Where I live, /6 engines are a bit rare as old Mopars were shred due to rusting. My knowledge and interest in the slant 6 was because my 1956 Lone Star Cruiseliner 2 boat came with one, but needed work. Not many were made with the Slant 6 225 inboard/outboard setup. The boat is big and heavy. So I had thought about changing it out for a bigger engine, but decided to squeak out as much as I could from the old Slanty. But the project gets sidelined a lot. And now I am at a cross roads. Continue? Sell it all? Just get it back together and enjoy it?

Now then, I have before me an opportunity to get a running 1970 198 out of a Duster. I was thinking of using the rods in my 225. But maybe just rebuild it stock.

If I do get the engine, what is the going rate for the 198 rods, If I were to just part the engine out? What about the Crank?

Thanks all for your feedback. Fyi, I am in Central Canada.

Brian.

Hi Brian,

Thanks for posting and glad Slants are on your mind. The boat sounds cool, and I'm sure you can pep up the Slant if you put in some time and effort (and $). How good are you with basic mechanics and such? Do you have a good/reliable engine machine shop near you? Doing some mild "bowl porting" of the head can get you substantial HP and tq gains without a ton of work. I am guessing this is the "tilted flat" engine so your intake and exhaust manifold and carb options are either limited or totally fixed.

It's not clear how much (if any) 198 rods help you, but they have been used by many. Probably a set of core rods is worth $200-400/set depending on the buyer. Crank is not worth much, but you can always post them here and see. They are certainly very rare these days, but Slant folks are also cheap... If you use the 198 rods, you will need different pistons than stock (with lower compression height - around 1.6" instead of the stock 225 at 1.74". Unless you are doing a high compression, high HP engine, the 198 rods are really not worth the trouble and expense.

Happy to discuss more parts of your projects as you think through things...

Lou Madsen

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Home of Slant6-powered fun machines since 1988

Yes and no. Substantial gains can be had when coupled with other modifications. When taken in concert, the torque curve can be widened and extended with long rods, but not inexpensively. There are other places, like Lou suggests where your returns may be greater. I don't know how much wiggle room you have with your out-drive to take advantage, such as carrying more prop. Boats are a different animal. Pushing the limits where it isn't as simple as parking over the fog line, and calling triple-A isn't an attractive option to me. I like to row, but paddling a power boat isn't on the top of my favs.
There are applications where the 198 parts can be used to good effect. The crank can be used for a 210 low deck but that isn't going to be appropriate for a boat, unless the out-drive can be modified to take advantage of higher revs, the rods are a healthy part of a racing engine, and could be used with a selection of parts to make either economy or power. Just depends on what you are trying to accomplish.
Last I remember when Doc was doing mad-scientist stuff, he was building a high deck with a 170 crank and long rods for a high fuel efficiency commuter. I don't know if that survived the fire. If I remember right, the rods were a lot longer than 7.005" to get zero-deck. Be great to hear from him again!
I ran a long rod with a stroker crank, and it made for an over-the-top torque curve. Lots of machine work, and clearance work for 250 CI and 10.8:1 I put $10k into that engine. I don't know that I would want to be at the mercy of river current depending on that engine. It ran a quarter mile at a time, and runs to this day. There were "idiosyncrasies" that would made it unsuitable for cruising. It still bumped the fuel economy up just over 20mpg so long as the revs stayed down. ~2500 which was stall, so it made heat in the trans.
Doc ran a stroker that would tow and cruise in his wagon. He used to tow his toad all over the west coast with it. But.. 225 rods. There are some benefits for dwell time the better rod ratio 7.005" rods provide, but that is clearly debatable as you talk to other camps.
If K1's had been available back when I built my long rod, I would have used them. Not that the stock 198 rods had any particular weakness, but availability was better with the K1's for the better part of the last two decades. Finding serviceable 198 rods is like finding hens-teeth, so don't damage them. The ones I have were machined and lighened, with the small end resized to fit smaller pins in order to play nicely with my pistons of choice. One-offs. I have another set of 198 rods in the wings, and a new set of K-1's I bought when they became available.

5 cents, as pennies are obsolete...
CJ

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Part of Tyrde-Browne Racing

Would a 198 have a different cam in it when compared to a 225? Would there be any changes there if I used that, probably stock, bump stick, when compared to lets say the 80s dodge truck or possibly the unknown one in the boat engine? I am curious but would have to take some measurements off of that one to compare specs.

I think I have concluded, at least for myself, that I am not going to long rod this project, based on input here, but I am not sure I want to even invest (time and $) on saving these 198 rods from the shredder. It sounds like there are aftermarket K1's? for such a build now for those inclined to put the $$$$ into an engine.

No difference in the cam, head, or block. There was a guy not long ago that was looking for 198 bits, and there will be again, I'm sure.

If you decide to work the chambers and ports, read up. There is water very close to the ports. Doc posted some photos showing how close. Going to oversize valves doesn't require sizing up on the seats. You can easily hit water cutting for a larger seat, and it won't matter for the valve sizes that won't shroud on the bore. There are options like Chevy small block valves, and Ford 300 valves, though you may find you have to turn the Ford valves down a bit for clearance. I turned mine down, and back cut them. Oversize valves generally won't give big gains until you are .060 over with the bore. If you choose to use larger valves, make sure to check chamber alignment of the bores. You risk tagging the deck with a valve, especially when cutting the deck and/or head for compression. Generally speaking, there aren't a lot of gains in over-bores, as it doesn't add many cubes. 0.100" over is a 238. The only way to make a big difference in cubes is stroking, but the 225 is already well under-square, so Piston to wall speeds, and reciprocating forces are not ideal for high rpm operation with a stroke. If the operating envelope is kept in check, that can be OK. The head won't flow well enough to take advantage of higher revs anyway. A stock 225 can survive at 5500 as long as it's put together right. Some folks run higher, but tend to find weaknesses rather quickly. A 225 at 6000rpm is a time bomb. 7000 rpm is asking for trouble right away with all but the top drawer builds. With a stroker like mine, you can hear the engine getting unhappy anytime it's over 5000 rpm. At 5500 it makes unpleasant noises. It's also a bit pricey to stroke the slant substantially if your not going for a smaller rod journal. I had my crank welded up to get back to the standard rod journal with a 4.475" stroke. I don't know what Doc did with his 4.500" stroke crank. With four mains, even the forged crank is going to flex at higher revs, so surface hardening or coatings aren't going to live long. I didn't bother. The bearing area is huge, so as long as your oil and filter habits are good, it won't make much difference in life expectancy. Once up above 4000 rpm operating ranges, oil pump drive gears become "a thing." There's plenty to read on this forum about ways to mitigate oil pump gear failures. At lower rpm, aeration and cam walk are lesser. With a boat, higher rpm may not be desired. Of course if your going with Stainless or surface piercing drives, cavitation might be less an issue.

Compression, careful head work, cam selection and ignition system would be what I would concentrate on. All selected for the desired operating envelope, and fuel grade. Bigger is often not better.

Enjoy the journey!

CJ

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Part of Tyrde-Browne Racing

Sounds like a keen boat, but…1956, you say? If that's the right build date, then none of them was built with a Chrysler Slant-6. That engine didn't exist until late 1959, and wasn't marinized until 1964.

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Too many people who were born on third base actually believe they've hit a triple.

I'm not going back in my current/6 anytime soon but I sure wish someone had a set of 198 rods they didn't know what to do with when I was in early stages of that build