4.6L crank in a 3.5L?

[Kassi]

Hi,

for increasing the stroke of my 3.5L it should be possible to fit a 4.6L crank, which has a stroke of 82 versus 71 mm. But has larger diameter main bearings. Has anybody fit this crank in enlarging main bearings in the block. Recondition companies should have the machining equipment for doing this.
Do I need at any rate a different timing cover in addition?

Best regards
Christof Kass

[russell_ram]

Christoff,

Yes it's possible but not easy.

Assuming you are going to use 4.0 or 4.6 rods to match the crank BE pins (which are much larger than 3.5), you will find that the B/Ends will foul the block during crank rotation and will need some serious relieving. How bad the fouls are depends on the age of the block.

By the time you have line bored the block to take the larger main pins there won't be much main cap thickness left. The later blocks have much deeper, X bolt mains caps. You risk durability issues in the caps - even factory small bearing 4.2s are marginal and commonly fail main cap 4.

Alternatively you can get the crank mains machined down to 3.5/3.9 size. Obviously this reduces the pin overlap and reduces the strength of the crank.

You have to use factory 4.0 or 4.6 or after market rods to match the bigger BE pin size. 3.5/3.9 ones aren't big enough to open out to the larger bearing size.

Next problem is that you won't have any of the std pistons to fit. The 3.5 pistons won't fit the 4.6 rods and the 4.0/4.6 pistons won't fit the 3.5 bore without a re-sleeve. (Using a 3.9 block would fix this problem though).

EASIEST bit is the front cover - you can use any of the available ones. ie SD1 by fitting a spacer in front of the crank pulley to take out the extra crank nose length, or any of the later LR concentric pump types using a later poly V/serpentine belt system with or without distributor.

HTH

Russ

[robinj66]

I know that the 4.6 crank had a longer "nose" (bit with the woodruff key in it) than the 3.5. Means you may be restricted as regards pulleys. Also the 4.6 had cross-bolted main bearing caps - cross bolting may not be possible with earlier type main bearing caps.
Don't think you would necessarily need a different timing cover but it depends on the type of oil pump/distributor drive on the crank.

To be honest, my info is a bit limited and I would recommend you talk to the guys at www.v-8.org.uk who are a mine of information and very helpful (as of course are the guys on here) :thumb:


I see I was beaten to it by someone more knowledgable

[Kassi]

Hi Russ,

thank you for your answer. It seems there is no solution without a risk.
What's with the stroker kits. The huge increase up to 86 mm for me seems to be a risk, too.
Do you think the 3.5L block could last this load and the engine could last revs up to 6000 RPM in the same way as before?

Thank you for reply again.

Best Regards
Christof

[tonym]

If it's any use, this is the path I took. I bought a second hand 3.9 block (£50) and had it bored out +20. I bought a second hand seized 3.5 engine and used the sump, front cover, heads (for exchange), distributor, front pulley etc. I then bought a crank and associated bits from John Eales Developments (not JE Engineering :thumb who machined the crank to fit my block. That way I had a cheapish large capacity RV8 and still had my old 3.5 to sell on complete as a turnkey engine which helped offset the other costs. HTH :thumb:

[Kassi]

Hi there again,
it is, of course, a rational solution using a 3.9 block. But my car is an oldtimer and from the outside nobody( i.e. the technical inspection agency) may see, that there is a 3.9 block in my car. This is not allowed in Germany. 3.9 blocks have never been used in an SD1.

However, usage of 3.9 cylinder liners in a 3.5L engine is the next question I will ask here.

Best regards
Christof

[tonym]

But isn't a 3.9 block externally identical to a 3.5? An unscrupulous person could even take the engine number off and replace it with a SD! number, not that I'm advocating that sort of thing, you understand :thumb:

[Kassi]

I would never do such act as you would't, of course.
Possibly a friend of mine could do it, without my knowledge, naturally.

:thumb:

[robinj66]

Apparantly it is not advisable to bore out the 3.5 block to allow fitment of 3.9 liners - leaves too little wall thickness in the block . Therefore Tony's suggestion is probably the best. You can also use a stroker kit to get 4.2 using the 3.9 block (I think)

[TINKA]

Sorry to disagree but it is far better in my opinion to get a 3.5 block and bore and reline than to get a 3.9 block. As far as I am aware they are based on the same casting and due to some fuelling issues some 3.9 are prone to having cracks or loose liners due to over heating. I had a 3.5 block relined with 3.9 liners and it ran great.