3.9 to 4.2 conversion

jamie · Post by **jamie** » Thu Apr 03, 2008 9:18 pm

I'm wanting to upgrade my 3.9 to a 4.2. Am i right in thinking that to do this i need to swap the crank for a 4.2 crank and skim the pistons? If this is the case can anyone help me locate a 4.2 crank and some (3.9) 8.13:1 pistons for skimming? Also can anyone advise me on how much to skim off the top and bottom of the pistons?

Thanks in advance for any guidance.

CastleMGBV8 · Post by **CastleMGBV8** » Thu Apr 03, 2008 10:16 pm

Jamie,

I am just in the middle of doing something similar although mines a little more complicated because I'm using Chevy rods and pistons to get 4.35L and a pair of Buick 300 heads.

You can get a crank from John Eales JE Developments, he has a few 4.2 cranks reground +.010 and tuftrided for £130 + VAT but do make sure you get a good one, the first one he sent me was not good, but he replaced it and was full of apologies.

Well thats the easy bit, you can't use the 3.9 pistons because they have a compression height of 49.5mm the 4.2 version has 44.795mm which means you would have to machine 4.705mm off the crowns and that is not feasable.

The 3.9 rods are the same length though, are there no 4.2 pistons available as that would be the simplest thing to do.

I believe the compression ratio of the 4.2 is only 8.94/1 so you could do with picking up a little more compression, 20thou off the heads would help.

You will probably need to have it rebalanced as well.

Kevin.

jamie · Post by **jamie** » Thu Apr 03, 2008 10:28 pm

Cheers for the reply kev.......i tried JE developments, their website was advertising the crank at £120 but when i called i was quoted £200 + vat plus about £ 18 delivery.....i didn't know if there was anywhere cheaper to get them from? On the pistons....i was advised that with the 8.13:1 pistons the machining wouldn't affect them too much and gave a slightly higher compression??

However if you've gone through this i'm sure your advice is good, and i thank you......i'll get on the hunt for some 4.2 pistons. Many thanks!

Jamie

HairbearTE · Post by **HairbearTE** » Thu Apr 03, 2008 10:30 pm

I have not heard of this being done before but it may be possible. I would guess that 2mm would have to be removed but the best thing to do would be to assemble such a combo then you can see exactly how much must be removed from the top of the piston and if any needed to removed from the bottom for clearance purposes. If you have to remove enough material from the piston crown to actually remove the dish completely then I personally wouldn't bother. I think that would weaken the piston too much for comfort. Of course what you're suggesting could also allow you to run a quite high CR if you use the right heads and gaskets. And of course piston to valve clearance will have to be checked. If you do need to clearance the skirts of the pistons then complete the machining to the crown first then get the digi kitchen scales out and try to keep the set as closely matched in weight as is practicle.

HairbearTE · Post by **HairbearTE** » Thu Apr 03, 2008 10:36 pm

Just read kevs post. If nearly 5mm has to come off then it's a non-starter

I too am building a 4.35 with 4.2 crank, 3.9 block, chevy style rods and buick heads! looking forward to seeing how your combo performs Kev.

jrv8 · Post by **jrv8** » Fri Apr 04, 2008 9:39 am

Hi Jamie,
I've a full & running 4.2 engine here that I'll be breaking later this month.
If you're still on the lookout for pistons , give me a call, or PM.

Regards
Jim

CastleMGBV8 · Post by **CastleMGBV8** » Fri Apr 04, 2008 6:17 pm

HairbearTE wrote:Just read kevs post. If nearly 5mm has to come off then it's a non-starter
I too am building a 4.35 with 4.2 crank, 3.9 block, chevy style rods and buick heads! looking forward to seeing how your combo performs Kev.

What pistons are you going to use because getting the compression up to abot 9.8/1 is the trickiest bit you really need a flat top but there were none available. I had to use the chevy 305 piston 3.736" 94.89mm which is a flat top but has a 5cc cut out for the chevy valves.

We have had to mill the heads 30thou and the pistons were 66thou down the bores so are milling the decks by a further 40thou and will use tin gaskets to get around 9.7/1, so a lot of work involved.

Jamie,

JE had the 4.2 cranks on special offer I know he had quite a few so try to speak to him direct they were definitely £120 + Vat + freight.

Kevin.

seight · Post by **seight** » Fri Apr 04, 2008 8:04 pm

Mine is a 4.2 that needed a rebore so after advice from Glen Towery in the US who has done loads of them I am using 3.9 low comp +.020 pistons which have .090 off the tops and 1/8 inch off the bottom with composite gaskets and 34cc heads. Should be 10:1 CR ish. Maybe a little lower due to valve pockets too.
Hopefully it will hold together if I ever get the rest of the car finished an start it up

Mike

Here are some pics:

CastleMGBV8 · Post by **CastleMGBV8** » Fri Apr 04, 2008 10:54 pm

Thats very interesting, I know of Glen Towery and he has a good reputation the curious thing is the Land Rover engine data sheet gives the compression height of the 4.2 piston as 44.795 and the 3.9 as 49.5 it doesn't specify any difference in compression height for the low compression piston its just the size of the piston bowl thats different that provides the lower compression.

If that were true then your pistons should be would be out of the top of the bore by approx 2mm depending on the original deck clearance which obviously they are not assuming the rods are on the crank. The con rod lengths for the 3.5 3.9 & 4.2 are all the same

It's possible there is a mistake for the spec of the 4.2 piston but it would be interesting to know for sure.

I'd also like to know how deep the top ring land is after machining the piston tops.

Kevin.

HairbearTE · Post by **HairbearTE** » Sat Apr 05, 2008 12:04 am

CastleMGBV8 wrote:
HairbearTE wrote:Just read kevs post. If nearly 5mm has to come off then it's a non-starter
I too am building a 4.35 with 4.2 crank, 3.9 block, chevy style rods and buick heads! looking forward to seeing how your combo performs Kev.
What pistons are you going to use because getting the compression up to abot 9.8/1 is the trickiest bit you really need a flat top but there were none available. I had to use the chevy 305 piston 3.736" 94.89mm which is a flat top but has a 5cc cut out for the chevy valves.

We have had to mill the heads 30thou and the pistons were 66thou down the bores so are milling the decks by a further 40thou and will use tin gaskets to get around 9.7/1, so a lot of work involved.

Jamie,

JE had the 4.2 cranks on special offer I know he had quite a few so try to speak to him direct they were definitely £120 + Vat + freight.

Kevin.

Kev i'm biting the bullet and going for custom pistons. I've had the heads annealed and will be welding up the chambers and cutting in new, much smaller ones myself. For this reason I won't be buying my pistons until the heads are finished - I want to match the dish to the new chamber. Having said that there may not be much of a dish in the new pistons as I have to get the CR up to a level where my cam will function. Using custom pistons also allowed me to run a longer rod. My pin will probably encroach into the bottom ring land with the projected compression height of 1.13" I have to use to squeeze in the 6.125" rods, but thats ok, at least the pistons will be nice and light.

That's the first clay model of the new chamber design, it's not very good actually as the walls of the chamber around the inlet shrouded the valve a bit and there's one or two other minor details I will improve upon too. I am of the opinion that taking a little off the head surface is going to give me a better starting point so that is what I am going to do before I get stuck into the heads again.

CastleMGBV8 · Post by **CastleMGBV8** » Sat Apr 05, 2008 5:43 pm

Hairbear,

I am looking forward to seeing how it will perform but am held up at the moment by Real Steel who have a 6 week backlog on balancing work so the engine will not be assembled until mid May

Your last post showing your intentions for the heads is very impressive, but an incredible amount of work, what do you intend using the engine for.

The Buick 300 heads as standard will flow 149cfm at .50 lift for the inlets and 115cfm for the exhausts with 1.625" In, valves and 1.312" Ex. valves.

I decided to fit new valves and have fitted the usual big valves with waisted stems for the Rover heads at 1.63" In. and 1.35" Ex. will bullet the guides and just clean up the throat area. The original seats which are quite hard will just be reground with the usual three angle valve job and that should improve the flow enough for my needs.

Re pistons and rods, unless you are going full race and need around 13/1 compression you could use the same 5.85" rods and chevy 305 Keith Black Hypereutectic pistons as I am. I got my rods from V8 Tuner but he is now out of stock but Real Steel may have a set for £250.00 the pistons are about £220.00 including rings from Summit Racing in the US.

You would only need to close up the combusyion chamber a little to get the compression where you want it, alternatively Keith Black do a piston with a raised crown that might work for you.

Kevin.

CastleMGBV8 · Post by **CastleMGBV8** » Sat Apr 05, 2008 5:45 pm

Hairbear,

I am looking forward to seeing how it will perform but am held up at the moment by Real Steel who have a 6 week backlog on balancing work so the engine will not be assembled until mid May

Your last post showing your intentions for the heads is very impressive, but an incredible amount of work, what do you intend using the engine for.

The Buick 300 heads as standard will flow 149cfm at .50 lift for the inlets and 115cfm for the exhausts with 1.625" In, valves and 1.312" Ex. valves.

I decided to fit new valves and have fitted the usual big valves with waisted stems for the Rover heads at 1.63" In. and 1.35" Ex. will bullet the guides and just clean up the throat area. The original seats which are quite hard will just be reground with the usual three angle valve job and that should improve the flow enough for my needs.

Re pistons and rods, unless you are going full race and need around 13/1 compression you could use the same 5.85" rods and chevy 305 Keith Black Hypereutectic pistons as I am. I got my rods from V8 Tuner but he is now out of stock but Real Steel may have a set for £250.00 the pistons are about £220.00 including rings from Summit Racing in the US.

You would only need to close up the combusyion chamber a little to get the compression where you want it, alternatively Keith Black do a piston with a raised crown that might work for you.

Kevin.

HairbearTE · Post by **HairbearTE** » Sat Apr 05, 2008 7:29 pm

Kev it's gonna be mainly for racing, as long I can get to and from the strip with occasional ace cafe action then I shall be happy. 13.1 is in the ballpark for my compression and even with a lot of overlap its likely to need 50/50 premium/race gas. It's by no means a daily driver engine. Then only parts I haven't got yet are the bits I need custom made which are basically the pistons and the pushrods, and I can't get either of those done until I finish the heads. The pistons are expensive and i've got many other other things on the go at the mo so i've not put a date on completion of the car or this engine.