Finishing Up Forged Cp Piston Rb30 Build, Few Q's

[jarrod83]

Hey fellow skyliners, having a few problems with my engine builder regarding a few things. I have 9:1 compression cp pistons at 20 thou over and spool up rods.

After talking to a few people and searching the forums i decided to run 3.25 thou piston to bore clearance using the measured diameter of each piston, instead of the recommended 3.5 thou from C.P, but my machine shop doesn't want to run the clearances this tight and in fact want to run a clearance of 4 thou.. but would prefer to have them tight as possible to hopefully minimize the amount of oil it breathes... so i told them i would double check the clearance and get back to them.

I have searched for the forums for clearances and pretty sure the 3.25 thou is suitable for what i want.. which is about 25psi with a gt3582 with the 9:1 compression and hopefully around 500rwhp at the wheels, the head is mildly ported and the combustion chamber has been polished with the sharp edges removed. I'm a little worried though since I'm not running oil squirter's (heard that forged pistons run a bit colder and probably not needed) and running some descent boost and compression to whether or not the slightly tighter than recommended piston to bore clearance is still suitable or whether theres a possibility that the pistons might get a little too hot and seize up.

Cheers for any advice

Jarrod

[GTRNUR]

Piston manufacturers supply clearance spec sheets with their pistons for a good reason. They KNOW that a piston of X diameter will expand Y amount when up to temperature. As the iron cylinders expands at a slower rate to the pistons, the piston manufacturers calculate the required clearance based on piston and cylinder material so they get the best cylinder seal.

The wiseco's in my most recent engine are machined to 2.75 thou, as per the manufacturer specs. I cant remember the grade of alloy they are made of, but it is different from CP's material. Because CP use a different type of forged alloy, the same diameter piston will expand at a different rate, which is why the clearances are different.

Out of interest, air cooled porsches run a 0.001" clearance between the piston and cylinder as their cylinders are nikasil coated aluminium, so the piston and cylinder expand at the same rate. The reason they get away with such close clearances is that everything is machined properly.

So long as the block is bored and honed when a torque plate is bolted on to spec (supply your ARP head studs and torque spec details if your using them), then there is nothing at all wrong with going 3.5 thou. If they are boring without a torque plate, that could explain their desire to run 4 thou. It makes it "safe" for them, but certinly doesnt help your leak down/blow by.

Follow the spec sheet for filing your rings as well. Same principal as per pistons result in ring gaps being what they are, so that when hot the close up that gap so there is a near perfect seal.

Hope that helps.

Edited January 5, 2010 by GTRNUR

[Spoolup]

Exactly - CP spec those pistons for around 1000bhp and 30psi, so follow their recommendations.

[jarrod83]

cheers for the advice, with the build moving from a stock build to a forged build i forgot all about the boring to be done with a torque plate instead of the cheap method.. that could be the reason why they want to cover their asses with the extra 0.0005" clearance..

In regards to the piston to bore clearance, i was under the impression that the clearance's specified from cp pistons was the minimum clearance needed to cover a wide range of conditions, and if you machined the bores to the individual pistons and were going to the extremes you could run a slightly tighter clerance.. though im fine with using the 3.5thou clearance... just definately dont want to be using the 4 thou clearance they suggested.

Does anyone know if these engines will run fine without the oil squirter's? or is ceramic coating needed?

Thanks alot of your input guys.. ill call the builder and hit him up about the torque plate boring

Jarrod

[GTRNUR]

Ive seen 625rwhp from an 30/25 with no internal coatings or squirters. Ceramic crown coatings will keep oil temps down though, and IMO i'd be using them if you dont have squirters. Saves your oil cooler from having to work as hard to keep oil temps down. While your at it, get the skirts moly coated too.

Matching cylinders to pistons is pretty extreme. Ive never heard of anyone being quite that anal before. Save your desire to go to the extreme when filing your rings. File the ring sets to suit each cylinder and do one piston/cylinder at a time, numbering 1-6. Measure the ring clearance at the position in the cylinder the they will be when the piston is at TDC. Just use a vernier to measure the ring land heights on the pistons, and then push the ring down the cylinder that far when gapping.

If you pull out a micrometer and measure your pistons up they should all be very closely matched. You will notice that they arent round though. The shape of the forging and the type of re-enforcing in the piston along with the material type determine how much the piston expands and in what direction. They are supposed to become round once they are up to temp, which hapens within the first 30 seconds of idling.

[Spoolup]

Jarrod,

The pistons are designed with no oil squirters in mind.

Ive seen 1154 rwhp with no oil squirters - though it was no enduro car. Just a dyno queen.

Though my engine has been going strong for nearly 3 years with no squirters.

Oil squiters were really designed for non forged internal engines, Nissans way of ensuring longevity from cast pistons in a production car. Great if you have them but not worth the effort and expense in an RB30.

Ceramic coating is a excellent option and relatively cheap now.

Edited January 5, 2010 by Spoolup

[jarrod83]

Thanks heaps for the advice guys, been worrying what I'm going to do in terms of the oil squirter's for days, ill probably not worry about the ceramic coating as my budgets getting a little stretched and don't want to be completely broke in case anything did go wrong =].

I don't actually have a oil cooler yet, it's one of the few external mods i don't have, if i do change my mind with regards to the ceramic coatings im guessing id have to get it done before the piston to bore clearances were measured and machined?

Cheers again for the info

Jarrod

[GTRNUR]

Yes you generally get your coatings done before the block machining. Especially if you have the piston skirts moly coated.

[STATUS]

i dont run cp's @ 4 thou, would never consider it especially on a streeter there is definately room to tighten them up. All our premium engines are are hone to fit, ie each bore is finished to suit each piston especially as some of clearances we run are half what you posted on certain pistons..

[brada31ke70]

not being a engine builder, but after building a rb26/30 i will be the first to admit that some of my theories are not quite orthordox as mr uras knows after i suggested just starting a new engine even though i am sure that motor companies would not go through your process when starting every new engine assembled, better to be safe than sorry i guess.

talking about filing rings, is this really necessary, surely the ring end gap would be detirmined by the piston to bore clearance, eg the bigger you make the bore the longer the distance around the cylinder the bigger the ring end gap? and wouldnt the manufacture of the pistons and the rings calculate this? and how accurate can someone be with a file? sorry to add questions into you thread jarrod 83, good luck with your build, its a great feeling when you finally drive it.

[GTRNUR]

The correct procedure is to measure the pistons, then bore the cylinder to the appropriate clearance.

Rings from the piston manufacturer are manufactured in bulk for a specific bore size by the thousands and then a set of 6 is shipped with each piston. They dont go to the effort of measuring each piston (which can have variences up to a few thou, and then adjusting the rings accordingly. Doing so would be a waste of time as every machine shop and builder does things a little differently.

So, the rings shipped while having some gap must be appropriately modified to suit each cylinder bore for the application that the engine is being built for. Eg, Street turbo or NA, drag, NOS...

As for accuracy, my machine shop works to a 0 thou tolerance for everything they do, and using a good diamond ring file there is no reason someone that knows what they are doing cant achieve the same level of tolerance. It just takes time and patience.

Not gapping at all, and using the rings straight out of the box could result in cylinder damage when the rings heat up, or broken rings, and then damaged piston ring lands.

[proengines]

It's worth checking the sizes on the pistons but with manufacturing technology being what it is today and diamond tooling usually used to turn pistons rather than grinding them as they used to, there is very little chance of having different sizes on the pistons. Maybe .0002" (two tenths of a thou) variation over a set might be possible though Wiseco's are always spot on when I measure them.

Don't run less than what the piston manufacturer gives you as a minimum clearance, there's a reason they give that and it is dependent on piston material, geometry, pin offset and a number of other factors. You do need to check the ring gaps, never just assume they will be right out of the box, they normally wont be. Most performance rings are supplied as "file fit" rings which require the gaps to be set. "file fit" doesn't mean you get out a file and hack away at the ends of the rings, it means they need to be machined to set the proper gap. I made a small electric ring grinder to do them as most of the hand crank rotary ring files you buy have far too much slop in them and a wheel that's made for grinding cast iron rings, not the steel top rings you'll usually run in one of these type of engines and they are much too coarse.

The bore size has a big effect on ring gaps. .001" difference in bore diameter will give you .003" difference in ring gap (Pi x diameter).

Have the block honed with a torque plate, it is more important for ring sealing to have a round bore than small piston clearances.

Cheers,

Greg

[rockabilly]

i have cp,s and spool rods in mine, its on the spool website,cars done 10,000 miles now with a gt4094r on it..it has 650 at all four wheels, built mine myself and 3.5 thou clearance. my old rb26 which i also built was at 4 thou thats now done 15000 miles. bernie uk

[jarrod83]

Cheers heaps for the info guys, definitely going to settle with the recommended clearance especially since i like the occasional burnout

I asked my machine shop about the torque plate boring and they mentioned some other method they use that i cant remember but they claimed to be accurate :S, they said they don't do it but can do the torque plate honing with the cradle? i thought you also needed a plate that bolts to the top of the block, so ill have to confirm if they definitely can do it properly, they haven't been the most helpful people on the phone.

lucky you mentioned not to grind the rings with a hand file, i was actually going to do that. Is there anything that you can easily get to file them down? or is it easier to just send them to the machine shop

Cheers

Jarrod

[BezerkR32]

torque plate honing is done when you are boring a sleeved engine, ie alluminium block with steel liners. it is done to prevent the liners moving. pistong ring filing should be done with a little tool called a pistor ring grinder. you turn the grinding wheel by hand and the ring sits on either side or as greg mentioned an electric one. it doesnt hurt to measure piston to bore clearance by feel. you can do it with a long set of feeler guages. try each piston in each bore and you will get a feel of which fits best in each cylinder. i havent built an engine with 4thou piston to bore clearance for about 10 years.

Cheers heaps for the info guys, definitely going to settle with the recommended clearance especially since i like the occasional burnout

I asked my machine shop about the torque plate boring and they mentioned some other method they use that i cant remember but they claimed to be accurate :S, they said they don't do it but can do the torque plate honing with the cradle? i thought you also needed a plate that bolts to the top of the block, so ill have to confirm if they definitely can do it properly, they haven't been the most helpful people on the phone.

lucky you mentioned not to grind the rings with a hand file, i was actually going to do that. Is there anything that you can easily get to file them down? or is it easier to just send them to the machine shop

Cheers

Jarrod

[proengines]

Torque plate honing is done with a plate bolted to the top of the block to simulate having the head bolted down. The idea is to finish the bore round while it is distorted by the head bolts so when the engine is assembled the bore will be round. If you don't use one, the bore goes out of round by up to .001" when you bolt the head on. Image hopefully below:

This is what I use to do my rings:

[jarrod83]

Thanks for the both the pics proengines, ill be able to setup something like that rather easily for the ring gap filing and know a little more about what I'm talking about when discussing the torque plate honing with them

Cheers

Jarrod

[jarrod83]

Just a update and also need some further clarification

Not able to do the torque plate honing as the engine builder doesnt have a torque plate for the block but said he can torque up the cradle? is it worth doing that at least?

Secondly im getting back mixed opinions again from the clearancing between the pistons and the bore since. I've decided to ceramic coated all the internals including the pistons crown and skirts. I've spoken to the machine shop and the ceramic shop, the ceramic shop said to set up the 3.5 thou clearances first then theyll ceramic coat them and the tighter clearance from the coat makes up for the fact they run alot cooler with the coating. I told my machine shop this a few weeks ago and they seemed to understand.

I've since picked up my pistons to be ceramic coated assuming the block was done, and when i talked to them about whats left to be done they said they were just waiting on the pistons so they could do their final 3.5thou clearance honing, the ceramic shop told me the skirt coating is about 1 thou thick and with my machine shop adamant that this is correct my fear is that the pistons will run with a ridiculous amount of clearance.

Has anyone got any experience with what is the correct way to have this done, i know someone mentioned further up as well that the clearancing should be done after the coating but considering the pistons run colder it doesnt seem like its the correct approach

Cheers for any info

Jarrod

[GTRNUR]

Bolting the cradle on and torqueing to spec will have no effect on the cylinder shape. The bolts and supporting material are too far down the bore for it to matter, as well as the entire bottom area around the crank is heavily reenforced and has ribs etc (in a 26 it does at least).

The only reason you want to go messing around with the cradle is if your using a cradle from another engine, if you have partially grout filled the block, or you are using ARP main studs in the block.

About the coatings.... measure measure measure. Its fine and dandy to assume the coatings will be even and consistent on all pistons but you only get once chance to assemble the engine properly. Its not a big deal to measure up each piston to be sure. About the cooler temps and lower expansion rate due to that fact, it would be negledgable. I'd still be inclined to run them at the recommended factor clearances.

The order I do things is:

1. Measure crank mains after having it machined and balanced etc. If using ARP main studs skip to 3.

2. install main bearings in cradle and block, install factory bolts and torque to spec. Measure for roundness and to determine overall clearance.

3. If out of round, then the cradle needs to be shaved 1-1.5 thou and then re-installed in the block and line bored.

4. If round but too loose, consider using the 1/2 thou undersize ACL bearings in either just the cradle, or in the cradle and block depending on how much clearance change you need.

5. crank with mains, and then install an old rod and piston in cylinder 1. measure deck, then repeat for cylinder 6

6. calculate the diference in angle level out the cylinders, and have the machine shop deck the block accordingly so all cylinders have the same CC.

7. Measure you pistons to make sure they are all ok, gudgens to. Then get your coatings done. Finally re measure them all and determine the largest diameter.

8. Add your desired clearance to that (2.75-3 thou?) then have the block honed to that diameter.

I highly recommend either taking the engine to a place that can torque plate hone it, or try to borrow a plate from someone that can. You might have more success asking for an RB30 torque plate too.

Hopefully I havent left anything out. I hope that helps.

[Swiper the Fox]

Ive tightened up the old Cp's a quarter of a thou...no issues at all. But the newer ring package they come with seals a lot better and really there is no need to run 3.25 thou. 3.5 thou will allow you to run as much boost as you like as well.

beware though...some guys use mild steel torque plates but they do not flex like an allow head does and doesn't replicate whats really going on when a head is fitted to the block.