4K Bores

[Mitch-12]

hey can you bore a 4k to take 5k pistons or do the bores get superthin i was thinking maybe the 4k is a 5k with really thick bores so maybe you can go out to 5k size pistons

[Figjam14]

You would have to bore it around 200thou. so ill guess the walls would be to thin.

But could be wrong :hmm:

[filfrederick]

You would have to bore it around 200thou. so ill guess the walls would be to thin.

But could be wrong :hmm:

 

i bored out my 4k to 81mm but i sonic tested it and had to reposition the bores but i still have about 120thou wall thickness.

[Mitch-12]

hmm ill have to go 5k or have little pistons

[KENut]

4K out of a forklift have thicker bores.

 

Or just sonic test blocks till you find a usable one :P

[Mitch-12]

it's got boost so i need thick i just thought maybe 5k was a 4k block just bored but it seems they have thicker walls

[altezzaclub]

Does that mean they have smaller waterjackets??

 

I assume the bore centre spacing is exactly the same, the outside dimensions of the block are the same, so effectively the 5K is an 80mm 4K.

 

They would have to make smaller waterjackets to have thicker walls. It seems a nice idea to me, any time you want to rebore a 4K you make the most of it with 80mm bore and get a 1500cc.

 

The flat-top pistons exist, and I saw that someone used Mazda 626 80mm pistons.

 

filfre, how far did you move the bores? Across or along the block?? What pistons were 81mm??

 

 

Ah well now.. I see!

4k block+4age head

bored out the block by 6mm, had to move the bores around a bit to get them in the middle of the casting, still about 3mm wall thickness.

 

Edited June 10, 2011 by altezzaclub

[altezzaclub]

Actually, the 5K shallow-dish pistons make this idea even more attractive.

 

The stock 4K has a compression ratio of 9.4 to 1, so if we bore the 75mm out to 80mm we will compress more air/fuel into the head thusly-

 

cyl vol(367cc)+head vol(31cc)+gasket vol(7.5cc)+piston dish(6cc) into head+gasket+dish

 

So compression ends up as 411.5 to 44.5 or 9.2 to 1

 

Which sounds pretty good- A set of shallow dish 5K pistons in a 4K give 1500cc, great torque and a grunty road compression to suit a cam and carbs.

 

Can someone please measure the shallow dish piston carefully and post up the result with a photo of the piston??

Edited November 28, 2011 by altezzaclub

[Mitch-12]

THE BORES END UP TO FIN IN A 4K BORED THAT FAR

[ninja-philbo]

it doesnt make sence since the 5k block is identical to the 4k block and someone has done it so it can be done

[love ke70]

mitch is right from everything ive read on this forum over the past 5 years.

unless you get the odd 4k with thicker bores

might get away with it na, but no good for turbo

[Mitch-12]

yes its possible but 4k clock has thinner bores and when you bore it 5mm you end up with super thin bores so anything with good cyl pressure blows the bores out you can do it but i will beat you in a race with 4k pistons and other work done cause it will have mega blow by

[altezzaclub]

Well, if someone can measure a shallow-dish 5K piston at least I could see if it is easy to do.

 

Really, very few guys turbo their 4K, nearly all of us are running NA, and if you're going to have to rebore your 4K why stop at 20thou.

 

So, where do we go to find out if 5Ks have thicker walls?? Who woud have measured that sort of thing?? Would anyone know if they changed the moulds when they cast them??

[SLO-030]

Would anyone know if they changed the moulds when they cast them??

 

The 5K blocks are different in a few ways, so i don't see why they would have kept the outside of the bore the same when the inner is bigger by around 2mm?

[Evan G]

how about resleeving the 4k block with thicker or 5k sleeves?