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Author Topic: Bump clearances  (Read 8558 times)

rleonard

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Bump clearances
« on: May 05, 2008, 07:34:33 PM »
I am to the point of setting the piston to head clearances on the 16-2 I am working on.  Per Lister specs, the clearances are different for the 6 and 12/2 and the 8 and 16/2.  They are .045-.050 and .075-.080 respectively. 

I was thinking about this.  The 8 and 16-2 do not have changeover valves and therefore have a fixed compression ratio.  Is this the reason for the different specs?

Bob
 
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Tom

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Re: Bump clearances
« Reply #1 on: May 05, 2008, 08:58:32 PM »
The 6/1 roid does not have a change over valve and uses the .045 to .50 spec too. So I would guess the difference may be to the use of aluminum pistons in the 8/1 expanding more.
Tom
2004 Ashwamegh 6/1 #217 - ST5 just over 3k hours.

rleonard

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Re: Bump clearances
« Reply #2 on: May 05, 2008, 11:30:06 PM »
Tom,
I thought that thermal expansion would not be a consideration.  So I did a little math.   The coeficiient of expansion is 24E-6 per degree C at 20C  So say a piston runs 150C and is 150mm tall, 150 X 150 X 24E-6 = .54mm or .020 inches.  I guess it does grow.

Thanks.

Bob
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Tom

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Re: Bump clearances
« Reply #3 on: May 06, 2008, 01:35:58 AM »
Gee, you did all that math, I just did a SWAG.  ;D
Tom
2004 Ashwamegh 6/1 #217 - ST5 just over 3k hours.

rleonard

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Re: Bump clearances
« Reply #4 on: May 13, 2008, 03:10:06 PM »
I was chasing differing clearance values on the right and left side of the piston.  They were all off .005 right to left.  Measurements were taken with lead solder over the wrist pin held in place with grease.   I even surface ground the heads to be sure that they were flat.

I learned from my Lister expert that he always averaged the readings to compare against the spec.  That done, I was withoin spec.

Bob
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Stan

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Re: Bump clearances
« Reply #5 on: May 13, 2008, 03:37:20 PM »
It's pretty easy to change bump clearances by just putting in or taking out paper gaskets from the bottom of the cylinder.  I'm experimenting with that right now.  I've been torquing to spec, waiting overnight and then retorquing before doing measurements.
Stan

Quinnf

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Re: Bump clearances
« Reply #6 on: May 20, 2008, 05:14:59 AM »
Bob,

While I was going through the Kit Engine build, http://www.utterpower.com/kit_engines3.htm I found that the piston crown was machined "off" 0.008" from one side to the other.  All the other dimensions were parallel to the bottom of the skirt.  Ring grooves and wrist pin hole.  But when I got to the top, it was off.  With a 'roid, that can be the reason for the bump clearance to be off from one side to the other.  I finally concluded that was the reason for the half-gaskets beneath the cylinder base that I and several others have found.  The Indian assembler was attempting to even out the bump clearance side to side, not knowing that by doing so he was cocking the cylinder off axis, setting up a lateral force on the piston and wrist pin.  All of which doesn't apply to your engine, but I thought I'd just mention it for benefit of any 'roid owners who find their clearance is off from side to side.

I can't imagine that would be possible with an English engine, but it's something to check just for grins.

Quinn
Ashwamegh 6/1, PowerSolutions 6/1 "Kit" engine, and a Changfa R175a that looks like a Yanmar I once knew

matt

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Re: Bump clearances
« Reply #7 on: May 21, 2008, 02:33:40 PM »
The 6/1 roid does not have a change over valve and uses the .045 to .50 spec too. So I would guess the difference may be to the use of aluminum pistons in the 8/1 expanding more.

Hi Tom,

I thought the aluminium pistons were only an Indian 'roid option for increaced temp caused by those units equipped with the Direct Injection heads.

Don't the 6/1 and 8/1 IDI units all have the same pistons?

Have I missed something along the way?

Matt

Tom

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Re: Bump clearances
« Reply #8 on: May 21, 2008, 04:39:24 PM »
I believe Dursley Listers started using aluminum pistons in the 8/1 & 16/2 due to the higher rpm.
Tom
2004 Ashwamegh 6/1 #217 - ST5 just over 3k hours.

oliver90owner

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Re: Bump clearances
« Reply #9 on: May 21, 2008, 05:49:18 PM »
Matt,

Cast iron/aluminium pistons do have a piston speed related argument for choice.  However, my 4 3/4" bore by 6 1/4" stroke tractor engine running at 1125 rpm at max power would indicate that CI is good for quite a few rpm above 650.  Can't remember, and am not looking for, the generally accepted cut-off point but suffice it to say that there were probably other reasons for changing to aluminium pistons. Reciprocating weight and minimal crankpin lubrication come to mind along with cost, cost, cost.  CI was probably only kept for replacements due to minor changes in balance or problems of running aluminium pistons in Listard cylinders (if there is an issue).

Regards, RAB