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Old 07-08-2012, 10:37 PM   #1
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Default Main Bearing and Ring end Gap Clearances

I'm doing a stock internal 1.8L rebuild prior to blowing some boost on it.

The block is from a '96 Miata.
The crank is from a '95 Escort GT and has been "polished" by a machine shop (they farmed it out to a crankshaft grinding specialist).
Standard size King bearings.

FSM states main bearing clearances should be 0.0007" to 0.0014" with a max spec of 0.004".

I plastigauged the mains and they are all at 0.002".

Is this too much? I know loose is good, but am I going to have any oil pressure at idle?

Did the polishing take off too much from the crank? Should I just slap it together and not worry about it?

Also - another question - this one about ring end gaps.

I have some Hastings chrome rings. I put them in the bore and pushed them down about half a piston length (loose - not attached to a piston). I used feeler gauges to measure the ring end gap. Both the top rings and the middle rings were in the 0.015" to 0.019" range.

The FSM states top ring 0.006" to 0.012", and middle ring 0.012" to 0.018".

So, it looks like the middle rings are pretty much in spec, and the top rings are really loose. But, is this just a Hastings thing? If the middle rings are close to spec, that means the bore can't be too big, and Hastings just makes the top ring gap bigger?

Maybe this is fine for boost? Do I want an even bigger gap for boost? I'm only planning on ~220-250hp.

Also, one of the Hastings rings was messed up right out of the box. It has like 0.250" ring gap, and you can see where a piece of the ring was broken off somehow. The rings were brand new still in the box, so weird.

I am going to call Hastings and see if they can send me one ring to replace it. I suppose I will ask them about the ring gaps at the same time.
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Old 07-09-2012, 10:34 AM   #2
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First.. are you building a stock engine? If yes, then stock clearances will work.

But sounds like you are doing a NA to turbo build.. co clearances need to be adjusted for the extra heat and thermal expantion.

.002 on the mains will be fine

What is the oil clearance on the rods?

Now for ring end gap

Look here and follow the formula for figuring your ring end gap.

Depending on your boost level you end up running, use the factor for "street-Moderate turbo" or for high boost levels use "Blown Race only"
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Old 07-09-2012, 10:57 AM   #3
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Main bearing clearances are fine. I had the exact same when I built my engine. My rod bearing clearances were 0.0018".
Ring gap (and piston-wall clearance) is determined by piston manufacturer.
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Old 07-09-2012, 01:31 PM   #4
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I haven't checked the rod bearing clearances yet. Will do that and report back. It is good to hear that they aren't completely out in left field.

So, based on the Wiseco website, it looks like they recommend these ring gaps:
Top Ring 0.016"
Middle Ring 0.018"

So, I might be looking pretty good already. I will double check them again and file fit if necessary.

Thanks guys.
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Old 07-14-2012, 09:17 AM   #5
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Well, I called Hastings about the bad piston ring that came in the box. Apparently, they don't ever deal with customers directly - only through distributors. I had bought these on ebay 6 months ago, so there was no way the distributor (some dude in his basement on ebay) was going to figure this out for me.

So, I had to order one ring directly from Hastings. $28 shipped for one piston ring. The entire ring set only cost me $50, so this was a little shocking. In any case, this was still the cheapest way to get me to a full set of functional rings.

Not sure I will ever use Hastings rings again, as this all has left a bad taste in my mouth.

Now to re-check the ring gaps.
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