RPO Code Clues

[HomeMadeSin]

New here, but have lurked for some time. A number of years ago, I effectively bought a 1967 C20...effectively in that at the time I just wanted the rear axle for a 4x4 project. I now want to rebuild the truck and put it to good use.

The truck has been stored at a friend's house all this time and I am currently ~2,000 miles away. I plan to pick it up and haul it East next month. Let the games begin...

Anyway, I was told it has a 327. But in what little info I have, I can't verify that (will physically check when I get there). Seems the 283 was the standard v8 for that year and model, and I don't see anything on the SPI sticker that would suggest otherwise.



So, it also was standard to have the HO52 rear axle. But the cast stamping indicates HO72 (#3883379). Holding one rear wheel (years ago) and measuring turns we figured it is a 4.56 axle w/o the locker of course.



What is really shocking to me anyway is how the RPO codes are listed in several places but virtually NONE offer any real clues about what it means (an incomplete description does little). For instance, most sites indicate a M20 is a 4 speed manual. Awesome. Is it the SM420 or SM465? What's the point of an RPO code if you can't boil it down to a specific model or feature?



I'm hoping the engine is at least a 327. Supposedly it has a spun bearing but will run. Short term is to get her running and with discs up front. Lots to do for sure, but I don't mind building a 327 and then later dropping a fuelie in. I have an LQ9 in my arsenal that is begging to be used...

The good news (I think) is that it should haul some serious weight. The F51, G60 and F60 RPO codes and HO72 indication bode well I think for high GVWR. This works well for my planned goal. And I like the original paint color spec.

Thanks in advance for any assistance.

Z

[samkost]

Check out member Jeff's great web site:
http://outintheshop.com/options.html
Very useful info and his services are top notch.

[HomeMadeSin]

Originally Posted by samkost

Check out member Jeff's great web site:
http://outintheshop.com/options.html
Very useful info and his services are top notch.

Thanks, I had found that during my searches. Very exhaustive list but summaries only. My option list may be short, but when I get the truck here I can least tell people what transmission the M20 refers to....

['68OrangeSunshine]

1967 was the last MY for the SM420. SM465s came on MY 1968 trucks.

[HomeMadeSin]

Ok, thanks. Good to know so that is sorted. Now, how about the engines? I've read the only way to determine the difference between the 283 and 327 (visually) is the web/rib between the passenger head and coolant riser. If a finger fits the recess on the head size, it's a 327. If not, its a 283.

Well, this one has no web:



Any clues? Thanks

[HomeMadeSin]

Oh, and it has a killer payload....



Score on that value!

['68OrangeSunshine]

Not that up on the ins-and-outs of Early V8s.
I'm an Inliner.
But you should find the stamped digits on a machined pad on the block, just under the head, passenger side, forward [I think?] on the SBC.
If not in front, look on the block at the end of each head.
Something like:
F0101ABC
F = Flint MI, engine assembly plant
0101 = Date "Jan 1st''
ABC = engine dress codes. These will tell you displacement, year and possible applications like auto/manual trans, etc. These codes can be found in the LMC catalog.
Would a 327 have a 2-Bbl intake?

[HomeMadeSin]

This thing couldn't have a 366...could it?!?

Edit: supposedly that was not available on 1T or less.

[HomeMadeSin]

Reading the heads, appears to be the "I" type, circa 1971-1976 and indicative of a 350 or 400 from that era. So likely not the original engine, but a tepid mid-70's small block.

['68OrangeSunshine]

Originally Posted by HomeMadeSin

This thing couldn't have a 366...could it?!?

Edit: supposedly that was not available on 1T or less.

Wasn't the 366 an underbored big block? Your engine is definitely a small block by its valve covers.

['68OrangeSunshine]

Targetmaster or Mr Goodwrench dealer 350 crate motor, most likely.
A lot of sellers will say it's a 327, if they don't think you'll believe ''it's a Corvette engine."

[68bowtie]

This will give you some good info. If it's not listed on the SPID, it came with the "standard" equipment.

https://www.gmheritagecenter.com/doc...olet-Truck.pdf

[special-K]

That motor is not original or in original form. A '67 283/327 will have the oil fill in the intake by the thermostat. As far as your SPID goes, it doesn't list the optional 327, but VIN states V8, so it is an original 293 truck.

To ID the engine to see if that's what came, or could have come, in it you need to get the suffix code (letters at end) of the engine stamping on the pad at the driverside front of the block. Usually 3 letters but sometimes only 2. Are there bosses (squared off pads) on the heads for alternator mounts and A/C? Looks like it. If so, those are newer I think. It looks like the alternator still uses the original brackets off the water pump.

[HomeMadeSin]

Originally Posted by 68bowtie

This will give you some good info. If it's not listed on the SPID, it came with the "standard" equipment.

https://www.gmheritagecenter.com/doc...olet-Truck.pdf

SCCCCCCCCCOOOOOOORRRRRRREEEEEEEE!!!!

Thank you, that was simply awesome and definitely something I was looking for.

Thanks again,
Z

[HomeMadeSin]

Originally Posted by special-K

That motor is not original or in original form. A '67 283/327 will have the oil fill in the intake by the thermostat. As far as your SPID goes, it doesn't list the optional 327, but VIN states V8, so it is an original 293 truck.

To ID the engine to see if that's what came, or could have come, in it you need to get the suffix code (letters at end) of the engine stamping on the pad at the driverside front of the block. Usually 3 letters but sometimes only 2. Are there bosses (squared off pads) on the heads for alternator mounts and A/C? Looks like it. If so, those are newer I think. It looks like the alternator still uses the original brackets off the water pump.

Yep, the owner prior to the one I am buying said it was a 327. Wrong answer. Now I'll sort out what it is when it gets extracted. And then the planning beings in earnest.

Thanks,
Z

[special-K]

Cure our curiosity. Could you clean up that flat spot on the block under the alternator in front of the head and get the suffix code? We can determine just what you have with that. It could be a 327, we don't know that yet, do we? And yes, a 327 could be 2bbl or 4bbl.

On the rear. That casting# is also found on HO52s.

Yeah, I never got why they didn't use different RPO codes for optional components like engine and transmission when there was more than one choice.

[Keith Seymore]

Originally Posted by HomeMadeSin

What is really shocking to me anyway is how the RPO codes are listed in several places but virtually NONE offer any real clues about what it means (an incomplete description does little). For instance, most sites indicate a M20 is a 4 speed manual. Awesome. Is it the SM420 or SM465? What's the point of an RPO code if you can't boil it down to a specific model or feature?
Z

You are mixing metaphors.

The SM420 or SM465 are model numbers, as described by the supplier. Just like a New Process A833 is a transmission type, recognized across the industry.

The RPO codes (the three digit codes) are specific to GM. Within that you have a "merchandised" code (ie, the one the customer or dealer chooses) and a "non-merchandised" code (ie, the one that is chosen by the engineering release tables based on model, powertrain, GVW and other specific vehicle contstraints).

It doesn't really matter if there is only one trans of that type available. For example, if you say "I want a three speed manual with the granny low" then you chose RPO MM4 (merchandised code) and the engineering specifications release a RPO M20 (which is an SM465 or whatever).

If you say "I want a three speed manual with an overdrive" then you chose RPO MM7 and the engineering charts select a MY0 New Process A833, which is the only one available.

However - if you say "I want the three on the tree manual" then you chose MM3 and the engineering charts dictate either an M62 Saginaw three speed, or an M64 Saginaw three speed with a different low gear, based on the other considerations mentioned.

Same thing with automatics (MX1 or MX0).

K

[Keith Seymore]

Originally Posted by special-K

Yeah, I never got why they didn't use different RPO codes for optional components like engine and transmission when there was more than one choice.

Excellent question (assuming there is a question there).

The answer is:

Remember those "dummy codes" that appear on later SPIDs? They used to contain useful information.

So you might have a specific engine RPO (LS9 in the example attached) and the "AC" suffix provides additional detail as far as clutch type or whatever. Any further detail would be captured by the specific 7 or 8 digit GM part number so that the correct part would make it to the build.

Eventually that level of detail fell away and the process went straight from the RPO selected to the 8 digit part number. During the transition those additional spaces were filled with characters (like a "1") until they were just left off the SPID.

But the 8 digit number defines the specific engine requirements within the RPO:

12345678 RPO LS9 manual trans no air Federal emissions
12345679 RPO LS9 manual trans no air California emissions
12345680 RPO LS9 manual trans w/air Federal emissions
12345681 RPO LS9 manual trans w/air California emissions
12345682 RPO LS9 automatic trans no air Federal emissions
12345683 RPO LS9 automatic trans no air California emissions
12345684 RPO LS9 automatic trans w/air Federal emissions
12345685 RPO LS9 automatic trans w/air California emissions

In the example above the all get the same RPO code because they are all the same base engine: cubic inch displacement, compression ratio and HD vs LD (4 bolt main or 2 bolt main or whatever).


K

[special-K]

Great info, Keith. Thanks! I wonder where this guy went. I was hoping we could keep at it a get to which engine he has.

[HomeMadeSin]

Apologies...been learning what I can and did n't get back here sooner. I doubt I will get a solid answer on the block until I get on-site. It's not in the best location to get access to currently, and will require some effort to extract.

Anyhoo, I didn't think I was mixing metaphors. I just wanted what 68bowtie had but I was not able to find. Perhaps my searching skills needed work, but I was able to find every generic description for the RPO codes without any means to tie it to a specific model. If I was going to inquire about what parts I need to adapt an LS motor to my M20 RPO truck, simple stating "4 speed manual" is next to worthless.

And I'm very literate when it comes to generic vs specific part numbers, inventory codes, etc. Been working with manufacturers in that regard for some time. But usually, it's both a functional effort and proprietary (purposefully hiding the specifics). I just thought 50 years and as many Chevy truck fans the data would be readily available. But thanks for the additional info.

Z

[pritch]

Don't the 327 have a by-pass hose? The one in the picture does not.

[Steeveedee]

Originally Posted by pritch

Don't the 327 have a by-pass hose? The one in the picture does not.

That might be true for a vehicle without heater, but I've never seen it to be the case. Weird stuff does crop up, though.

[Keith Seymore]

Originally Posted by HomeMadeSin

Anyhoo, I didn't think I was mixing metaphors. I just wanted what 68bowtie had but I was not able to find. Perhaps my searching skills needed work, but I was able to find every generic description for the RPO codes without any means to tie it to a specific model. If I was going to inquire about what parts I need to adapt an LS motor to my M20 RPO truck, simple stating "4 speed manual" is next to worthless.
Z

Be advised that the RPO codes do get recycled. As a result it is better to use contemporary literature, like the Heritage Center document which was linked, rather than the generic online decoders which can sometimes lead you astray.

If you tell folks you have the four speed with the "granny low" gear anyone worth their salt will instantly know what you are talking about.

K

[HomeMadeSin]

Well, finally made the journey home (Savannah) last night. Arrived around 10pm, after hauling the truck from Denver with about 3 hours sleep at a rest area. I hope to never do that again.

I will post some engine pics later, but here's some from the trip:









Z

[HomeMadeSin]

Originally Posted by '68OrangeSunshine

Not that up on the ins-and-outs of Early V8s.
I'm an Inliner.
But you should find the stamped digits on a machined pad on the block, just under the head, passenger side, forward [I think?] on the SBC.
If not in front, look on the block at the end of each head.
Something like:
F0101ABC
F = Flint MI, engine assembly plant
0101 = Date "Jan 1st''
ABC = engine dress codes. These will tell you displacement, year and possible applications like auto/manual trans, etc. These codes can be found in the LMC catalog.
Would a 327 have a 2-Bbl intake?

How's this?:





Z