hello, I have some question for my new engine in these few month.


B18c run with 81.5mm ctr piston, that a high compression piston , it should be reach over 250 psi by compression test . But i Just got 15X-16X across four cylinder

I go back to machine shop and ask how many clearance over bore for the cylinder he did? he says over 0.05mm for it, i found that's the service limit in manual.

Now, Should I honed the cylinder one more time to go for 82mm piston? how about the reliability if I go 82mm. how many should I bore for the clearance.

Thanks for all give the help

 

0.05 mm is 0.0019 inches. I believe most people here shoot for 0.001 - 0.002 inches on aluminum cast pistons.

What were your ring gaps? If your piston to wall (ptw/p2w) is too large your ring gaps would also be large and out of spec. Search around for ring gaps. I believe the first ring gap should be around 0.014".

You can also go 82mm. I have ran 82mm b18c with no problems.

Good Luck.

 

hello , bros, thanks for your opinions
I has took the short block back to machine shop, the mechanic measured to the cylinder , he said the cylinder is fine, the clearance 0.03- 0.04mm only.
but I measured the piston ring end gap, it's not in spec, the first ring is 0.55 mm (near the service limit), the second ring is 0.85mm ( exceed the limit)

is that mean the issue is the ring set failure? ring set is NPR , I buy it a set with piston

thanks help

 

Okay looks like p2w is in spec. Yes your ring gaps are way too large. I have only used Hasting rings. All the Hasting rings I have bought were within spec or had to be filed to get the correct gap. I would recommend you get Hasting rings.

If it turns out your Hasting rings are also too large then your p2w is definitely too big.

 

thanks, i buy a set of hasting

 

If ring gaps are unusually large without filing, this is an indication that the bores are too big for the piston/ring combination being installed. It is also possible that the ring set you have is the wrong size for your application... as in too small.

OP, just so you are aware of the process... it is common that piston manufactures make their pistons SMALLER than the specified bore size. In fact, the piston diameter (measured at the specified point on the piston skirt) PLUS the "suggested" piston-to-wall clearance EQUALS the bore size... or VERY close to it.

Let's use a practical example: You are building an engine with a 81.50mm bore. The recommended piston skirt-to-wall clearance is .002". This means that the pistons will likely measure something very close to 81.45mm. IF a machine shop simply bores the block over .002" for a total of 81.55mm, the piston skirt-to-wall clearance will be .004-005"... which is WAY too big. This error in bore will be magnified by Pi... so a +.002-003" increase in bore translates into an increase in ring gap between .007-.010" !!!

 

JRCivic1, very good point.

I assumed everyone knew you need to measure the piston first then bore to spec.

 

thanks you for u guys help,, at least you're guided me the direction

 

I got a set hasting ring, I measured those of rings, first ring gap 0.43mm and second ring is 0.48-0.5mm , but oil ring gap clearance are too big 0.9mm, So I would like to try Hasting compression ring with NPR oil ring.
Do you think those of clearance will running normal??
and I seem the edge of design not same to the OEM , I cant find the edge on the second ring , there are just round of silver of the ring and no any words stamped on the ring.

 

First and second ring gaps look good. I can't remember on Hastings what tells you what side is up. It should have came with instructions. There might be a dimple in the ring that tells it is up or maybe it doesn't matter. Sorry I can't remember.

Yes oil ring gap is too large. 0.9mm is 0.035". I believe you want that around 0.020" or 0.5mm. I have never mixed and matched rings. Maybe someone else can chime in.

 

Yes, I dun know what happen in my case that's NPR compression ring gap too large and Hasting oil gap too large ,SO. I now to install combination hasting compression ring with NPR oil ring...HAHA

anyway, thanks for your help

 

Any luck with the mixing of rings? Update....