240 to 260? pistons and boring

[clutchdust]

any issues i should be aware of when taking the factory block out to 2.6L? '73 block, all original to my knowledge. i want to take it out to 2.6 just to clean it up and a slight bump in compression and displacement. could this engine be taken out any further? if so, what's the max? could the block handle 85.0mm? has anyone ever tried the L18 pistons in a bored 2.4L block?

a company i buy alot from for my work also sells speed pro so i can get a good deal on a set. my speed pro catalog lists one set of pistons for the L6. this is the information they show:

L6: 2565cc 2.6L

83.0mm (3.2677) <---no idea what this spec means

1972-75 car (non-USA)

recessed head 1.20mm deep x 71.5mm dia. 8.3:1 C.R.

(available sizes) standard, .50, .75. 1.00, 1.50mm

rings: 2-2.00mm, 1-4.00mm <---don't know what this means

comp. dist: 38.2mm <--- ?

pin diameter: 21.0mm

there are a couple specs there i don't know what they mean. if you have any ideas, let me know.

i think i want a bit more compression than that, though. i was thinking with an aluminum head, decent ignition and headers, i should have no problems with 9 to 9.5:1CR. these are also cast, and i wanted to get them in hyper. i'll see if that's available but i want your input first so i only have to ask questions once. one question is, do these engines only utilize two rings?

the other question above was about the L18 engine. I show a nearly identical piston (only difference in the specs being the dish is 1.3mm instead of 1.2mm) but it's an 85.0mm piston. it's also available in hyper with moly rings whereas the 2.6 L6 is cast with iron rings. maybe i'll do some experimenting with this unless anyone has tried this before.

[JMT240KGTR]

There are many options.

- bore the cylinders out 3mm = 2.6L or

- retain standard bore, fit in 260/280 rods and crank, = 2.6L or

- bore cylinders 3mm AND fit 260/280 rods and crank = 2.8L

To increase compression, factor in the head cylinder volume, if it's large capacity fit flat top pistons, if not, use dished (if thats what you already have) and shave the head .030" (max recommended before other items require mods). Also ensure the head has large enough valves to enable the motor to breathe; high comp, no breath, bad news! You want to get the hot gas out!

Remember that L24/26/28 used dished & flat top pistons, so your combinations will have to factor these in.

If you want to stay the L24 route, then I'd bore the block 3mm and stick with the L24 rods, as opposed to the standard 2.6 configuration of longer stroke with L24 bore; advantage is retaining quicker revving engine with slight increase in displacement as opposed to losing rev's to the 2.6/2.8 crank.

There is a limit to these basic mods though and you won't compare to a fully worked up L28, but you will gain significant power. Build it with high comp, free flowing head, extractors, twin carbs, elec dizzy, suitable cam and you will have a quick car; the L28 is good, but the old L24 hasn't expired either.

James

[JMT240KGTR]

Oops, the pistons use three rings, top compression ring and two oil rings.

[clutchdust]

thanks. that's the kind of input i'm looking for.

give you a little backround on what i have in mind, i just want to bump the power a bit without a great deal of $$$. later on in life, the little L6 will give way to a 4.3 chevy v-6/t-5, so i don't want to inject a great deal of cash in this engine.

what about milling the block? is there enough meat on the deck for any noticeable bump? what about a thinner head gasket? if i mill the deck or head or both, what will that require about the chain? different chain or some type of tensioner mod?

i like the idea of a high-winding engine so a little longer stroke (ala 2.6) is not really what i wanted to do. not to mention, like i said above, i should be able to get a good price on pistons, but not so much the rest of the go-fast goodies. i do want to go with a set of 3-2 headers and a cam but i will be sticking with the factory intake and the round-tops for the time being. i'm not afraid of a little port work so i can bump up intake and exhaust efficiency that way. i just need to bump the compression to take advantage of that.

[JMT240KGTR]

I've never milled the block before, but the head yes.

On an L24 I built with the old man a few years back we used L24 rods and crank, bored the cylinders to 86mm (280 bore), L28 flat top pistons, shaved the head .040", used 240Z round top carbs, extractors, 2.25" exhaust, elec dizzy, and suitable cam. Also fitted new chain and guides and deemed cam tower shims unnecessary.

This engine was powerful, love to rev and was very responsive.

I personally wouldn't mill the block, you get the same results if you do the head and if you do both, you'll find you chain will be quite slack; you'll then need to fit cam tower shims, sutiable length valves and springs, rocker arms, lash pads etc etc.

There is a site somewhere on the net, maybe someone can point it out but it's more relevant to you seeing your in the states, and it refers to U.S parts/specs and combo's available.

James

[clutchdust]

good to hear. hopefully, someone knows the site your talking about. it's very common to mill the block .010"-.020" to clean up any warpage on most American iron. is this not an issue with the nissan? i don't have any experience with aluminum heads but my experience would make me more comfortable milling the block. i'll go with this forum's more experienced opinion though. is there more meat on the head to mill? how much mill do you recommend and what does that give you for compression?

my first thought was to bore out to 2.8 but some people have said this is a bit too much for the stock L24 block to handle. if this works, i'd rather do that.

what i figure is pull the engine, new oil pump, fresh bearings, reuse crank and rods (barring significan wear or damage), bore cylinders, new chain and tensioner, check the head/valves/guides, minor porting, and new cam and springs. externally, i'm thinking pertronix, home-built cold air intake, the aforementioned headers w/2.5" exhaust and aluminum radiator.

not looking to blow the doors off any vettes, vipers or porsches but i would like to show tail lights to the rice boys in their slammed civics and preludes.

[JMT240KGTR]

I guess milling the block to "true up" warpage is justified, but yeah as I said, as a compression raising exercise I'd prefer shaving the head.

There's a fellow here in Oz who used to work for the Datsun racing team and he's been boring out L24-L28 .120" (3mm) for years; if it can be done with an L28 to fit 89mm 200SX pistons with a stroker combo, why not an L24 to 86mm?

Personally we have done and have friends who have bored out L series 6's .120"+ and they've never had problems; all depends on the application I guess.

As for a guide to milling the head and compression gains, this all depends on the initial chamber volume, Nissan heads had an assortment of chamber designs, very similar but size wise can differ considerably; take the readily available N42 off an L28-chamber size 44cc's and compare that to the P90, comes in at around 52cc+ or thereabouts. Then you have the early version heads with smaller chambers 38-41cc's but downside is smaller valves-can be remedied by having larger valves and seats installed.

If you want to stick with the L24 set up then I would do this:-

get yourself an N42 head (comes with the large 44mm inlet valves, and is cheap, easy to find), mill/shave it a maximum!!!! of .030"- .040" (any more than this is not recommended unless you want to fit cam tower shims etc etc), bore the block .120", use L24 rods and crank, flat top pistons, new rings, bearings etc, cam, carbs and so on.

The above should produce a C/R of 10-10.5:1, nice and healthy!

Opinions vary on the boring capacity of the L series but if the former Datsun Team mechanic has done it over and over as his daily business, then it's got to be pretty damn close to being O.K.

James

[Phred]

Just a few things to consider.

When the factory increased the L24 to an L26, They did it by increasing the stroke. The 24/26 have the same bore. The 26/28 use the same crank. They all rev quite freely, and when you put in a fat cam you loose bottom end. the increased stroke will help increase torque in the low to mid range which helps balance everything out. Summary: keep the stock bore plus .040. The extra cyl. wall thickness will help maintain strength and transfer heat. Run the L26 crank, rods, & pistons.

Your first piston question:

83mm=3.267inches

thats the actual diameter of the piston.

The stock bore is 3.270, which gives you .003 pist/wall clearance.

The question concerning rings:

The available oversizes are .5mm, .75mm, .1mm, & 1.5mm.

Referance: 1mm= .03937 inches (or .040 overbore)

The other specs you mention as 2-2.0mm and 1-4.0mm indicates the ring set includes 2 rings which are 2mm thick which would be installed on the 1st and second grooves. The other being the oil ring which is 4mm thick.

The compression distance you indicated as 38.2mm (1.503inches)

is the distance from the centerline of the piston pin to the top of the piston.

The factor that determins how much you can mill the block deck is the distance known as "piston to head clearance". as an example, if you had a "zero deck" meaning your pistons were even with the top of the block at TDC, your piston to head clearance would be the thickness of the compressed head gasket, which is around .043/.047 for a factory gasket. This is the minimum that should be considered. Or the piston might smack the head at high RPM. And that will spoil your day.

Phred

[clutchdust]

hi phred, can't imagine owning a Z in gresham. i used to live in wilsonville, then vancouver, so i guess your Z lives inside quite a bit!

still, good input. i really don't want to get into buying new cranks, rods, etc, etc. pistons are not a big deal since cleaning up the bores are good for the oil consumption anyway. the cam i'm looking at (comp cams 280S) is a higher RPM cam anyway.

i already have a SBC so it'll be nice to have something that really likes to wrap up to 6k.

from what you guys have advised so far, this is what i'm thinking. 86mm pistons. check the block and only mill if required to true the deck. mill the head (and block, if required) no more than .040" (total). fresh bearings, chain and oil pump. new cam, springs, lash caps and rockers (do i need to replace the rockers?). pertronix, MSA 3-2 header (coated) and some port/polish to intake and head ports.

like i said, i'm not looking to go out and spank any vipers (yet), that'll come with the SBC. right now i'm just looking to "wake up" the stock engine. i'm trying to get the most bang for the buck and i'm setting a target budget of $1k for this little overhaul.

JMT, what would have the N24 head you refer to? if i go to a bone yard, what am i looking for?

[JMT240KGTR]

The N42 head you're looking for is usually off Z series of cars, although I'm not sure whether the U.S market had other cars with L series 6's in them; if so, these cars may also be fitted with this head.

To narrow down your search even further these heads would be found 99% of the time on an L28, don't believe they were released on 2.4/2.6L motors.

To identify the casting (N42) look on the spark plug side of the head, underneath/between no.1-2 plugs and you'll see a cast number, if it says N42, there you go!

James

[clutchdust]

thanks. next time i'm in the bone yard i'll keep my eyes open.

in the meantime, i'm trying to find out if the pistons speedpro sells for the 2.8L are on the shelf or if they have to make them at the time of order. if they have to make them then, i'm going to request no dish. should be able to bump the CR pretty significantly without having to mill anything, and that's a good thing.

[Dan Baldwin]

I believe that the L24 can't be overbored as the L28. 3mm isn't a problem with the L28, but isn't recommended for the L24. This is only what I've heard/read, but from very reliable sources. I don't think I'd overbore more than 1mm with the L24 block.