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Help with boring motor


Whittie

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the car has been down on compression and i've just got it at the mechanics now and he said that the bore is worn a bit and it'll need to be bored. this means oversized pistons, but i have no idea what i need.

 

i'ts currently an l28 F54 block, 2 mm shaved p90 head, some rods (prob standard, dont know), and flat top pistons. i race the car (read: spends most its time at 5000-6500 rpm) so i need some pistons that are going to be able to take this abuse, but i and my mechanic are not sure what to get. anyone got any good ideas? i am sure someone has done this before but a search proved fairly useless.

 

I am in autralia so if i can get some stuff locally that would be good, but if it has to come from the us, thats cool too. price isnt tooooo much of an issue, they just need to be good lasting pistons, but i dont want to be spending thousands either.

 

Thanks,

 

Tom.

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done some more searching and found that 87 and 88mm are popular over bore sizes. i'm leaning towards 88mm cause of the extra displacement but i couldnt find anything that said what pistons to use for this size bore, only that it was popular.

 

forged pistons aren't out of the question so long as thet are proven to be significantly better, i just need to do some fast talking to run it past my dad (partnership in car as we race it).

 

Thanks,

 

Tom.

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"Worn Bores"

 

That's a first! Usually the rings will wear before the bores will even be worn to the high end of acceptable NEW tolerance.

 

Low compression in one of these engines is more an function of worn/weak rings, or leaking valves.

 

New pistons on a racer is a good bit of insurance nonetheless, but I would be suspect of "worn bores" based only on low compression. Chances are it only needs new rings and a deglaze of the walls, not an oversize boring.

 

I'd look a bit closer at the options, unless you are looking for an increase in displacement for those reasons...

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I'll have to agree with Tony. I've gone through alot of so called "worn out" motors picked up cheap from here and there and usually find the bores within tolerance. I have even used .005 over "file to fit " rings to compensate for marginal bores with great sucess. Granted, my experiences are not with Z motors but durability has always been a high point for Datsun blocks of all types in those days..

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"Worn Bores"

 

That's a first! Usually the rings will wear before the bores will even be worn to the high end of acceptable NEW tolerance.

 

Low compression in one of these engines is more an function of worn/weak rings, or leaking valves.

 

New pistons on a racer is a good bit of insurance nonetheless, but I would be suspect of "worn bores" based only on low compression. Chances are it only needs new rings and a deglaze of the walls, not an oversize boring.

 

I'd look a bit closer at the options, unless you are looking for an increase in displacement for those reasons...

 

dry compression test shows up a range of about 100-120 psi, and wet brings that to 120-150 psi. just did a valve grind about 1000 miles ago and according to the mechanic all of the rings are fine. i'm not 100% sure of the details as i haven't had a chance to speak to my dad about all of this yet (he's been in contact with the mechanic more than i have) but he is telling me that the bore is worn and needs to be bored out, so i came here to find out my options.

 

I will take a further closer look at the motor and get all of the measurements and let you guys know the results of that, then i suppose we can go from there.

 

Thanks,

 

Tom.

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"Rings are Fine" is not how I would phrase those results. The numbers are fine for a street engine, but given the raise in compression when wet, that would normally indicate wear on the rings, or the piston ring lands causeing excessive blowby.

 

I would be wary of someone saying Nissan Bores are "Worn"... is this engineer familiar with te Nissan L at all? The reason I'm asking, and you can verify this all over, is that even in racing applications it's far more common to deglaze and re-ring than to EVER have to rebore the block.

 

"BIG SAM" the "samuri" campaigned in the UK is a well know "Commonwealth Example"---magazine articles of the day were amazed that even after thousands of racing miles, the most the car ever required was a de-glaze and re-ring.

 

Of course, doing machine work will raise internal profits, through outsourcing the job and makring up discounted labour....but that is a different story...

 

In itself, for a racing engine, reboring is usually done for displacement increase....and that is fine. But barring something like what happened in our Bonneville Engine (the gudgeon pins worked free on the pin end, and scored every bore 0.080"!!! Every machinist said "never seen that before") the most I have ever seen was a need for a deglaze and re-ring.

 

Choice of pistons is up to you. Forged will give you an RPM limit over 7000 for extended periods. More RPM will equate to more HP if set up properly.

 

So think about your options carefully. If you price the difference between new pistons in cast or forged....... Long term for high rpm use, I'd presonally go with forged....... the decision becomes difficult.

 

Putting in new cast pistons on a unbored, but re-honed bore...

Putting in Oversized pistons on a rebored oversize bore...

 

Either way, rings will be needed. From what I have experienced, a wet test increas that little is usually tired rings on a Nissan, not a bore out of tolerance.

 

Good Luck.

Cheers!

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Just gone and had a look at the car and the engineering shop where the pistons where x-rayed says that they are worn and need to be replaced anyways, and they've scored the bore so i'll be boring the motor anyways.

 

now comes the hard part, how big to bore it and what pistons to use. ofcourse i want to go as big as possible (why the hell not) so i was thinking 88mm is safe from what i have read (i believe that would raise displacement to 2.95 litres?), but then what pistons can i put in that will still keep the high compression ratio? chatted to a few z savvy people around here and they have all said that stock pistons will be fine so what have people used that has worked well for them?

 

i believe the rods are from an l24 but i dont know for sure, will have to do a bit more investigating but for now work on the assumption that they are l24 rods.

 

does anyone know how hard it would be to track down an l28DE crank in Aus or is it not really worth all the extra effort and cost and should i just stick with the crank thats in there already?

 

Thanks,

 

Tom.

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does anyone know how hard it would be to track down an l28DE crank in Aus or is it not really worth all the extra effort and cost and should i just stick with the crank thats in there already?

 

Thanks,

 

Tom.

 

 

LD28, might want to change it. Some take offence to the wrong spelling.

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Guest d3c0y z3d

LD28 cranks are hard to come buy in Aus and can be pretty expensive - expect to pay around $600 min if you can find one.

 

Usually the easiest way to get hold of one will be to get one shipped over from NZ.

 

The way i am going to go is KA24 pistons (Navara) which give you the 89mm bores - stock L28 crank shaft and L20A/L20ET rods.

This give you pretty much on 3.0L (2.96ish from memory).

 

It will also give you nearly 11:1 compression too. (I hope 1mm positive deck height is OK!!)

 

On top of this you get a better rod/stroke ratio so it will rev better than a stroker

 

In my opinion it is easier to find an import L20 than a stroker crank.

 

What sort of carbouration are you using? Don't forget you want that to be able to supply enough fuel and air for a big engine. You will also want a bigger camshaft so put aside money for a regrind too (not much).

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Just finalised all the details with the mechanic today and this is what we are going to do:

 

bore was already .04" over and so we are going to .06"

pistons arrived today

crank is being knife edged

head is being further ported and polished

keeping rods (l24)

going to larger cam again

porting valves to put bigger ones in

 

i think thats the lot. the engine is running triple 45 webers with a high flow bosch fuel pump, so fuel is no problem, and with any luck i will now be getting some decent power from the car when i get it back and tuned again.

 

Thanks for your help everyone!!

 

Tom.

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