Max piston to wall clearance on Wiseco 2618 pistons?
11-12-2013, 12:03 PM
#1
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Max piston to wall clearance on Wiseco 2618 pistons? Short question:
Is .0065 too much clearance for a 2618 piston that will see 15psi of supercharged boost and make ~750whp?
More info and details:
Pistons are Wiseco 4.035 bore with coated skirts. Wiseco tech recommends .0040 for my application, which is also their min clearance. The piston is designed for this .004 clearance before the skirt coating, which is ~.0005, so piston should measure 4.0320 assuming +.5 thou for coating on each skirt.
Onto my measurements: Piston = 4.0320 with outside micrometer; With bore gauge zero'ed at 4.0320 with same micrometer, I get .0065-.0068 clearance across all 8 bores. Machinist said the clearance was .0050 when I picked it up.
Block was honed without torque plates. My measurements were made without the mains or heads torqued, ie bare block. Cylinders were round with the exception of a slight hourglass shape on the pin side of the pistons of -.0005.
Wiseco tech didn't come out and say to add any additional clearance above the min .004 for my 15psi of boost but agreed that .0050 would have been ok but .0065 would have ring flutter and wear badly. However, other piston manufacturers, such as Diamond and JE, clearly state on their spec sheets a min .0040 clearance and additional .001-.003 for boosted applications on their similar 2618 slugs.
Is .0065 too much clearance for a 2618 piston that will see 15psi of supercharged boost and make ~750whp?
More info and details:
Pistons are Wiseco 4.035 bore with coated skirts. Wiseco tech recommends .0040 for my application, which is also their min clearance. The piston is designed for this .004 clearance before the skirt coating, which is ~.0005, so piston should measure 4.0320 assuming +.5 thou for coating on each skirt.
Onto my measurements: Piston = 4.0320 with outside micrometer; With bore gauge zero'ed at 4.0320 with same micrometer, I get .0065-.0068 clearance across all 8 bores. Machinist said the clearance was .0050 when I picked it up.
Block was honed without torque plates. My measurements were made without the mains or heads torqued, ie bare block. Cylinders were round with the exception of a slight hourglass shape on the pin side of the pistons of -.0005.
Wiseco tech didn't come out and say to add any additional clearance above the min .004 for my 15psi of boost but agreed that .0050 would have been ok but .0065 would have ring flutter and wear badly. However, other piston manufacturers, such as Diamond and JE, clearly state on their spec sheets a min .0040 clearance and additional .001-.003 for boosted applications on their similar 2618 slugs.
11-12-2013, 02:42 PM
#5
IMO, that's too loose. I've used .004" for engines all the way up to 850whp.
Coating the pistons is an option as well. Polymer Dynamcis (PolyDyn) here in Houston charges about $150 for a set of 8 pistons and they can coat the skirts about .001" thick for a total clearance reduction of .002".
Coating the pistons is an option as well. Polymer Dynamcis (PolyDyn) here in Houston charges about $150 for a set of 8 pistons and they can coat the skirts about .001" thick for a total clearance reduction of .002".
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11-12-2013, 03:53 PM
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Thanks for the opinions guys, that is what I was afraid of. 
Unfortunately I could not find a local machine shop that had LS torque plates. I was skeptical but went with it hoping for the best. Machinist had the pistons and was told of Wiseco's recommended .004 clearance. I was told the clearance was .005 when I picked up the block and pistons from the shop.
When the bores 'distort' once mains and heads are torqued, in which direction do they move? Do they twist? Bulge? Elongate? I am using studs for mains and heads which I read don't distort the block as bad as bolts do. I will toque down the mains and a set of heads to take measurement from the bottom to see the difference. What should I expect to see, smaller bores or inconsistencies in roundness?
When the bores 'distort' once mains and heads are torqued, in which direction do they move? Do they twist? Bulge? Elongate? I am using studs for mains and heads which I read don't distort the block as bad as bolts do. I will toque down the mains and a set of heads to take measurement from the bottom to see the difference. What should I expect to see, smaller bores or inconsistencies in roundness?
11-12-2013, 04:12 PM
#9
I assume you are NOT going to put this together? Anything over .005 I wouldn't waste your time. I'd take the entire mess back and stick it down his throat! Make the guy show you its right!
I've seen this so many times only worse, I actually seen a machine shop double the piston clearance thinkin it meant .004 all around! That's .008 plus clearance. The engine ran so bad it was insane.
At almost .007 its going to sound like 8 hammers and blow down the crankcase, It won't be long before its in pieces.
In retrospect it would probably be cheaper to get a short block that done right from one of the sponsor's.
I've seen this so many times only worse, I actually seen a machine shop double the piston clearance thinkin it meant .004 all around! That's .008 plus clearance. The engine ran so bad it was insane.
At almost .007 its going to sound like 8 hammers and blow down the crankcase, It won't be long before its in pieces.
In retrospect it would probably be cheaper to get a short block that done right from one of the sponsor's.
Last edited by RockinWs6; 11-12-2013 at 04:18 PM.
11-12-2013, 04:24 PM
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As of now, no. I want to get all the information I can before I take it back and ask him to measure it with me using his tools and mine. If my measurements end up being correct, hopefully Wiseco will take the pistons back and credit me towards a custom set.
11-13-2013, 06:08 AM
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Sounds like none of you guys have ever torn down a badly worn engine. I've torn down 350's and 454's that had such a big ridge at the top of the bore that it broke rings when I knocked 'em out. Still running fine, not smoking, no outward signs of wear. One standard bore block was so bad gone that a .040 overbore wouldn't clean it up. Still running fine, just a bit of clatter at start-up...which seems standard for LS engines.
I ain't saying I'd take this as-is, just that a couple of thousandths isn't going to make the engine run like ****. The pistons don't seal against the cylinder wall, that's the job of the rings. Boosted, you'll want a bit extra anyway. And an extra .0015 isn't going to make it rattle a damned bit worse. Skirt coatings would be the way to go, IMHO.
Or wait 'till the engine has about 30K on it, you'll have just about .0015 extra wear - is it going to be so bad that you'll have to tear it down again and rebuild it?
This is cylinder-to-piston clearance, not friggin' bearings or valve lash we're talking about. A thou and a half over ain't ****.
I ain't saying I'd take this as-is, just that a couple of thousandths isn't going to make the engine run like ****. The pistons don't seal against the cylinder wall, that's the job of the rings. Boosted, you'll want a bit extra anyway. And an extra .0015 isn't going to make it rattle a damned bit worse. Skirt coatings would be the way to go, IMHO.
Or wait 'till the engine has about 30K on it, you'll have just about .0015 extra wear - is it going to be so bad that you'll have to tear it down again and rebuild it?
This is cylinder-to-piston clearance, not friggin' bearings or valve lash we're talking about. A thou and a half over ain't ****.
11-13-2013, 08:11 AM
#14
Sounds like none of you guys have ever torn down a badly worn engine. I've torn down 350's and 454's that had such a big ridge at the top of the bore that it broke rings when I knocked 'em out. Still running fine, not smoking, no outward signs of wear. One standard bore block was so bad gone that a .040 overbore wouldn't clean it up. Still running fine, just a bit of clatter at start-up...which seems standard for LS engines.
I ain't saying I'd take this as-is, just that a couple of thousandths isn't going to make the engine run like ****. The pistons don't seal against the cylinder wall, that's the job of the rings. Boosted, you'll want a bit extra anyway. And an extra .0015 isn't going to make it rattle a damned bit worse. Skirt coatings would be the way to go, IMHO.
Or wait 'till the engine has about 30K on it, you'll have just about .0015 extra wear - is it going to be so bad that you'll have to tear it down again and rebuild it?
This is cylinder-to-piston clearance, not friggin' bearings or valve lash we're talking about. A thou and a half over ain't ****.
I ain't saying I'd take this as-is, just that a couple of thousandths isn't going to make the engine run like ****. The pistons don't seal against the cylinder wall, that's the job of the rings. Boosted, you'll want a bit extra anyway. And an extra .0015 isn't going to make it rattle a damned bit worse. Skirt coatings would be the way to go, IMHO.
Or wait 'till the engine has about 30K on it, you'll have just about .0015 extra wear - is it going to be so bad that you'll have to tear it down again and rebuild it?
This is cylinder-to-piston clearance, not friggin' bearings or valve lash we're talking about. A thou and a half over ain't ****.
The normal wear you speak of is completely different. The entire piston and bore is not worn to .007 over..........
I also would NOT want coated pistons in a street car. There no logical reason to accept anything but what you paid for. Luckily you checked your clearances or it would be I JUST BUILT A ENGINE AND IT USES OIL AND CLATTERS LIKE SANTA's REINDEER ON THE ROOF!
I don't know what your engine would sound like at .007 but the engine I heard run at .008 was INSANE, no one would run but for more than a few mins.
Last edited by RockinWs6; 11-13-2013 at 08:16 AM.
11-13-2013, 10:01 AM
#17
When the bores 'distort' once mains and heads are torqued, in which direction do they move? Do they twist? Bulge? Elongate? I am using studs for mains and heads which I read don't distort the block as bad as bolts do. I will toque down the mains and a set of heads to take measurement from the bottom to see the difference. What should I expect to see, smaller bores or inconsistencies in roundness?
when the plate was put on, and this was only the top 1-1.5 inches of the
bore. Remember these engines pulled threads from the deck. One of GMs
reasons for having the head threads near the bottom of the cylinder barrels
was to minimize bore distortion so tightening up the PTW clearance wouldn't
cause interference issues. It also allowed the low tension/less friction rings to
save fuel (better mpgs) and use less oil.
It's not uncommon for shops to add a full thou clearance to be safe and few
shops let the block cool for measuring when creaping up on the number. So
the block gets heated (friction from honing) and the fourth cylinder on a
bank will measure the same as the first but shrink when cooled as the temp
of the block has now been elevated. So they just add a thou and ship it.
In your case creeping up on 6 is just about a full 1 more than tolerable for a
boosted app. IMO.
11-18-2013, 03:47 PM
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For an update, I torqued the mains, took measurements then torqued heads and took another set of measurements. I was getting hopeful after the mains as the clearances started looking better but actually got worse after torquing the heads. The tightest clearances are .0061 thrust/.0062 90* to a loosest of .0069 thrust/.0072 90* with the majority of the clearances falling in the .0064-.0066 range. Here are the actual measurements I got...
12-16-2014, 07:33 PM
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my ls3 build Im in similar situation right now
416 ls3 aluminum block 914 rwhp 20lbs
im putting wisco pistons in for the cheap 4032 mayle
4.070 bore
machinist honed the block with plates and has .006 piston to wall
sending pistons back for 4.075 pistons..going to rehone for. 004 clearence he prefers
416 ls3 aluminum block 914 rwhp 20lbs
im putting wisco pistons in for the cheap 4032 mayle
4.070 bore
machinist honed the block with plates and has .006 piston to wall
sending pistons back for 4.075 pistons..going to rehone for. 004 clearence he prefers
