piston protrusion (WTF, mate?)

[krayton]

Ok, i dont know much about building engines, thus the machine shop building it for me.

apparently my piston head is sticking pass the block deck by 25 thousandths. whats up with this?

RB25 build up bottom end with wiesco pistons and rb26 eagle rods. the machine shop is trying to figure this out, but seeing if anyone can shine some light on this.

as of now im basically leaving it up for him to do something since i really dont know.

anyone?

[minivan_don]

did you get rb26 pistons AND rods ??

or rb25 pistons and rb26 rods??

there is a difference on the rb25 to rb26 pistons...

[BoostFab]

they may have over-decked the block. check for valve-piston clearnace

[krayton]

they havent decked it, and it really didnt look like this engine was touched inside.

its rb25 pistons and rb26 rods.

[Sil240]

The wrist pin of the RB26 piston must be lower.Guessing because the 26 has a lower C/R

[Carl H]

sounds like some one shipped you 26 pistons...there is a diffrence in wrist pin location between the 2 even tho bores are identical...

[BoostFab]

say hello to 1.5+mm headgasket or retun the pistons

[Sil240]

or do like those Vdub guys do make a motor plate. lol

[krayton]

say hello to 1.5+mm headgasket or retun the pistons

thats exactly what i was looking into. my question with that is that measurement of aftermarket HGs, is that how much it raises it?

and im still trying to find what my cometic hg raises if any. but im looking into these other apex/ greddy ones.

ohhh, the machine shop did a quick measurement and he thinks the piston heads are exactly like my stock ones

[Darius]

Wait. I think people are confused (including myself). You are saying that the piston at TDC is sticking above the block by .025" (.635mm)? The pistons are obviously not RB26 or the pistons would be recessed in the bore 1.5mm.

If that is the case, I would just install a thicker head gasket if you are uncomfortable with it. The squish/quench volume should only be up to 2mm deep (if I remember right) so we're not talking a huge amount of play here. With a 1.5mm aftermarket (non-Cometic) gasket, you have .865mm.

[Sil240]

If you need RB26 pistons there's a guy in Canada thats selling brand new ones for $450 I'm guessing Canadian.

[krayton]

doing some research im not too sure which part is screwed up. if the machine shop is correct and the pistons are the same as stock, then i can only guess either the rods or machining the crank caused this.

i think im gonna talk to him about HGs, so anyone know which the thickest aftermarket hg is?

[krayton]

sounds like some one shipped you 26 pistons...there is a diffrence in wrist pin location between the 2 even tho bores are identical...

so wait, are rb26 pistons gonna raise or lower compared to stock?

carl is saying one way and darius is going the other

[Sil240]

I tried looking it up for you but could not find anything.

Does the FSM have specs for it??I'm at work don't have it handy

[Darius]

The wrist pin in the RB26 piston sits about 1.5-2.0mm higher than the RB25 so the RB26 piston will sit lower in the bore at TDC with the RB25 crank.

Trust me, I have two sets of stock RB26 pistons and my stock RB25's. It can be found by the difference in stroke since the rod lengths for both motors are the same. If you want a set of RB26 pistons, I will give them to you for $120+shipping.

If you have aftermarket rods, I would double check those for accuracy. It's going to be hard to determine where the discrepancy is coming from, but I highly doubt that it's the bearings or crank machining since .025" is a ton of material in those areas. Nobody in their right mind would take that much off of a crank unless the journals were totally jacked up.

You do have a few options...

1) Run a slightly thicker head gasket 1.5-1.8mm2) Machine a tiny bit of your quench pads out of your head3) Get RB26 pistons and run 8.3:1 compression

[krayton]

thanks for the help guys.

im looking into different HGs, try to get something as big as possible.

sort of want to keep the pistons and rods since theyre forged.

[Darius]

Excellent choice.

[eh?]

Why do you think there's a problem?...

[Sil240]

"apparently my piston head is sticking pass the block deck by 25 thousandths. whats up with this?"

Pistons stay in the block.If they stick up that much valves GO BOOM.Compression ratio is CRAZY.There might be other BAD things too.

[Darius]

There's not so much concern with the valves hitting pistons as your valves should not be sticking out very far when the piston is at TDC. The true concern is the interference between the flat areas on the edge of the pistons coming in contact with the quench pads on the head.

[krayton]

Why do you think there's a problem?...

the machine shop is the one that brought this up. at first he thought i was trying to increase compression like crazy so he called me. but i told him in fact im actually lowering it.

hes worried that it is too high, in fact near the danger limit. hes things high RPM itll be able to reach the valves

[Darius]

0.025" is not going to cause piston to valve interference. Refer to my previous posts.

Tell him to quit being a ***** about the compression ratio too. The wisecos are 8.5:1 right? Stock is 9.0:1 right? fuggetaboutit. It's only dangerous if you don't tune it and run 87 octane gas at 16 psi, but only retards do that kind of thing.

[Sil240]

0.025" is not going to cause piston to valve interference. Refer to my previous posts.

Tell him to quit being a ***** about the compression ratio too. The wisecos are 8.5:1 right? Stock is 9.0:1 right? fuggetaboutit. It's only dangerous if you don't tune it and run 87 octane gas at 16 psi, but only retards do that kind of thing.

What????Hope your kidding bro. lol

Figure out what your Compression Ratio is going to be:

CR= .785*Bore^2 + Stroke ---------------------------------------------------------- Combustion Chamber Vol + HG Vol + Piston Vol

This webpage may help you:http://www.ajdesigner.com/phpe...o.php

[krayton]

talked to him today. he thinks (after measuring) that thats what its suppose to be.

he just wants me to make sure when i set it all up i crank it by hand to double check that nothing is hitting.

also he doesnt think that changing to something thicker will matter all that much since my cometic is close to the other sizes.

so thanks again guys, hopefully this will turn out ok

[Sil240]

Ask him for the measurements so you can calculate it all for yourself.Before you get it back and have to bring it right back to the machine shop.

Good Luck!!

[Darius]

Compression ratio is not going to make or break your project. You are still going to tune based on AFR and knock (possibly EGT) with your fuel and ignition timing maps.

BTW if you want to use Sil240's link, the stock RB25 has a piston deck volume of -11.5cc assuming a stock gasket thickness of 1mm (.785cc) and a stock CCV of 62cc.

As you can see, decreasing the effective head gasket thickness by .025" increases compression by 0.05. Not a huge deal.

[Sil240]

Compression ratio is not going to make or break your project. You are still going to tune based on AFR and knock (possibly EGT) with your fuel and ignition timing maps.

BTW if you want to use Sil240's link, the stock RB25 has a piston deck volume of -11.5cc assuming a stock gasket thickness of 1mm (.785cc) and a stock CCV of 62cc.

As you can see, decreasing the effective head gasket thickness by .025" increases compression by 0.05. Not a huge deal.

Haha I was thinking .250Reading works.SOMETIMES!!! lol

[Darius]

krayton - You should be good with the setup that you're proposing. When you install your timing belt, you will have to hand crank it anyways. I recommend a different brand of metal head gasket, but other people can get Cometics to seal. Keep us posted on how things are going.

Sil240 - I'll give you a call Monday or something and we can get together for some beers.

[BoostFab]

remember to plastiguage the piston & valves just to be sure