I understand that K heads are interchangeable between K blocks...

 

Is there any issue with heads that are designed for blocks with a smaller bore going onto bigger-bore blocks? example: 3K or 4K head on 5K. Is there more to it than just bolting it on with a new gasket?

 

:harhar:

all engines (k/3k/4k) have the same bore size (75mm) apart from a 5k and 7k(both 80.5mm), which have dished pistons.

 

if you bolt a k/3k/4k head on a 5k (7k has extra headbolts and isn't interchangable) you will lose compression.

ok i knew all of that except you gain compression, thats the idea

ok i knew all of that except you gain compression, thats the idea

 

huh?

 

skimming your existing head gives more compression. putting an earlier head on a bigger bored 5k will lower the compression ratio. ok for forced induction.

ok now I'm confused, i was referring to heads like a 3K bigport, which gives 10 to 1 compression?

ok, let me get this right.

 

you are talking about putting an early 3k head on a 4k? if so, yes it will increase the compression. both have the same bore size..... stroke is different.

no no i'm talking about a 5K block :harhar:

well a 5k has about a 16cc dish in its pistons in standard form. you will drop the compression ratio to about 8.0:1 with an early 3k head.

 

attached is a pic of a 5k piston.

both have the same bore size..... stroke is different.

 

Ok. So you are saying that stroke made up the 100cc difference between the 3k and 4k. So, if i fit a 4k crank into my 3k block, I will then have a 1300?

If so, if I fit a 5k crank into a 3k block, will I get the same result (1500cc)?

 

So there was no changes in bore sizes, only stroke and compression?

 

Sorry to move in on your topic rjenman

3k = 75mm bore x 66mm stroke

4k = 75mm bore x 73mm stroke

5k = 80.5mm bore x 73mm stroke

 

 

if you put a 4k crank in a 3k and changed nothing else, the pistons would hit the head.

 

a 4k and a 5k crank are identical. in both the 5k's i have had they have "4k" cast on the crank in raised lettering.

7k block with earlier head apparently does work, you just use an earlier head gasket with a sealant (hylomar, permatex, silicon, etc) over the pushrod side and there's enough there to make it not leak. at least that's what i've seen on japanese starlet webpages

you'd at least need a 5k head gasket, otherwise the firing ring would overhang the bore.

ahh! the guy in question used a 5k head he had from his last engine, so that would make sense

but isn't the 4k block about 3/4" taller to make up for the stroke?

if I recall somewhere I read an article mention how to increase a 4K engine compression ratio to 10.1:1

 

the author use a 3K head + 4K block and porting plus polish the head :D