adequate as a stock diesel. As a gasser, it could hold up to 700+ HP

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I just have to ask why anyone would something "weird" out on the trail?
If it runs, and STAYS RUNNING, who cares?
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Old 07-27-2007, 12:56 AM   #20 (permalink)
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It sounds cool, but probably not worth it unless you want big horspower.
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Old 07-27-2007, 01:04 AM   #21 (permalink)
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20 8 hp briggs engines started by a steam engine would be more unique.
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Old 07-27-2007, 11:42 AM   #22 (permalink)
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the ansewer is in the post you quoted. if you swap out the heads for gas heads, they have the spark plug holes already.
i got that

i asked about the DISTRIBUTER
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Old 07-27-2007, 11:52 AM   #23 (permalink)
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BTW this brings to mind the buildup of the Silver Bullet, the Cat 3208 block they turned into a spark-ignition engine.

http://www.google.com/search?&q=3208+silver+bullet

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Heh, yeah right. This thing was barely adequate as a stock diesel. As a gasser, it could hold up to 700+ HP
Um, even more reason to turbo it? If you can keep the heads from floating at pressure, throw one on. I think of the silver bullet because my NA 3208 was a total dog... until I made a pedestal and oiling system for a turbo and threw one on. Literally, a totally different engine, and one that desperately needs the intercooler I've been too stupid busy to mount.

BTW this makes me wonder if the 6.2D is similarly built on the same piston centers as the 350.

Sorry for the hijacking - I think about and build this same sort of stupid shit all the time and this is fun to contemplate.
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Old 07-27-2007, 04:49 PM   #24 (permalink)
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I said I wanted something different.
Huh? So I'm guessing all the Olds 403 powered trail rigs are on the west coast or something?

I think I asked about that a few years ago on here. They can be built into decent engines, but need liberal application of the dollar, as with any weirdo engine that isn't a small block chevy/ford/dodge (go ahead and argue the semantics of "small" vs "big" block being incorrect in non-GM applications, I will ignore you).

Go ahead and build it up and report back. I'd like to see that.
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Old 07-27-2007, 06:37 PM   #25 (permalink)
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I haven't sifted through all the "you're dumb, you should just buy a 350 like everyone else" but I have an old 350 diesel in the barn. Its in good shape and I have thought about the same thing.

From what I remember, despite engineer reccomendations, they decided to use the same head bolt tooling which is why the head gaskets went fast. There were also some issue that would cause coolant to leak into the crankcase.

This combinded with newb technitions resulted in dissaster.

You do know they have the BOP bellhousing not GM correct? Gonna be hard to find a good tranny but there are dual pattern trannys out there if you know where to look.
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