Stroking a 3.5

chodjinn · Post by **chodjinn** » Wed Sep 24, 2008 3:33 pm

so the late spec 3.5 blocks are bqasically a 4.0/4.6 with 3.5 bores??

Wotland · Post by **Wotland** » Wed Sep 24, 2008 4:09 pm

This blocks continue to use small main journal like all interim blocks. Like last interim blocks they have large main caps (only the two vertical bolts drilled and tapped but possibility to be X-Bolted) and blocks are drilled and tapped for 10 bolt cylinder head fixing.

ChrisJC · Post by **ChrisJC** » Wed Sep 24, 2008 4:24 pm

Those late spec 3.5 blocks are the only block that can be built without any design flaws!!

As Wotland says, you can get it machined to take the large journal crank & have cross bolted main caps.

And because it has the smaller 3.5" bore, it doesn't suffer from the liner issues of the 3.7" (i.e. 4.0l and 4.6l) engines.

I've had my eye open for years, but never seen one at the right price.

Would make the perfect basis for a high performance blown motor.

Chris.

chodjinn · Post by **chodjinn** » Thu Sep 25, 2008 7:53 am

Well . . . I already have my eye on one of those! Already got a sump girdle, grp a rods, forged pistons, stage 3 heads, grp a rockers. Thinking of going for a mechanical cam/solid lifters as well, so just need to spec a cam and figure out which crank to use! Still tempted with a tvr 5ltire one tho ;/p

SC500 · Post by **SC500** » Thu Sep 25, 2008 12:11 pm

I may be able to help you out with a 5ltr bottom end but am not sure at the mo what our plans are for it.

chodjinn · Post by **chodjinn** » Mon Nov 17, 2008 7:34 pm

Anyone know the capacity of a 4.2 crank in a 3.5 block?

Wotland · Post by **Wotland** » Mon Nov 17, 2008 7:51 pm

Stroke 77mm and bore 88.9mm = 3823.6cc

chodjinn · Post by **chodjinn** » Mon Nov 17, 2008 9:11 pm

A useful bit more i guess. Any idea on comp ratio with 19cc pistons, standard rods and 36cc heads w/ comp gaskets? ta

CastleMGBV8 · Post by **CastleMGBV8** » Wed Nov 19, 2008 8:04 am

Olly,

If the rod piston combo you have is for a 3.5L crank then with the 4.2L crank the pistons will be above the top of the bores at TDC!

What length rods and what compression height are the pistons?

If you want a 4.2 crank John Eales has a few, I got one for £150 but the first one was unusable, but he swapped it for a good one without an argument, if you decide to go for one make sure you ask him to check it before he sends it, I think most of them are 10 thou undersize and Tuftrided which is fine.

I also find that just before Christmas is a good time to buy stuff cheap because the the trade is quiet and most of the trade guys welcome a bit of cashflow also e-bay on car parts is quiet and I bought parts with no one bidding against me

Kevin

chodjinn · Post by **chodjinn** » Wed Nov 19, 2008 10:08 am

What?!?

Right I'm proper struggling with this stroker crank thing. Is there anything I can use with my pistons and rods?? Pistons are 8:1 comp ratior IIRC and standard length 3.5 Grp A rods.

The way i figured, the only difference between a 3.5 and 3.9 is the bore/piston size. Therefore a 4.2 crank in a 3.9 can be used no worries in a 3.5 bore block, just providing 0.4l less capacity (ish)????? Same theory for the 5litre TVR but apparently not?? Meh. I hate this.

Looking increasing like I should just stick with a 3.5 because nothing I've looked at so far is suitable! Plus I have no idea how the rod/height etc works either, so if someone could explain that would be very helpful.

Wotland · Post by **Wotland** » Wed Nov 19, 2008 10:55 am

Olly,

as I said twice in previous posts you can use your rods and pistons with an 4.2 crank.... only if you machine the top of pistons.

Of course if you machine top of pistons you will decrease the dish capacity of your piston and increase the CR.

chodjinn · Post by **chodjinn** » Wed Nov 19, 2008 11:09 am

well i must have missed that, but still no one has explained why. I can't get my head round it, is it to do with piston height? This is a pain in the arse now, so much for a simple build lol. Since I can't get a straight answer out of anyone i'm not going to bother.

CastleMGBV8 · Post by **CastleMGBV8** » Wed Nov 19, 2008 12:35 pm

Olly,

The deck height of a rover block is, centre of crank to top of cylinders is stated to be 8.96" within that dimension you have to fit the throw of the crankshaft which is half the stroke + the con rod length and the compression height of the piston, which is centre of gudgeon pin to top of piston.

Example, 3.5 standard engine 2.8" stroke x 1/2 = 1.4" + con rod 5.66" + compression height of piston at 1.88" Total = 8.94" which leaves the piston 20 thou down the bore which is about average.

So if you fit a crank with a longer stroke you have to compensate with either a different shorter rod length or piston with with a reduced compression height or a combination of the two.

Hope that helps.

Kevin.

Post by **Rossco** » Wed Nov 19, 2008 12:37 pm

Sounds just like the 3.8 supercharged engine I ran

chodjinn · Post by **chodjinn** » Wed Nov 19, 2008 12:58 pm

Aha! now I get it! before i totally give up on this idea, i'm gonna measure the pistons to see how much needs to be taken off to make it work with a 4.2 crank. Also gotta consider the increase in CR as well as per Dimitri's comments.

Rossco, you have PM