Thread: 4.2l stroker crank into 3.9l?

If it is a factory 4.2 crank then it will fit, the LSE RR's used the 3.9 block with a stroked crank to give 4.2 ltrs. Even if its an aftermarket it should fit since all rover V8's used the same size main journals i believe.

I am thinking of doing just this so if u are successful can u post please.
I have several standard blocks and a 4.2 crank coming and I also have a Buick 300 crank and pistons as well . The pistons on this are ford 170hp mounted on rover rods and the standard bore needs to be 10 thou bigger, BUT and a big BUT the rear main oil seal is different, however there is a mod kit availible from America. I will maybe one day get it all done or I may just sell it all as I dont have a lot of time to stuff about with this unfortunatley.
Brad

my first question is,, will the crank miss the block??
14mm is a fair wack---

Good question, I don't have the block the 4.2 crank came out of at the moment, so I'm not sure if it's been machined inside.

Was talking to a bloke today who reckoned the 4.2 crank would fit into the 3.9 block fine, but I'd have to buy new pistons as the stroker kit pistons have the gudgeon pin closer to the crown. And another good point he made was that crankshaft would have been balanced to be used with those pistons, and using heavier 3.9 pistons would unbalance it, meaning metal would have to be added to the crank.

I've been told that liners from a 4.2l Jag can be used in the 3.5l block, and they are better as they are flanged at the top, so don't drop into the block like the standard ones sometimes do. Though I'm not sure if that is correct - if that engine has an 88.9mm bore. If it is, Jagspares has these liners for about $40 each, which is heap better than the $800 I was quoted for standard 3.5 liners.

Curious about this one too. When it comes time to re-condition the 3.9 I was thinking about a 4.2 stroker crank to help the beast along a bit without doing a complete transplant.

Keep us informed about your progress.

I think you're right PLR - only the new Jag S-types with the 3.5" bore, so liners are out of the question now.

Been chatting to my Uncle who builds racecar engines and he recently built a 4.9l stroker from a 3.9 (for and open-wheeler!). He also said it shouldn't be a problem fitting the stroker crank into the 3.9 block clearance wise. He said the basically the same as the other bloke in that the only problem will be where the gudgeon pin sits in the piston - ie. if the stroker pistons are the same as stock and the conrods are shorter, all is good, but if the pin height is different I'll need new pistons.

So my options are:
- 4.2l bored another 10thou with new +0.40 stroker pistons
- 3.9l stroked to 4.7l using stock pistons / stroker conrods (depending on length)
- 3.9l stroked to 4.7l bored with new 94mm stroker pistons

I'm starting to lose interest, so whilst a 4.7l would be very nice, I'm just gonna go with the simplest and cheapest option so I can get the car on the road.

Err, I ment +.040 size pistons

Mate, the more info I can get my hands on, the better - so bring on the long posts! I'm no mechanic so a lot of it's new to me - have only done engine transplants before, no rebuilds. Definitely some useful information, so thanks.

Funny you should mention getting another 3.5 block and using that - as last night that's what I decided to do. I spoke to my Dad and he's got a few good condition 3.5 blocks lying around the place from about '86 vintage. I dunno why I didn't think of it sooner, as using one of those blocks with my existing gear will be far cheaper than any other method. The current block I have needs new cam bearings (the reason I pulled it apart in the first place), so I might also get away without having to do that using a newer block.

Looks like a bore / hone is all I'll be up for, so I'm stoked.
I recently spent a year overseas and am currently paying back my debts, hence me being a cheapskate with the rebuild.

At present my costs are:

- bore / hone
- reco'd heads
- reground cam & lifters
- rings
- big end bearings
- main bearings
- timing kit
- oil pump

All of this for a bit over $1000, so I can't complain. Think the block from my Dad will cost me a slab. Was thinking a crank regrind might be worth it too? Anything else worth getting done?

I'm gonna run the twin carb setup with gas rings atop an Edelbrock Performer manifold to start with and see how she goes, Genie extractors too. I don't see the point in buying a Holley/Weber if it's going to run on gas 90% of the time.

PLR - I'll measure up the gear for you on the weekend, can take some comparitive photos with stock gear too if you want.