Hi guys,

Need your expertise and advice here

I've bought Mazda B6 n/a flat top piston and I'm planning to rebore my stock A14 block (76mm piston --> 78mm piston) 2mm oversized..what are the modifications that will be involved in this mod (beside rebore)?how bout conrod etc..

anyone can advice me how to done this mods?cuz i've tried to look around but found nothing..beside this -->http://datsun1200.com/modules/newbb/viewtopic.php?topic_id=25614

so a zillion of thanks if sumbody can lite up my day

Thanks in advance.

The person that have no idea to do this mod,
Bituten.

anyone?

I've done this a few times now, from memory you have to machine about 2mm off the tops of the piston and take the little end hole in the conrod from 19mm to 20mm.
Do the little end mod first and assemble piston to conrod to crank and do a dry run up the bore without any rings on, when the piston is at TDC scribe lightly on the gudgeon side of the piston and that will give you an idea how much you will need to take off.
Hope this helps to get you started.
*I don't mean dry run literaly, be sure to cover all moving parts in oil.

In a standard block,we have to chop off 2mm of the top piston..

is it still ok if i have to chop off the top piston to 4.3mm??,cuz i've decked my block 2.3mm before(i've also chopped off the stock a14 piston 2.3mm)..is it still safe for the piston?

From memory you can't take 4+ mm off the top of the piston, you will get too close to the top ring groove.
Taking 2mm of is really only machining off the outer of the pistons dish.

You really need a block that hasn't been decked already to do this, sorry mate.

thanks a lots Lemonhead..

that really helpful

hmm interesting...

you've also got to deal with a 20mm piston pin vs teh nissan 19mm pin - that's a lot of metal to machine out of the rod little end (doesn't sound like it, but trust me it is)

so how durable the engine will be after rebore the block and the pin?

how long have u guys using this kind of mod in ur engine?

In all seriousness - I'd be very concerned about the engine durability. If it's a NA engine, you (typically) have to spin to higher rpms to make enough hp (and then get acceleration improvements from that alone, or even more still by swapping diff gears for 4.3-4.8:1 to get more torque multipliucation. Turning higher rpms is in fact more life threatening than a decent amount of boost at 'stock' rpms - so any weakening of the rods at all is questionable. It might be allright if you keep it under 6000rpm (i.e. 6000rpm max sustained rpm) it might be a little higher, - put it this way, I wouldn't be keen to try it at all - as the only way you'll 'know' how borderline the rod is (esp with 1mm taken out of the little end) is when it breaks.

If you weren't going to go NA, but went with boost instead, well the rpms can be lower and that's a good thing, but the 1mm reduction in bore wall thickness (2mm oversize means the bore wall is 1mm thinner all around). Now it seems the blocks will take this ok (as long as the block is in good condition and not massively corroded or the victim of major core shift during casting - which is admittedly pretty rare with nissan a-series engines afaik) - for NA the extra capacity would be worth some power, but with boost, the thinner bore walls will flex more, and ring seal won't be as optimal, so they can actually make less power than a std bore!!

My experience with enlarging rod little ends is with two different engine make/models - both of which I've had a few of each over the years. The first was the locally made hemi 265, which can be bored out to use chevy 327 pistons (a 90 thou overbore, and 5 thou increase in little end hole size) or 307 pistons (25thou overbore, and can be had in forgings for a decent boosted application cheaper than custom pistons, and again same 5thou rod enlargement). The 5thou (about 1/10th of a mm) enlargement of the rod little end makes no difference - plenty of people have spun them to extremely high rpm (for that engine) and not broken anything. The hemi rod itself is a very strong item to start with however.

The other was the locally made holden 6. With a lot of machining a ford 221 inline 6 crank can be adapted to it, to bring it out to 230ci (vs 202ci for the largest factory variant, but the mod is actually done to suit a 186ci holden block). These motors came with rods (the bathurst xu-1 rod, and then all holden 'blue' motors used the same spec rod in std form) that could handle 7000rpm all day long and 7500-8000rpm for a competition engine and stay together for a full race season. The conversion ended up using a ford 250 piston. This required a bigger rod little end cut. - just on (by coincidence) a touch over 1mm. They 'held up' ok on a streeter, but speedway guys who were trying them tended to see them coming apart with momentary blasts to 7000rpm.

On both of these engines the rods were quite decent from the factory and quite durable. but the 1.1mm increase in little end size on the holden rod meant that the rpm that could be run on a 'std' rod safely all day - i.e for a street car, couldn't be maintained for much shorter duration at that same rpm in a competition engine. To be fair, there was the added issue that the holden combination got its increased capacity from a longer stroke, so that would increase the stress on the rods all by itself.

Based on all that, if you never intend to take the engine above 5500rpm, then you'd probably never have any issues. At 6000rpm and above, you might. At 7000rpm and above I'd be surprised if the rod with 1mm oversize little end stayed together.

Then again, maybe it will last fine! I'd be very happy to find out that the rods are so strong and over-engineered that it handles the pin enlargement and 8000rpm with no problems! - I'm just personally not able to afford to find out for sure (i.e. to deliberately build such a combination and push it till it breaks and note what rpm and for how long at that rpm it lasted.