Destroking a 4.2?

Swaping 258 to 4.0, 4.0 parts to 258 etc!
RattleU
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Destroking a 4.2?

Post by RattleU »

Hey guy's, new to the forum. My old 258 ffinally decided to bite the dust last weekend, so I'm looking at rebuilding it.
With my transmission and gearing, i was doing 2500rpm at 70mph, and it really didn't like that.
Other than that it had great power and was outstanding offroad crawling and into the mid rpm range for mud.
But it was screaming at highway speeds.. So im thinking destroking the 4.2 with a 232 crank, i like a quick revving engine, and hope the shorter stroke wwould make 70mph more comfortable for the engine. I know alot about short stroke V8's and interchangeable parts, but not a whole lot on these inlines. I'd like to have a pretty good build to put out some decent power, but i know one thing i dont want is fuel injection or a 4.0 head, i want to stay with what i have for simplicity and reliability.
So where do i start?
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Re: Destroking a 4.2?

Post by SilverXJ »

I don't see how destroking will change the highway RPM.. nor do I see 2500 rpm to be a problem.
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Re: Destroking a 4.2?

Post by IH 392 »

2500 is "screaming at highway speeds"??, you've never wound an engine out have you! I've turned the QUAD 4 in my Olds to 9500 RPMs!!!!!!!!!!!!!!
If you want a 232 go for it, crank and rod are the ONLY! parts different from a 258 ('71 and later 232's, the '70 and earlier ones use the same rods and a different block!)
You can get more power out of ANY engine!!!
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Re: Destroking a 4.2?

Post by str8-6 »

2500 rpm @ 70 mph is screaming,wonder what You should say about my engine then : it revs 3300 rpms @55 mph's,don't do me nothing except the gas-bill's gets kind of high especially when gas in Sweden cost more per liter than You guys pay for a gallon...
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Re: Destroking a 4.2?

Post by Cheromaniac »

Destroking the engine isn't the answer. What you'll need to do is swap in lower ratio axle gears so that the engine runs at lower rpm on the highway.
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Re: Destroking a 4.2?

Post by RattleU »

Destroking wont change the highway Rpm, but it will make the engine more comfortable doing it by nature of the shorter stroke. I've wound out plenty of engines, and to me it seems destroking is easier and cheaper than regearing, which isn't the problem, i currently have 3.54's in the axles, with my sm465, 33's and no overdrive, they end up to be the best all around gearing. A ford 5.0 has a short stroke and can do 2500-3000 without screaming all day. But long stroke engines don't like high sustained high rpm, it causes them more wear and tear fron what I've read. My sisters 4.0 in her TJ cruises fine at 2500rpm ,there again... It has a much shorter stroke than the 4.2.
My goal again is to have the 3.50 stroke of the 232, but overbore or get more displacement somehow, someone mentioned usinga 4.0 block with the 4.2 head? Would thst work, since the 4.0 has the larger bores?
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Re: Destroking a 4.2?

Post by SilverXJ »

When the vehicle is cruising and not under a heavy load load it is not experiencing any high stress that would lead to premature failure. Destroking will have no effect and if anything it would make the engine work harder due to the decreased displacement and power output. 2500 isn't high RPM. These engines aren't really high RPM machines anyhow. If you were running 4000 RPMs, uphill, towing a heavy load and holding the throttle there for several minutes then I would be concerned.
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Re: Destroking a 4.2?

Post by JPrubo »

RattleU wrote:Hey guy's, new to the forum. My old 258 ffinally decided to bite the dust last weekend, so I'm looking at rebuilding it.
With my transmission and gearing, i was doing 2500rpm at 70mph, and it really didn't like that.
Other than that it had great power and was outstanding offroad crawling and into the mid rpm range for mud.
But it was screaming at highway speeds.. So im thinking destroking the 4.2 with a 232 crank, i like a quick revving engine, and hope the shorter stroke wwould make 70mph more comfortable for the engine. I know alot about short stroke V8's and interchangeable parts, but not a whole lot on these inlines. I'd like to have a pretty good build to put out some decent power, but i know one thing i dont want is fuel injection or a 4.0 head, i want to stay with what i have for simplicity and reliability.
So where do i start?
That statement is probably the opposite of what you really want. Is seems your complaint is that the 258 is overworked at 70 mph. My understanding is that the old carbed inlines where low speed pulling machines. The 242 head and intake swap with a fuel injection is a common change and to my understand makes the engines a little better on the upper end of things (hi way speeds). I would say instead of destroking and losing power through a lose of displacment why do something tried and true. Add a better flowing top end and more modern fuel delivery system. So give you the results you want. Destroking will drop displacement and lower compression and to get those back it will probably cost a lot more then a top end change or a total engine swap.
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Re: Destroking a 4.2?

Post by RattleU »

Well, i wanted to boost the compression anyways, the stock ratio on these things is ridiculously low.
As for the towing, a few times a year i tow my trailer loaded with an atv and dirtbike with full camping gear to west Virginia, and im doing 3k rpm pulls up steep long grades.
Is there any way to destroke and gain some cubes? Can the 4.0 block be used with the 4.2 head?
I guess if it really came down to it, i could go with the whole 4.0 head and block with my carb setup, but then ill need new adapters and have to deal with all the crappy 4.0 header designs that all crack eventually, instead of my hedman longtubes that sound and perform great.
As a side note, why don't they make longtube headers for the 4.0?
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Re: Destroking a 4.2?

Post by JPrubo »

well I punched some numbers into the compression calc. From what I roughly figured if you used the 3.5 stroke, .060" overbored flat topped pistons, decked the block for a .000" deck clearance, with 4.0 rods (6.123") and ran the comp cams 231-4 cam with and extra 2 degrees advance. You would have a 3.9 liter with 9.44 SCR and 8.29 DCR. That doesn't sound like horrible numbers but I have no idea how well it would rev or work. Maybe someone with a laptop dyno could punch in some numbers and give you an idea of how much it would put out. Would be interesting to see.
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Re: Destroking a 4.2?

Post by Cheromaniac »

JPrubo wrote:well I punched some numbers into the compression calc. From what I roughly figured if you used the 3.5 stroke, .060" overbored flat topped pistons, decked the block for a .000" deck clearance, with 4.0 rods (6.123") and ran the comp cams 231-4 cam with and extra 2 degrees advance. You would have a 3.9 liter with 9.44 SCR and 8.29 DCR.
With flat top pistons, zero deck clearance, and a higher valve lift from your proposed combo, I'd be concerned about the pistons smacking into the valves.
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Re: Destroking a 4.2?

Post by JPrubo »

Yeah and after looking at the numbers I doubt you could machine the block for a 0.0" deck clearance while using 4.0 rods. I mean that would be shorting the stroke by .4" and only adding .25" in rod length. You would still be .15" down in the hole and that would probably be too much to machine off. So you would either longer rods then even the 4.0 or pistons with more pin to top distance. Neither of which sounds cheap or desirable.
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Re: Destroking a 4.2?

Post by sly-jeeper »

with a healthy!...4.2 2500 rpm highway cruise isnt bad...whats left of your engine? are the crank and rods good? hell ive had a 210,000 mile beat! 4.0 live on the the limiter for over ten continuous minuets it was straight side piped and was very red hot all the way to the tip :o my buddy still daily drives with that motor and has gone back and forth to ohio with it lol
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Re: Destroking a 4.2?

Post by amchornet »

If you want a short stroke 6 with lots of power the best thing to do is use a 4.0 block and head with the 232 crank. In the end you still have a 4.2...just a short stroke one. Your 258 intake will fit on the 4.0 head with some very minor modifying. But you are right you will need a new header and the stock ones suck. So buy another aftermarket one for the 4.0. There are some real good ones out there.
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Re: Destroking a 4.2?

Post by xdark »

Correct me if im wrong but i thought the 232 crank and the 258 crank were the same specs but the 258 cast was stronger .
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