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Bensmc

Forged pistons and balancing

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Have you or know somebody that has forged pistons into an sr20det and not balanced it as a rotating assembly?

Reason I ask is because I have my sr20 that had alot if km on it and I checked the ring gap. The ring gab is out of spec, but I did measure it with the old rings. Long story short, thinking of getting oversized forged pistons, but not sure if it needs

To be balanced. Or spend the money and buy a new set of genuin rings and measure it, and if its within fsm I would run it. Car is going to be a daily driver, plus a couple of track days every year.

Engine specs Td05-18g -19psi running all

Supporting mods.

 

Ring gap Specs out of fsm

Top .0079 - .0118 inch limit .0154inch

Bottom .0138 - .0197 inch limit .0232

 

Actual

top .021 inch

Bottom .025 inch

 

 

 

 

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Used factory pistons would require balancing, especially if you change the oem studs with ARP ones, but quality forged pistons that are balanced from factory are most likely better balanced than what the machininsts will do. Remember, the parts get balanced separately, so if you're only reusing crank and flywheel/pressure-plate, then just get those balanced.

 

I paid for balancing on the complete rotating assembly and completely forgot that my pistons and rods were already balanced to +/- 1g. Mistake. Looking back on it, the reaction of the shop owner when I delivered them was "oh snap free money", and it was fairly clear that they didn't have to remove any material from the pistons or rods. So stupid me effectively paid a shop $150 to cover my clean pistons and rods with oil and metal shavings, then dick me around, bullsh*t me and waste my time.

 

So a word of warning: when you give a shop at least 1 week to balance some parts and they tell you it will be done well before Friday, before you book your leave from work, keep in mind that they probably did f**k all to them for the whole week, let the balancing guy leave at midday Friday to go on a long weekend, fail to call you back a bunch of times, then expect you to agree that the poor guy needed a holiday so it's ok.

Edited by pmod

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Lol, thanks for the tip.

I'm going to reuse the rods, crank, balancer, flywheel and I'm replacing the clutch... So all in all, the forged pistons should be perfect just to be assembled and installed into the engine with any issue. Besides the machining that will need to be done for the oversize.

I'm not expert of forged pistons, but when on opening the box of pistons do they come with a spec of how much bigger the bore needs to

Be for expansion on the piston or is it all up to the machinist.

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You get oversized pistons to suit the new bore diameter (e.g. bore out to 87mm, you get pistons to suit that new bore size), but be very careful with ring selection. For this reason I would suggest CP pistons, as my Wisecos came with random rings with mismatched gaps. I got CP rings and they were very consistent, but I don't know wtf is the deal with the second compresion ring gap sizing from these companies. I stated the numbers I needed in my order and my top ring gap was ever so slightly smaller than desired (but by so little I didn't need to file it), however the gaps on the second rings were bigger than desired. It wasn't too much, so having bought two sets of rings at this stage I just said "f**k it" and put them in, but I was most puzzled that they too weren't undersized.

 

If the engine was in good order with just a glazed or scored bore, then you could simply install the pistons and leave it at that. However it would be wise to balance your reused rods, flywheel and clutch pressure plate anyway. I'd suggest getting the crank and harmonic balancer done too. Once they're balanced, then future rebuilds will really only need the pistons/rods/flywheel/clutch balanced, unless the crank or balancer are damaged. Nissan didn't exactly go to town on the SR20, and at 20 years old there will exist some wear to correct.

Edited by pmod

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Fair enough. I got told something completely oppostie. I got told that when you buy forged pistons. You will give them to the machine/builder shop and the machinist would make the custom bore size suiting the piston depending on your application, eg, (drift, drag, street or daily) that makes things alot better.

Well the bores are in really good nick suprisingly. The bearings had very little wear on them. Number 1 had next to no wear at all. With the engine just clicking over 190,000km.

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I would measure the bores first to make sure there not overly worn and then decide if it needs oversize pistons or just a hone and new rings

Usually if there std piston and rings in it now and the gaps measure too big it means the bores are worn

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Also if you get forged pistons you should give them to the machinist with the block as he will know what size to bore it as forged pistons slightly expand with heat and ask him to set the ring gaps as they need to be larger for boosted applications

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Those people were talking about ring gap Ben, and from my research judging by the results of folks that have sized higher, it would be a mistake to go for anything other than the street specs on anything other than a drag car. To give you an idea.

 

http://www.wiseco.com/PDFs/Manuals/RingEndGap.pdf

 

Sizing ring gap isn't a complicated thing either; the bigger the ring, the smaller the gap, and there's a range of usability between bore and rings which is the piston to wall clearance. If you're concerned, then follow pru0m's advice and first get the bore checked for size and taper (which you would do anyway), as that will determine whether you HAVE to get it bored. If it's on the larger side of acceptable, then you could indeed get some 20 thou oversized pistons and request a bore to the piston diameter plus 2x piston-wall-clearance. Fine-tuning that clearance is the only advantage I can see to buying the pistons before getting the bore. The downside is that if they screw up the bore and go too big, you need a new block or you're out of pocket on the pistons. I doubt the shop will cover the cost for their mistakes.

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Also if you get forged pistons you should give them to the machinist with the block as he will know what size to bore it as forged pistons slightly expand with heat and ask him to set the ring gaps as they need to be larger for boosted applications

 

after hours of research, iv decided to get forged piston. Send the block and the head away to get decked. Block to suit oversized pistons and honed. Crank polished, balanced etc etc.

one thing that I'm not 100% about is the bearing clearance of the mains and the rods. My idea was to purchase acl bearings with a standard bearing clearance. Apparently a stock sr20 bottom end car handle 350hp comfortably. :/ but there is a choice of .001" larger bearing clearance. Pretty big, most likely being for extreme racing conditions. But being a daily/track car. Standard would be more

Appropriate and reliable. Correct?

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No no no. Get it all measured first, THEN buy the gear. Buying parts with fine tolerances based on guesses is a recipe for disaster. Measure bore. Crack test crank. Measure crank journals. Check factory size references on crank and block. Work it all out, then buy the parts.

 

Read all of this first then come back to us. You can get the same info from the FSM, but this is convenient and has pictures.

 

http://forums.nicocl...et-t309576.html

Edited by pmod

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No no no. Get it all measured first, THEN buy the gear. Buying parts with fine tolerances based on guesses is a recipe for disaster. Measure bore. Crack test crank. Measure crank journals. Check factory size references on crank and block. Work it all out, then buy the parts.

 

Read all of this first then come back to us. You can get the same info from the FSM, but this is convenient and has pictures.

 

http://forums.nicocl...et-t309576.html

 

I don't see the point on measuring a bore that I'm just going to end up boring out larger. If the bore was that out of spec, that I couldn't put 86.5mm pistons in because the bores were too big already. There would be some major piston slap happening on the standard pistons. Pistons had no scuffing on them, 3 out of the 4 pots had a decent amount of cross hatching visible but defiantly needed a tidy up.

Crank was in spec, runout was fine. Just had some minor stains around it from poor oil changes. Bit of a lip from the rear main.

The bearings were practically brand new, someone would almost say put them back In and reuse them. So based on the tear down, there should be nothing wrong with the tunnel, nor crank. The deck height was within spec but because I'm going to use a metal head gasket, I wanted a dead flat surface, so decking on head and block is a must for me. I have not yet crack tested the block or crank, iv never herd of standard sr20 crank cracking before under standard working conditions. . I have some flaw check that I will apply on it tomorrow. But I'm sure it will be fine.

Edited by Bensmc

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The point is to know, rather than guess or assume, and the crank and block weren't blueprinted to exact specs by Nissan. For instance, the bore on my rebuild engine was smaller than 86mm from factory, so with light wear and a hone it hit 86mm exactly. The crank bore and journals can differ in size too, hence you check the numbers stamped on the block and crank, compare them to Nissan's chart and work out what the original bearing thicknesses were. Nissan also make a .2mm oversize factory piston, which although it's unlikely, you could potentially have in a 20yo imported engine without knowing it.

 

I get where you're comming from, but flying blind isn't really the best practise.

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Posting so i can find it at a later date.

Lots pf great info in here thanks Pmod etc

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^ same here very good info here pmod - i know who i need to talk to soon enough :)

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Cheers, but we all know STR8E180 and Donny are the real goto guys when it comes to rebuild information.

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Also check out duncan foster engineering videos on utube.

 

They will make all the pistons and rods equal in weight, or at worst within 1g.

 

Then they dynamically balance the crankshaft by putting bob weights on the crankshaft to simulate the weight of the piston/rod.

 

Dont bother having the balancer or clutch done with the crank because you may change them in the future.

 

For what youre doing u should probably just put new standard stuff back in as long as the piston to bore clearance is ok.

 

If not i would have it bored and put forged 86.5mm pistons and eagle rods in it

 

 

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Information related to the Forged pistons and balancing are very hard to find and you shared the best information that is authentic and we can follow it. Writing the freshessays review about these issues and its solutions can tell the people what should they need to do.

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