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Author Topic: Big bore questions (72mm) edited version  (Read 1233 times)

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Offline Don R

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Big bore questions (72mm) edited version
« on: October 23, 2021, 04:47:37 PM »
   This old drag bike has 72 mm pistons, it also has a copper head gasket which I annealed and sprayed with copper coat then re-used. (old racer trick) During a leak down test on #3 I found air leaking between the 3 and 4 cylinder fins. Enough to blow around soap bubbles. 
   The piston was at TDC, the valves didn't seem to be leaking but it was leaking way over 20% at only 40 psi.
   The dyno cover was removed, so air wasn't coming from the crankcase.
   I'm thinking it was leaking at the top of the sleeve, probably due to my re-use of the copper head gasket. It appears smooth, has a circular pattern  from the sleeve flange. Anyway now I'm thinking of trying a mls gasket. Make sense?
   I understand this still does not seal the cylinder from crankcase pressure. The PO had tried to stuff O rings in the cylinder bottoms. This may explain why it was thrown back together and not properly re-assembled.
« Last Edit: October 23, 2021, 05:41:27 PM by Don R »
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Offline Don R

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Re: Big bore questions (72mm) edited version
« Reply #1 on: October 23, 2021, 05:45:12 PM »
 A trip to cyclex may be in order. I read at one time they could bore a cylinder for big sleeves and hone the cylinder to fit the sleeves properly, any breakthrough could be welded and re-honed.
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Offline dragracer

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Re: Big bore questions (72mm) edited version
« Reply #2 on: October 24, 2021, 07:02:23 AM »
Dang, if it's leaking between cylinders that's definitely not good.  You may want to run a straight edge across the surface of the head and then the block to be certain it's flat. A light resurfacing might be needed.. MLS won't fix imperfections.
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Offline MRieck

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Re: Big bore questions (72mm) edited version
« Reply #3 on: October 24, 2021, 07:30:04 AM »
Anytime sleeves are replaced the block has to be decked.....that's what I've found. Well worth the 50.00 or so.
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Offline Don R

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Re: Big bore questions (72mm) edited version
« Reply #4 on: October 24, 2021, 09:35:52 AM »
 This thing is a riot. If I was looking for a challenge, this would be the bike for me. I do appreciate the input, I noticed the sleeve/deck height isn't perfect and wondered how that can be.

  I've done enough big bore VW's I keep writing 92mm not 72mm. George discreetly sent me a YFU message. 
« Last Edit: October 24, 2021, 08:29:03 PM by Don R »
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Offline Tracksnblades1

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Re: Big bore questions (72mm) edited version
« Reply #5 on: October 24, 2021, 10:06:28 AM »
Quote from: Don R on October 24, 2021, 09:35:52 AM
This thing is a riot. If I was looking for a challenge, this would be the bike for me. I do appreciate the input, I noticed the sleeve/deck height isn't perfect and wondered how that can be.

Big Kitties (Caterpillar) specs no more than .002 adjacencies for their sleeves. I install them in a ramp instead of high low adjacently. Big Cube Old IH 800cu and plus, I can’t measure accurately enough to detect a difference. One is still in business though…
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Offline Don R

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Re: Big bore questions (72mm) edited version
« Reply #6 on: October 27, 2021, 09:51:09 PM »
Quote from: MRieck on October 24, 2021, 07:30:04 AM
Anytime sleeves are replaced the block has to be decked.....that's what I've found. Well worth the 50.00 or so.
After a good long look this one appears not to have been decked. Yet.
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