. . . in a GMC motorhome. 40+ year old engines in heavy duty use is an interesting subject, especially engines that weren't originally designed for it in the first place. (Un?)Fortunately for me, I'm looking to possibly buy one for hauling 12,000+ pounds around.

My Olds 307 was pretty stout for what it went through with me until 270,000+ miles. I know to watch out for cooking around the EGR valve in the middle of the left head, leading to cooked oil and gaskets nearby. I never had an issue with it. Not any other gotchas in this engine that I know of. I never towed more than 5,000 pounds with it, but with a 2.41:1 rear end it probably felt like 9,000 to the engine.

The 455 appears to be some kind of undersquare over-stout torque monster. I drove a badly beaten one whose transmission was shifting too early and it didn't seem to care much that it was accelerating 11,000 pounds at 20MPH in 3rd gear with a plugged up air filter. What are the gotchas in this one?

The 403 appears to be an oversquare siamezed V8 with some potential for overheating. In my searching, I haven't seen much mention of overheating in these in the motorhomes, but I saw a bunch back when they were newish that had heating issues. Maybe the big radiator makes it okay?

Anyways, are there any gotchas in these that you have experience with? What should I look out for in these?

The usual complaint about the 403, besides the siamese bores, is the 'windowed' mains.

I don't have any personal experience with either the 403 or the 455 olds engines, just a little ancedotal info. My late father had a 78 Toronado with a 403, I drove it on several occasions and it had no problem pulling that heavy Toro around, no overheating issues with it. I'm originally from eastern Wa. state and back in the 80'*, the Olds 455 was the favorite engine powering flat-bottomed jet ski boats, I take it because of the low-end torque, the jet boat propulsion system is fairly inefficient fuel wise at least. If you could find a marine 455, that might be the ticket, probably lower hours/equivalent miles, and the marine heads would be a big bonus too.

Windowed mains. Didn't know about that.

I read around after reading your post and the consensus seems to be that as long as you don't boost them a lot they hold up well. There are a few mentions of racing them as a less-expensive (and a bit faster) alternative to a SBC. One guy said he swapped rods for Dodge 360 rods to increase compression and raced them for years. Not quite dragging a motorhome but still stressful.

I guess if one of those pops up in my path I'll be cautious if it'* been power-added somehow.

Cool, thanks for the info. I dug around the Interwebs with this and found that the blocks are probably the same throughout 455 production, but heads differed over the years. Many say the cranks are stronger in the marine motor, and some say the head gaskets are stainless steel. They look to be pretty rare, and if I find one it would probably be well-used and need an overhaul. My concern would be that these are designed to run 4,000RPM all day long where I'd want something that would have low-end and run 2,500-3,000RPM all day long. Good to keep in mind.

I'm hoping to find an original or a rebuilt-to-original unit. We'll see.

Here'* a random conversion/swap thought: https://www.uhaul.com/TruckSales/equ...24028/JH1990K/

Carry on.

I've always wondered the story of how this came about. The 403 is an oddity, not having the larger crank and connecting rod bearing shared with the 400/425/455 (and 350 diesel), since it is the smaller or shorter decked block. I've wondered if it was 'victim' of the same cost cutting that weakened the bottom end of the Pontiac 301 (and 265, I assume), as it crossed divisions to be the T/A 6.6 engine? Seems like such an odd thing to do.

Blegh.

163,000 miles as a U-Haul is like 163,000,000 miles as most other things. Maybe it'* the same one as:

Yeah, I had no idea about the windowed block. My dad says they never had trouble with them when he worked the dealership in that era. He only saw them in cars though. For me, the design combined with the years of production tells me "yes, CorvairGeek is probably right, it'* a victim of GM'* shenanigans of the mid-to-late 70'* design direction", but I also think it might have been one of the lucky ones like the Chevy 305, the Olds 307, and the Pontiac 265 V8 for that matter.

I own a 265, and even at 150,000 miles it happily buzzes away and the oil is almost new looking every 3,000 miles. I know what'* it'* been through in its life (at least since the late `80'*) and it (and its Metric transmission) have had many excuses to come to pieces and haven't. It has no torque though, and I don't blame it considering its heritage and redesign.

Anyways, back on track: My guess is they were abandoning the big-block architecture but still needed a "big" engine, so in typical 1970'* GM form someone said "Let'* talk to the Oldsmobile guys and see how big we can make their small block, and we'll keep it around long enough to survive the transition until we're all driving roller skates or until the legal guys beat California and the EPA into changing their minds."

. . . . then someone at Pontiac got wind of this effort mid-design and said "hey we could use that in the Firebird, if only they could lighten it a little."

. . . . then the engineers at Oldsmobile, already frazzled about the effort so far, said " The freaking block is already siamezed and as small as we can get it and they want it lighter? What the heck else can we take out of it? Maybe they want plastic pistons so it can be lightweight? Why don't they install a paper exhaust system to save weight if they're so worried about it!" then their boss gave them a stern look and they picked up their crack pipe and got back to work.

I've been looking around more since my last post on the issue and still find folks who mention that [the windows weaken the structure so be careful/leery/cautious/etc.] but no mentions of bottom ends coming apart.

Maybe I'll get lucky and find a good unit with a 455 and not have to worry about it.

[QUOTE= Anyways, back on track: My guess is they were abandoning the big-block architecture but still needed a "big" engine, so in typical 1970'* GM form someone said "Let'* talk to the Oldsmobile guys and see how big we can make their small block, and we'll keep it around long enough to survive the transition until we're all driving roller skates or until the legal guys beat California and the EPA into changing their minds."

. . . . then someone at Pontiac got wind of this effort mid-design and said "hey we could use that in the Firebird, if only they could lighten it a little."

. . . . then the engineers at Oldsmobile, already frazzled about the effort so far, said " The freaking block is already siamezed and as small as we can get it and they want it lighter? What the heck else can we take out of it? Maybe they want plastic pistons so it can be lightweight? Why don't they install a paper exhaust system to save weight if they're so worried about it!" then their boss gave them a stern look and they picked up their crack pipe and got back to work.
[/QUOTE]

That'* about how I imagined it too.