Crankshaft compatability? 2T crank 3T block

[Radar]

I was curious If I could use a 2T crank in the 3T block? (long rod motor)

[w810sc]

Yes you can, but if you think the 3T has a taller deck height, you better think again. They both use the same length connecting rods and the 3T pistons only compensate for the 78mm stroke crank. Use the 2T block if you already have it.

[Radar]

Much appreciated

[jfallen]

stating the obvious, why not use a 3T crank in a 3T block get an extra 8mm stroke (unless this is for a 1600CC racing class?)

Cheers
Jordan

[white_sandshoe]

Going on from this question, (hope it's not seen as hijacking... i was wondering the same thing the other day and would love some clarification on this...)

Would putting the 2T crank into the 3T block, and using the 3T pistons, it'd pretty much kill the compression ratio right??? same with putting the 2T pistons with the 3T crank???

If anyone knows, how much would it drop compression ratio to???

[Radar]

Its not for compression, its for the Rod/Stroke ratio..

Bored out to 88mm using the 2T crank with 5.400" rods, you get a 1.7L with a R/S ratio of 1.96 (High) which from what I've been reading makes for one hell of a Revvable motor..

But you cant use the 3T pistons with the 2T crank, I'm guessing this is because of piston pin offset? I'm also under the impression now that they don't make forged pistons to suit the 2TC anymore, well not for the size or pin offset you would need for the long rod motor.

Using the 3TC crank with with the longer size rods (5.400") and 88mm pistons gets you a 1.9L with a healthy R/S ratio of 1.75 so that's still more than acceptable so I'll just stick to that plan..

**This is all theory**

[styler]

with t series blocks they are identical for crank/piston combination purposes,

differences are narrow/wide bearings, some early blocks need grinding for 3t crank,
twincam models have oil galleries blocked off, turbo model has oil return, and maybe some other minor differences?

blocks take crank + matching pistons as a set as follows:

2t crank + 2t pistons

3t crank + 3t pistons

these are the only combinations possible as:

2t runs a shorter stroke crank with longer pin height pistons
3t runs a longer stroke crank with reduced pin height pistons

so you need the crank and pistons as a set cant interchange them.

there are various 2t and 3t pistons with different dome designs and volumes...

measure your own actual head volume as it could have been decked flat and its the only
reliable method of compression calculations

do your own compression calculations based on your figures of your actual parts,
overboring, valve reliefs, decking head or block flat, different motor pistons, dome volumes,
headgasket thickness and deck height all can change compression in a second!

[styler]

2t crank is 70mm stroke and 4 counterweights,
with matching std pistons = approx 1600cc
(1588cc)

3t crank is 78mm stroke and 8 counterweights
with matching std pistons = approx 1800cc
(1770cc)

[Radar]

I understand about the size of the motor and the crankshaft its the basics.. Im asking why you cant use the longer rod with the 2T crank and a Custom set of pistons with a reduced pin height so it doesn't go shooting out the block. ( This is all assuming the: dish, size, pin offsets of the pistons are correct and the headgasket and stuff are correct)

[styler]

Its not for compression, its for the Rod/Stroke ratio..

Bored out to 88mm using the 2T crank with 5.400" rods, you get a 1.7L with a R/S ratio of 1.96 (High) which from what I've been reading makes for one hell of a Revvable motor..

But you cant use the 3T pistons with the 2T crank, I'm guessing this is because of piston pin offset? I'm also under the impression now that they don't make forged pistons to suit the 2TC anymore, well not for the size or pin offset you would need for the long rod motor.

Using the 3TC crank with with the longer size rods (5.400") and 88mm pistons gets you a 1.9L with a healthy R/S ratio of 1.75 so that's still more than acceptable so I'll just stick to that plan..

**This is all theory**

theory indeed... wow

bored out to 88mm is fine

2t crank is fine

this will need custom pistons as there are no oversize 2t pistons

5.4" rods = custom rods

5.4" rods are 14mm longer than std so wheres that 14mm gonna go?

R/S ratio do you know why you want this? do you want longer tdc time?

i believe 3t piston skirts would collide with the 2t crank on the piston downstroke
if memory serves me correct due to the lower pin height on the piston.

piston pin offset is on the horizontal plane and is a factory feature on some motors,
piston pin height is on the vertical plane and is mandantory obviously.

forged 2tc pistons were ultra rare is ever made, forged 2tg used to be common in
oversize, now forged 3tc is super common, custom orders for pistons are also avaliable.

14mm of extra rod! thats a lot of pin height change, go check out a piston to see where
it might go? 3tc pistons at 4mm extra throw from the crank put the pin housing right up
against the bottom of the ringlands... as far as i know i havent seen a pin run through
ringlands before but hey...

[Radar]

I understand about the R/S ratio to a certain degree but I'm limited to what I have read.
I assume its better to have a much higher rod/stroke ratio when it comes to a more hard tuned engine, the sidewall loading and crank angle is reduced so a lot of friction is taken away allowing more RPM.
The longer rods may weigh more, but the reduced weight of 2T crank compared to the 3T crank should make up for that..
The piston stays longer at the top cylinder the longer you make the rod and higher you push the pin into the piston, allowing it to get all the grunt of the explosion.
The trade off might be poor VE but with forced induction you shouldn't have to worry as much.
As for the extra 10mm higher pin height needed, BRD offer a off the shelf set of longer rods and custom pistons to suit the 3TC.

**example** *pin height is into the oil ring I think*
AGAIN all theory

[styler]

okay i kind of see what you are trying to do and it might work but it would
be very expensive and very much to the extreme of compatible dimensions...

you want these long 5.4" rods:

std rods = 4.850"

so these new rods are approx 14mm longer...

so you need 14mm to shove somewhere in the crank or piston and still
have everything clear and rotate and not pop out the block...

2t crank + custom 3t pistons with a short skirt to clear 2t crank = 4mm

so that leaves 10mm to shove up the pin upwards into piston and i really dont see
it happening looking at a 3t piston right in front of me...

theres a 1.5mm gap between OD of pin and bottom of ringlands...

[styler]

Also the piston stays longer at the top cylinder the higher you push the pin into the piston

yeah thats about right except for this, its the longer the rod is that actually causes this, the rod sometimes has to be used with said piston to be compatible dimensionally, but its not the pin height that causes longer dwell at tdc. look at long rod examples with crank rotation in degrees to get a better picture of tdc period changes.

[Radar]

3T crank 78mm : 2T crank 70mm
rod: 4.850" std

According to BRD I can get 5.400" rods 14mm longer to suit the 3T crank(modified) and pistons.
So I guessed seeing as the 2T crank has 8mm less throw, with the 14mm longer rods I could push the pin height of the 3T piston up 10mm anyways and get away with a much higher rod stroke ratio..

[Radar]

yeah thats about right except for this, its the longer the rod is that actually causes this, the rod sometimes has to be used with said piston to be compatible dimensionally, but its not the pin height that causes longer dwell at tdc. look at long rod examples with crank rotation in degrees to get a better picture of tdc period changes.

Sorry I should of been more specific I understand the rod is what changes it, but the pin height allows the rod change..