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Old 04-02-2010, 05:28 PM
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Bore size confusion

I just decided to measure the bore size on my 383 short motor (supposed to be 40 over) and it measured 4.035" using my dial caliper. The cylinders have been finish honed. The pistons are JE/SRP 138094.

Does this sound right?? I was expecting the bores to be exactly 4.040".
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Old 04-02-2010, 05:45 PM
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Something doesn't sound right, I would measure the piston diameter to find out what size the bore should be. measure the piston across the skirts at the same level as the wrist pin, then add the recommended clearance figure from SRP for those pistons to determine what the block bore size should be.

Most piston companies size pistons to fit with the correct clearance at the nominal overbore size, so a 4.040" overbored block would have 4.033-4.035" piston measured diameter to fit in the .040" overbored hole at the correct clearance, for example, but pistons can also be special ordered in any overbore size you may want so it could be a 4.035" bore with a 4.030-ish" piston.
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Old 04-02-2010, 05:49 PM
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I see SRP says this is for a 4.040" bore, maybe you need to verify your measurement of the bore and make sure you got it measured correctly.
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Old 04-02-2010, 06:08 PM
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Quote:
Originally Posted by v8hed
I just decided to measure the bore size on my 383 short motor (supposed to be 40 over) and it measured 4.035" using my dial caliper. The cylinders have been finish honed. The pistons are JE/SRP 138094.

Does this sound right?? I was expecting the bores to be exactly 4.040".
Be sure the calipers are zero'ed. You can get a more accurate measurement using a snap gage in the bore, and measure THAT w/the calipers. Won't be perfect, but should be better than just using the calipers in the bore.
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Old 04-02-2010, 06:12 PM
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Thanks guys. Think I'm gonna stump-up for a dial bore gauge to make a reliable measurement possible. Using calipers isn't ideal. I've also got my eye on a good used micrometer set on eBay with calibration pieces. I do hope I haven't got a mish-mash of bore size vs pistons. The pistons were installed (with rings) in the block when I got it, but don't suppose that proves anything.
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Old 04-02-2010, 06:17 PM
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Quote:
Originally Posted by v8hed
Thanks guys. Think I'm gonna stump-up for a dial bore gauge to make a reliable measurement possible. Using calipers isn't ideal. I've also got my eye on a good used micrometer set on eBay with calibration pieces. I do hope I haven't got a mish-mash of bore size vs pistons. The pistons were installed (with rings) in the block when I got it, but don't suppose that proves anything.
If the pan's not on it, pop one or two of the rod/pistons out and check them over for piston clearance, ring end gap, bearing clearance, etc..

Might want to do that anyway, pan or not- if you didn't build it. Unless you know and trust the builder...
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Old 04-02-2010, 07:34 PM
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Quote:
Originally Posted by cobalt327
If the pan's not on it, pop one or two of the rod/pistons out and check them over for piston clearance, ring end gap, bearing clearance, etc..

Might want to do that anyway, pan or not- if you didn't build it. Unless you know and trust the builder...
Oh, I've completely torn apart the motor for exactly that reason. Already, I've found the brand new crank in need of grinding (the guy who assmebled it didn't rifle brush the oil holes by the looks of things and I found a badly scored journal). Also, I had the rods checked and there are issues with them too. All I need now is for there to be a mismatch between the machined bore size and the supplied (forged) pistons! Don't think I'd ever purchase an engine I hadn't built myself again, or an engine that I can't see/hear running. My experience, so far, suggests it's a false economy no matter how good a buy you think you're getting.
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Old 04-08-2010, 01:03 PM
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Thought I'd update this thread as I've now acquired a cylinder bore gauge and a 4"-5" mic.

Pistons (JE P/No. 138094 forged) measure 4.0370" to 4.0375" measured ~.500" from the end of the skirt.

Cylinders measure 4.0402" to 4.0407".

So, my piston to wall clearance is .0027" (tightest) to .0037" (loosest), or, expressed another way .0032" average between all cylinders.

Sound about right? I don't have the JE spec sheet, but I read elsewhere that JE quote minimum clearance as .0025-.0035". I need to double-check that though.

Thanks.

Last edited by v8hed; 04-08-2010 at 03:04 PM.
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Old 04-08-2010, 01:35 PM
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Now, you're cookin' with gas......as my dad used to say.. He was raised on the farm with a wood stove, so when he was able to move up to a gas stove, that was a big deal.
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Old 04-08-2010, 01:38 PM
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Here's JE's data sheets, save you some time researching...

I had to shrink them, you can expand them if needed.
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Old 04-08-2010, 02:20 PM
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Quote:
Originally Posted by v8hed
Thought I'd update this thread as I've now acquired a cylinder bore gauge and a 4"-5" mic.

Pistons (JE P/No. 138094 forged) measure 4.0370" to 4.0375" measured ~.500" from the end of the skirt.

Cylinders measure 4.0402" to 4.0407".

So, my piston to wall clearance is .0027" (tightest) to .0032" (loosest), or, expressed another way .0032" average between all cylinders.

Sound about right? I don't have the JE spec sheet, but I read elsewhere that JE quote minimum clearance as .0025-.0035". I need to double-check that though.

Thanks.
In the case of piston to wall clearance, the average isn't what you're looking for. I'm not quite sure how you have a minimum of 0.0027" and a maximum of 0.0032" and still have an average of 0.0032".

Did the machine shop have the spec sheet when they did the bore/hone?

In any event, it's the individual cylinder/piston combination that needs to be within spec. Sometimes this means juggling the pistons to match the holes, although I always ask the shop to mark the slugs to the hole they belong to, then double check that. This only goes so far- it's used to 'fine tune' the clearances- IF needed- not to make up "large" differences.

If the pistons are already on the rods, this limits the cylinders where the pistons can go (each rod/piston assembly can go into one of four cylinders), because the rods have to stay in their proper relationship- if not the exact hole they came from. The exact hole isn't as important- as long as the rod side clearance and bearing tang/bevel orientation is correct.

Last edited by cobalt327; 04-08-2010 at 03:01 PM.
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Old 04-08-2010, 03:08 PM
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Quote:
Originally Posted by cobalt327
In the case of piston to wall clearance, the average isn't what you're looking for. I'm not quite sure how you have a minimum of 0.0027" and a maximum of 0.0032" and still have an average of 0.0032".

Did the machine shop have the spec sheet when they did the bore/hone?

In any event, it's the individual cylinder/piston combination that needs to be within spec. Sometimes this means juggling the pistons to match the holes, although I always ask the shop to mark the slugs to the hole they belong to, then double check that. This only goes so far- it's used to 'fine tune' the clearances- IF needed- not to make up "large" differences.

If the pistons are already on the rods, this limits the cylinders where the pistons can go (each rod/piston assembly can go into one of four cylinders), because the rods have to stay in their proper relationship- if not the exact hole they came from. The exact hole isn't as important- as long as the rod side clearance and bearing tang/bevel orientation is correct.

Sorry, there was a typo in my post... the largest clearance was .0037".

Pistons are off the rods.

Looks like I may have to have the cylinders honed +.001" as .0027" is .0008" tighter than the minimum clearance stated by JE.
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Old 04-08-2010, 03:09 PM
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Quote:
Originally Posted by 68NovaSS
Here's JE's data sheets, save you some time researching...

I had to shrink them, you can expand them if needed.

Thanks for that! Are these specs common to all JE forged pistons?
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Old 04-08-2010, 03:10 PM
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Quote:
Originally Posted by techinspector1
Now, you're cookin' with gas......as my dad used to say.. He was raised on the farm with a wood stove, so when he was able to move up to a gas stove, that was a big deal.
We use that expression here too
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Old 04-08-2010, 03:23 PM
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Ah, I've just noticed that the specs kindly posted by 68NovaSS relate to 2618 alloy. My 138094s are made of 4032 alloy, which I understand is less prone to thermal expansion and, therefore, can run with tighter piston to wall clearances.

So, my .0027" to .0037" clearance is probably about right on the money.

Edit: Just found the spec sheet...


Last edited by v8hed; 04-08-2010 at 03:30 PM.
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