4.2 crankshaft in a 3.5 rover

Paul V8 · Post by **Paul V8** » Thu Oct 25, 2007 4:18 pm

Hello, strange question i have a 3.5 v8 with a cam, heads the usual. I think a 4.2 is a 3.9 with a different crank, so would a 4.2 crank fit in my 3.5? what size would it make and what would it do to the driveability and power output. Is it worth the trouble?
Thanks.

chodjinn · Post by **chodjinn** » Thu Oct 25, 2007 4:24 pm

I think you'd need to get 3.9 liners put in first as the bores are smaller on a 3.5 (my current engine and the other that's in the machine shop are both 3.5s relinered to 3.9). Other than that no idea, although the gains vs. expenditure are probably not worth it, spend your money on other bits TBH.

ChrisJC · Post by **ChrisJC** » Thu Oct 25, 2007 7:09 pm

I have a feeling that the longer stroke can give clearance issues (that can be addressed by removing some metal).

But assuming you can get pistons to fit, then I think it will be OK.

Should give a useful increase in torque, but not do anything for top end.

Chris.

kiwicar · Post by **kiwicar** » Thu Oct 25, 2007 8:17 pm

Should be about 3.77 litres I am not sure about the main bearing sizes but if they are the same then no problem, try it. The con rod length or piston pin height will have to be sorted, or the pistons will pop out the top. If you need to clearance the crank in the block then I think the rover has quite alot of space so should be Ok (you need at least 40 thou, 60 is ideal)
Mike

ChrisJC · Post by **ChrisJC** » Thu Oct 25, 2007 8:44 pm

The bearing sizes will be OK. The 3.5,3.9 and 4.2 litre Range Rover engines were all the same sizes. It's the 4.0 and 4.6 litre crossbolt engines that have the larger journals.

Chris

kiwicar · Post by **kiwicar** » Fri Oct 26, 2007 6:13 am

Well shove it in and start hunting through the piston catalogs

. It would be better to change piston crown height rather than using shorter con rods. You might find one of the low comp 3.5 pistons might be Ok but I think the rover relies on deep dish not lower piston crown height to set compression. I would definatly make up a bottom end brace though.
Best of luck
Mike

kiwicar · Post by **kiwicar** » Fri Oct 26, 2007 6:25 am

Just found this http://www.seight.com/engine.html the power figures quoted are a bit up with the fairies but the crank throw numbers are usefull. the extra 6mm stroke means only an extra 3mm for the piston to come up the bore I don't remember how far the piston normally sits down the bore on a 3.5 but must be worth investigating.
Mike

ChrisJC · Post by **ChrisJC** » Fri Oct 26, 2007 7:03 am

The piston is about flush with the top of the bore at TDC for all comp ratios. Yes Kikicar is right, it's the bowl size that sets comp ratio.

I would also stick with 4.2L conrods. Then, as already stated, just find some pistons that suit..........

Chris.

Wotland · Post by **Wotland** » Fri Oct 26, 2007 3:51 pm

Look on old forum.
Rossco member had one 3.5 with 4.2 crank

minorv8 · Post by **minorv8** » Sat Oct 27, 2007 6:56 am

The piston is about flush with the top of the bore at TDC for all comp ratios.

Wish it would have been so, might have saved some cash. I had to machine a 4,6 block to get the factory pistons closer to the deck to achieve the advertised factory CR. Ditto with my other engine (3,5), the pistons are down the bore about 1 mm.

If you want to fit it, check that crank has space to rotate, then check that crank/rod combintation fits. If that checks out then fit the std pistons to see how much they stick out of the block. Only a dry (or dummy) build will tell you how things actually work out.