basically i have an A15 and i'm interesting in porting and polishing my head,firstly, WHAT IS IT???and what does it do??? is it worth getting my bore size increased? and what is a stroker???
please help me
thanks ty

Try searching on each thing. Depend really what your aims are.

Many modifications compliment each other say a cam works better if you have a good carbie an extractor package already on the engine

Different mods suit different motors its all a balancing act really

Porting is removing material from the head for larger capacity and better air flow.

Polishing is smoothing the channels etc to achieve better head air flow.

Engines are just big air pushers, the better less resistance to pushing air the better they will perform.

As for getting your bore increased I dont know your engines well enough 'yet' to say.

A stroker is changinging the stroke length of the piston, it changes the capacity of the motor. This is generally achieved by changing the crank and/or changing the con rod for a different lenght.

Quote:

it might be a bit difficult to stroke an A15 because the A15 has the biggest stroke out of all the A series'. does anyone else know if there are other cranks that fit in these engines that have a longer stroke? L series? J series?

boring the cylinders is the other way of increasing capacity of your engine.
most people will get their engine bored a few thousandths of an inch (you may have seen the term "5thou' over" or such, which means the bore has been honed out 5thousandths of an inch bigger than standard)
its not real wise to bore an engine much more than 2mm, as this takes to much "meat" from the cylinder walls.
the idea of an over bore is to clean up the cylinder walls to help the rings seal.

it is definatley worth gettin your head ported and polished. maybe even skim a few thou' off it too to help raise comp. a bit.

the larger the port size, at lower revs the velocity of air entering the cyclinder will be slower resulting in lose of torque, but higher in the rev range if your cam is matched, will flow more air.ie more power, more power=more air and fuel, just make sure the cam. carb and exhaust are all matched, porting is important, polishing isn;t, polishing or smoothing combustion chamber is, opening the bowl (behind valve) is most & more important than large ports. something Murray Coote told me along time ago when he did my old L series head.

hey with strokking an a15 if you really want you can get the crank ground and get some of set bearings made up that will increase by a little bit i got 60thou with that hope it helps...

To stroke an A15 crank you must be getting desperate, but it can be done with only standard parts.

Have the crank offset ground to the smallest allowable undersize. By offset grinding, I mean that ALL of the material that is removed from the crankpins is removed from the inside of the crank. No material is removed from the outside face of the crankpin. This effectively moves the center of the circle, that is the crankpin, outwards by half the amount that the pin is ground undersize.

For example, if the pin was reduced in diameter by 1mm [.040"] then the center of the pin is moved outwards by half this figure, which is .5mm [.020"]
This now means that the crankshaft will push the piston .5mm higher in the cylinder, as well as pulling it .5mm further down.
This then means that the distance that the piston moves up & down [the stroke] has been increased by 1mm.
You use standard issue bearings that are 1mm oversize. [the bearings are thicker, or 'oversize', for use on a smaller diameter, or 'undersize', crankpin]

Now that the piston pops up a half a millimeter more, we need to reduce the crown height by [nominally] the same amount that it now sticks up, so each piston will need to be machined to remove a half a millimeter from the top. This is to stop the piston from hitting the cylinder head, which makes it hard to start.

In our Datsun engines this is not a problem as the pistons have a dish, or bowl with a raised ring around the perimiter & this is quite thick enough to machine a small amount.

It will also be necessary to check that the pistons are not pulled down so far that they strike the counterweights of the crankshaft as this will also make it hard to start.

The standard A15 displaces. 1487cc
With a 1mm stroke job only, 1506cc

A balls to the wall engine with 79mm bore, 1607cc
The same engine with 79mm bore & 1mm stroke, 1627cc

As can be seen in this illustration, this more expensive stroker job yeilds only 19 & 20cc respectively which doesn't really add up to much bang for buck while the big bore job, possibly requiring 'O' ringing, yeilds 120cc Which is an 8.1% increase. The stroker job increases the stroke by 1/82nd of the previous stroke or 1.22% which would not really be felt as much increase in torque.
The bore job ,on the other hand, gives us an increase of piston surface area [4,536 sq mm versus 4902 sq mm] of 366 sq mm or 8.1% Thats 8.1% more surface area recieving the same combustion pressure to push that crankshaft around.

The stroker job illustrated in this explanation seems to fall into the realm of being at a 'point of diminishing returns.' In other words, in this particular example, you pay a lot more, to get a lot less, but other engines can use a crank from another model of engine, or even a whole different engine, & provide a big boost in capacity & torque for a much more realistic outlay.

I think that in practical terms, the factory already stroked our little A series block about as far as it should go in the A15 variant.

Hopefully this helps.]

PS. The 1200/A12 engine with A14 crank & 77mm A15 pistons gets a 10% increase in stroke & a whoping 23% increase in capacity.
Some combinations can work, & some are better left alone.

ty you are far better off to bore it out rather than stoking it.(not that you cant do both).Really on a street engine you just bore it out to get new pistions in there rather than get more displacement.

Porting is an art form in it's self and this is where most engine builders spend most of there time.Find a good porter(one with a flow bench)and then give them the specs of your engine cam, carbs powerband ect.That will help them size the ports flow ect.

Most of your power gains will be made in the head.It all depends on budget.

thank you all a lot,
i think i'm gonna just leave it until i save up enough money to get it ported, and re bored.
thanks ty