Piston to cylinder clearance

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I'm interested in how much piston to cylinder wall clearance some other people have, when boosting 30+ psi and fuel to mach?
The Bentley says .0012 inch new, and .003 inch wear limit.
Should the piston be looser in the boar to be able to grow with the heat and have room to exspand?

 

[rabbitman]

What Bentley is that out of? I'm like.....aaaaalmooost 100% sure the 77-84 Bentley says .001". I remember distinctly because, thinking diesel's were tighter than gassers I checked in both manuals and they were both .001" new and I can't remember the wear limit.

That said, you have a point, two years ago I was riding my snowmachine (600cc liquid) about 75-80mph for about 4 miles in a foot of fresh snow. All was good but I don't think the cooling system was up to the job, it siezed up so fast it skidded . I checked everything and it looked fine, after a couple minutes it loosened up, slowly got compression back and started up , I've driven it quite a bit since then it's got all it's power.

I think the pistons got too hot and expanded until they got stuck in the cylinder.

I'd feel weird rebuilding it much looser than the book calls for......

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Interesting about the ski. Was it a two stroke?

The measurments that I put up wear from the MK2, gray Bentley.
.0012 is just a RCH over .001 anyways. The worn out was .003.
 It would be nice to know what Aki-76, TurboJ and others are running for clearance.

I know that the milage aspect/ life exspectincy would be half, if over board, but this would be raced only.

[theman53]

.001" here for my build. Every book and place we looked the machinist said "yep .001 it is, I want to see it run though." He didn't think that there would be room for piston expansion, but these are 300.-- pistons...for 4 of them. I have bought 100.00 pistons for v8's and didn't have any toubles, but those were not turbo'd but still there are 4 more pistons there. I think that whatever VW is using in the piston alloy was designed not to grow a ton and that is why you see the .001 or .0012 . When the wear limit is .003 I wouln't want to go looser than .001" if possible, because you would just be that much closer to a rebuild.
It will wear in before it wears out

[Rabbit on Roids]

.001" here for my build. Every book and place we looked the machinist said "yep .001 it is, I want to see it run though." He didn't think that there would be room for piston expansion, but these are 300.-- pistons...for 4 of them. I have bought 100.00 pistons for v8's and didn't have any toubles, but those were not turbo'd but still there are 4 more pistons there. I think that whatever VW is using in the piston alloy was designed not to grow a ton and that is why you see the .001 or .0012 . When the wear limit is .003 I wouln't want to go looser than .001" if possible, because you would just be that much closer to a rebuild.
It will wear in before it wears out

thats what i was saying when this topic was brought up a couple weeks ago. you are basically building an engine to go less miles on purpose.

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True about breaking in just before its worn out. But, this engine will be used just for racing. So theres no need to worry about longevity. It'll probly blow up before that anyway.

Just tring to get an idea of what others are running for clearance on big pwr./boosted engines.

[truckinwagen]

there are lots of things done to racing motors that would never work on the street, some of which are done understanding that the motor will never break in properly.

look at ring porting,(drilling holes from the top of the piston into the upper ring land) it is done to force the ring outward and make a better seal, if the motor was broken in, there would be no real purpose. and if the motor ran long enough to break in, they would fill with carbon and stick the ring anyway.

race motors are purposely built to last as long as they need to, no longer.
for track cars that is a season, for drag racing that is one(or two) passes, for monster trucks they are built to last 90 seconds.

figure out how long you want the motor to last before a rebuild and go from there.

-Owen

[arb]

.001" here for my build. Every book and place we looked the machinist said "yep .001 it is, I want to see it run though." He didn't think that there would be room for piston expansion, but these are 300.-- pistons...for 4 of them. I have bought 100.00 pistons for v8's and didn't have any toubles, but those were not turbo'd but still there are 4 more pistons there. I think that whatever VW is using in the piston alloy was designed not to grow a ton and that is why you see the .001 or .0012 . When the wear limit is .003 I wouln't want to go looser than .001" if possible, because you would just be that much closer to a rebuild.
It will wear in before it wears out

Yeah, at 0.003" your pistons will be slapping the bore sides. I could only think of the extreme cold in Alaska as a reason for so much clearance. But if you are cold enough that you need 0.003" then it will likely be too cold to start anyway.

[truckinwagen]

when I had a motor that slapped its pistons(only when cold) it would get much worse the colder it got.

the piston has a higher expansion rate than the block, which means it will get tighter as it gets hotter and looser as it gets colder.

the motor that slapped when cold had pistons that were monumentally loose in the bores, I never measured the clearance, but you could rock them around in the cylinder with your hand and get them to "clunk" against the wall of the cylinder.

that and the skirts were heavily worn, like the bottom of the piston was .005 narrower than the crowns.

and it only made noise when bone cold

-Owen

[arb]

when I had a motor that slapped its pistons(only when cold) it would get much worse the colder it got.

the piston has a higher expansion rate than the block, which means it will get tighter as it gets hotter and looser as it gets colder.

the motor that slapped when cold had pistons that were monumentally loose in the bores, I never measured the clearance, but you could rock them around in the cylinder with your hand and get them to "clunk" against the wall of the cylinder.

that and the skirts were heavily worn, like the bottom of the piston was .005 narrower than the crowns.

and it only made noise when bone cold

-Owen

Yeah, I was thinking of all aluminum engines like aircraft engines... if they get cold enough, the pistions get stuck in the bore.

[sdwarf36]

I just did mine -it was my post a few weeks ago. I went with .0015. And even that makes me nervous. Of all the 100's of different things I've bored over the last 25+years, the only thing I've ever gone .001 is 4 stroke hondas under 100cc (and their bore is a little over 1/2 the VW's). I'm using Pro Topline pistons-if i had Mahles, i'd get braver. My thinking is the germans had a exact number in mind-for an exact conditon-not the day I try to make double the boost they had planned on-or the day the radiator hose pops + pins the gauge.   
 I was talking to my boss at the machine shop about tight clearances. He told my about if you try to build a stock Chevy 350 with .0015 that was called for on some brand cast pistons-and you bored+ honed the block the right way using a torque plate-when you go to assemble the motor, the pistons might not fit the bores from the difference in distortion from having the head bolts pulling on the deck to sitting there unstressed!  Not to worry with our motor-I measured things to death checking before+ after of torque plate/no plate. No change--but our head bolts pull from the bottom of the block-not threading into the deck.

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Well the pump is one Giles made for me.
I told him it will be used for racing. Full throttle all the time  . Well, most the time.
It will be intercooled, so that helps.
I think I'm going to go with the stock clearances, .001.
A loose engine is loose engine, and only making things more loose in my eyes.

[rabbitman]

Yeah, I was thinking of all aluminum engines like aircraft engines... if they get cold enough, the pistions get stuck in the bore.

Aluminum pistons will shrink more than the steel pistons making them looser as they get colder.

Back in the day when bush pilots were crazier and stayed the night out where they couldn't plug in they would drain the oil and bring it in the cabin for the night. Next day they'd get all ready to go, pour the oil back in and fire it up, about the coldest I've heard of is -60(ish)F .

[Fisher]

I fit all mine at .03mm, vw spec for new parts.  no problems ever.