175hp cylinder walls and absurd compression results

racerone

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Your measurements are questionable.-----The standard bore from the factory is 3.500" and after use you might find 3.502 or thereabouts.------Temperature does change the dimensions.----Highest cylinder is always #1 so the top on the starboard side is #1.
 

pastorbud

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Your measurements are questionable.-----The standard bore from the factory is 3.500" and after use you might find 3.502 or thereabouts.------Temperature does change the dimensions.----Highest cylinder is always #1 so the top on the starboard side is #1.

I thought the same thing on the 3.5”.

Will try a different cylinder. If that reads outa whack too, I’ll know I’m off base. What is it with me and absurd readings on this project?
 

pastorbud

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Second cylinder came in at 3.491 (at the bottom of the stroke, vertical) with the telescoping tool and caliper

I checked the caliper against some other known measurements and it was right on.

Even just measuring with the inside prongs at the cylinder head, it’s under 3.5”.

Is it possible someone Re-sleeved it and bored it under? Doesn’t sound sensible.
 

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flyingscott

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Take your block to a machine shop and let them measure the bores. They will also tell you what you need to bore the motor out to. Plan on a complete rebuild.
 

Fed

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I'm betting your 'touch' on the telescopic tool & caliper is not positive enough PB.
What I mean is your adjustments are a whisker lose.

I'd measure the rest of them before thinking about a re-build.

Pick up a cheapo compression tester.
 

pastorbud

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I'm betting your 'touch' on the telescopic tool & caliper is not positive enough PB.
What I mean is your adjustments are a whisker lose.

I'd measure the rest of them before thinking about a re-build.

Pick up a cheapo compression tester.

That’s what I’m leaning toward. I already have a compression tester, but I wasn’t using that right either! That is actually what started this particular adventure.

I had already ordered new head gaskets, so it is at least worth measuring all the cylinders as best I can, reassembling and re-testing.

I will see if I can get one of my mechanic friends down here to show me what I may be doing wrong with the telescoping cylinder diameter gauge. I know it is hard to get it centered and squared up, but I would expect an error like that on my part to give an over reading, not an under. I have it right on top of the piston in the center.

I’ll take a picture if I can.
 

oldboat1

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Why don't you get one of your buds to check/confirm compression for you before getting any deeper into it.
 

pastorbud

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Why don't you get one of your buds to check/confirm compression for you before getting any deeper into it.

Well, the heads are already off. It might have been a good idea to get help with the compression test first, but I really thought I had that right. I’ve done a lot of those before, after all.

So off came the heads!

How do those cylinder pics look to you guys, BTW?
 

Fed

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Are the rings free, can you wriggle all the pistons side to side a little?
 

Faztbullet

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The dark areas around port are wear....and you gave it a good try but you need inside mikes and pistons removed to check cylinder. You only measured the upper edge of cylinder that doesn't wear.
 

pastorbud

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Are the rings free, can you wriggle all the pistons side to side a little?

I checked the rings through the bypass cover and they looked good. I did not try to wiggle the piston. Should I try to wiggle it at the top, middle or bottom of the cylinder? Does it matter?
 

pastorbud

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The dark areas around port are wear....and you gave it a good try but you need inside mikes and pistons removed to check cylinder. You only measured the upper edge of cylinder that doesn't wear.

Thanks for checking out the pictures and analyzing. But I didn't measure at the upper edge of the cylinder. I know it's normal for there to be a "ridge" at the top.

I measured about 1/4 inch from the top, below the part that doesn't wear. I measured the top of the area that the rings touch and the bottom of the area that the rings touch.

I'll take pictures or videos soon to show how I'm doing the measuring and maybe someone can help me understand a better way.
 

racerone

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The pistons are cold.--------When they heat up they expand and the fit is different.----Even new pistons are tapered with clearance of about 0.010" near the rings.----About 0.003/4 at the bottom of the skirt.
 

pastorbud

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The pistons are cold.--------When they heat up they expand and the fit is different.----Even new pistons are tapered with clearance of about 0.010" near the rings.----About 0.003/4 at the bottom of the skirt.

Well, that’s a relief!
 

pastorbud

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Here’s the best video I could get of how I’m measuring the cylinders. In this case I used the bolt holes to get a straight diagonal across the cylinder, near the bottom of the stroke.
 

pastorbud

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This is how I’ve been locking and removing it for measurements. Hard to lock and video with one hand.
 

pastorbud

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Then I zeroed the caliper (not actually a micrometer) and got 3.497 inches. Maybe my caliper is reading under?
 
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