Rover 3.9 to 4.2 RV8

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garrycol
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Rover 3.9 to 4.2 RV8

Post by garrycol » Sat Mar 31, 2018 2:19 am

A friend of mine has a Land Rover 101 with a fuel injected 3.9 RV8. He rang me this morning and said that while on a 1000km trip to a Landrover 70th function his engine just stopped and is all locked up. The engine had not over heated and while the sump has not been removed to have a look we suspect a big end bearing cap has come off. The 101 has been trucked the 500km back to the owners mechanic and will be looked at next week.

It is likely that a new crank and other bits will be needed none of which are hard to get - however it just so happens that the mechanic has a 4.2 crank in his shed.

Now the crank which fits in the 3.9 block we are not sure about conrods and pistons. The 4.2 had the same bore as the 3.9 but stroke increased from 71 to 77mm. I am assuming the 4.2 uses different rods and pistons to the 3.9 but we are not sure what is what.

So to put a 4.2 crank into the 3.9 block what other specialist parts will be needed - what if anything off other RV8 engines will also fit?

Thanks

Garry



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Re: Rover 3.9 to 4.2 RV8

Post by unstable load » Sat Mar 31, 2018 5:32 am

It LOOKS LIKE all you will need is the 4.2 conrods, according to this article.
It lists all the engines' dimensions and from the 3.9 to 4.2, all that differs is stroke length.
http://www.gomog.com/allmorgan/40difference.htm
I used to have a really nice table with all the different makes of rods and pistons etc that could be used but my PC was zapped by lightning a while back and it is all stuck in my fried HDD.... :evil:
Cheers,
John

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Re: Rover 3.9 to 4.2 RV8

Post by DEVONMAN » Sat Mar 31, 2018 11:59 am

3.9 and 4.2 rods are the same. The 4.2 pistons have a shorter pin to crown height which allows the stroke to be longer. The wrist pin on the 4.2 pistons are offset slightly just like the later 4.0/4.6 cross bolted engines.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8. (10 things to make you smile)
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 54 year of driving. Same Car, 5 engine upgrades !!!


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garrycol
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Re: Rover 3.9 to 4.2 RV8

Post by garrycol » Sun Apr 01, 2018 12:47 am

Thanks for that information - looks like we can get away with just new pistons - hopefully the mechanic has them as well as the crank.

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Re: Rover 3.9 to 4.2 RV8

Post by DEVONMAN » Sun Apr 01, 2018 7:34 am

TVR use 4.2 pistons in their 5.0 engines but they machine the skirts to clear the 90mm stroke crank and they put valve pockets in them.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8. (10 things to make you smile)
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 54 year of driving. Same Car, 5 engine upgrades !!!


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Re: Rover 3.9 to 4.2 RV8

Post by sidecar » Sun Apr 01, 2018 8:37 am

DEVONMAN wrote:
Sun Apr 01, 2018 7:34 am
TVR use 4.2 pistons in their 5.0 engines but they machine the skirts to clear the 90mm stroke crank and they put valve pockets in them.


The short skirts are probably what makes the pistons slap in these engines! The conrod ratio won't be too clever either.

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Re: Rover 3.9 to 4.2 RV8

Post by DEVONMAN » Sun Apr 01, 2018 8:44 am

Yes indeed. I have forged sloppy ones in my 5.0 and they slap like crazy till they expand with heat.
1950 A40 Devon Hotrod with 5.0 twin turbo RV8. (10 things to make you smile)
EDIS8 wasted spark, Holley Injection.
Been as far as the Moon and back in 54 year of driving. Same Car, 5 engine upgrades !!!


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garrycol
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Re: Rover 3.9 to 4.2 RV8

Post by garrycol » Wed Apr 11, 2018 11:04 am

Just to come back to this thread and the outcome of the engine pull down.

Apparently a conrod around 4/5 cylinder broke in half - not completely sure but seems not at one of the ends but about mid length (I guess a nick in an earlier life from which a fatigue crack formed and finally gave away - not under any particular load just cruising down the road at about 2500 rpm.

When the conrod failed the bit that was still connected to the crankshaft gouged out deep cuts to the inside of the block and while these could be welded up it is not worth it so the block will be scrapped. On visual inspection the crankshaft seems OK but the camshaft is broken in three spots - valve gear seems ok but will require close inspection.

There is no point in going the 4.2 route so the mechanic is going to top hat a dead 4.6 so the new build will be around a top hatted 4.6 with low comp pistons (needs to be able to run on low octane fuel). the old 3.9 tweeked 4CUIX injection system, a remapped ECU and the 3.9 timing cover etc off the old engine so slot in with what is already in the vehicle. The old stuff is V belt so I assume some sort of spacer is needed with the 4.6 crank to make it work. He does not want to go serpentine belt at this stage.

So there - a broken conrod - while I had expected it to be a conrod, I thought that maybe big end bolts had come loose and the cap came off, was not expecting a crack the conrod shank. Also expected damage to the crankshaft but the camshaft being broken into 3 was surprising. When being told, I corrected the guy and said that he meant the crankshaft had broken at three spots but I was corrected - definitely the camshaft.

Garry

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Re: Rover 3.9 to 4.2 RV8

Post by ChrisJC » Wed Apr 11, 2018 8:35 pm

Interesting. Just down to a dodgy conrod you think? No evidence of the big end seizing up?

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garrycol
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Re: Rover 3.9 to 4.2 RV8

Post by garrycol » Thu Apr 12, 2018 2:00 am

No dodgy bigend - crack is across the shank of the conrod - maybe in an earlier rebuild the rod was handled roughly and hit against another bit of metal from which a fatigue crack developed over time. We will never know as it is not cost effective to get stuff tested and is easier to just move forward with a new short engine.

Garry

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Re: Rover 3.9 to 4.2 RV8

Post by ged » Thu Apr 12, 2018 7:36 am

garrycol wrote:
Wed Apr 11, 2018 11:04 am
Also expected damage to the crankshaft but the camshaft being broken into 3 was surprising. When being told, I corrected the guy and said that he meant the crankshaft had broken at three spots but I was corrected - definitely the camshaft.

Garry
I missed a gear & over revved a side valve V8 Ford once which threw a rod & also broke the camshaft in three places.

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