The V8 Owners Forum -

Hi,

I recently brought a 3.9 engine, mainly because it has the intermediate front cover, but i'm also thinking of using it as a base for rebuild.

I have taken it appart and the block is currently been pressure tested, so dependant on the result i trying to find out what to spend my money on. ideally i would prefer to rebuild a bigger capacity motor because the engine will be for a 4x4 racer and low down torque is where its at.

i know that the block will need a re-bore, crank grind and new bearings all round, therefore i'll need new pistons which has got me thinking. I also have a 4.2 engine that has knackered pistons so i was thinking of using the crank and rods in the 3.9 block. i hear you can't get oversized 4.2 pistons, so what piston/rod combination can i use with the 4.2 crank to get an engine that will produce good torque for offroad competition??

Thanks for any help

Rich

I have heard that the 4.2 crank has a nasty habit of snapping. I believe this is why the 4.0 / 4.6 engines went to a larger journal size.

Chris.

ChrisJC wrote:I have heard that the 4.2 crank has a nasty habit of snapping. I believe this is why the 4.0 / 4.6 engines went to a larger journal size.

Chris.

interesting, i wondered why they went larger journal

Cranks usually break after developing fatigue cracks. Said cracks result from application of torsional forces. The 4.2 has longer stroke, consequently the momentum at the same RPM is higher. To put it in layman's terms, in a stroker motor stronger force is trying to twist your crankshaft.

If you want to prevent crankshaft from breaking, you need to take every effort to minimize torsional vibration. Two areas need attention: balancing the rotating assembly and making sure that the main caps are not loose. While balancing can be done by any competent shop, the 2 bolt main caps are the limiting factor for 3.9/4.2 block. Using high quality studs instead of bolts to fasten main caps mitigates the problem to some extent. The next step would be a main cap girdle. Once you reduce the torsional vibration, even the small journal crank is strong enough.

In my opinion the 4.0/4.6 engines don't have crankshaft breakage problems not due to larger diameter crank, but due to 4 bolt main caps. Since 3.9 and 4.2 heads bolt right on, buying a 4.6 block may be a more practical solution. Stay away from 4.0 blocks as they are lower grade blocks with thinner walls between cylinders. The 4.0s have liners fall out very often.

Tomcat27 wrote:Hi,

I recently brought a 3.9 engine, mainly because it has the intermediate front cover, but i'm also thinking of using it as a base for rebuild.

I have taken it appart and the block is currently been pressure tested, so dependant on the result i trying to find out what to spend my money on. ideally i would prefer to rebuild a bigger capacity motor because the engine will be for a 4x4 racer and low down torque is where its at.

i know that the block will need a re-bore, crank grind and new bearings all round, therefore i'll need new pistons which has got me thinking. I also have a 4.2 engine that has knackered pistons so i was thinking of using the crank and rods in the 3.9 block. i hear you can't get oversized 4.2 pistons, so what piston/rod combination can i use with the 4.2 crank to get an engine that will produce good torque for offroad competition??

Thanks for any help

Rich

Rich, to answer you original question, the 3.9 and 4.2 blocks are identical and only the stroke of the crank increases the capacity.

I have just built an engine using a 3.9 block with a 77mm crank (4.2) together with chevy 5,85" rods and Keith Black Hypereutectic pistons to give a capacity of 4.35L so it is possible, but its not cheap especially with the current exchange rate.

It might be more cost effective to find a good 4.6 bottom end to rebuild as this will give more torque and with a mild cam and stage 1 heads around 260 BHP is possible.

If you still wan't to consider the 4.2 based engine I'll be happy to advise on whats needed.

Kevin.