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poor compression

dellatorre6708
Posts: 62
Joined: Fri Oct 27, 2006 3:00 am

Re: poor compression

Post by dellatorre6708 »

Just an update. I screwed up and used the wrong hose for the leak down I used the compression tester hose with the valve that's why I thought I had a good test. Did another with the correct hose and have a large leak past the rings.
Frank,
NYC

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Flx48
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Re: poor compression

Post by Flx48 »

Hi Frank-
Your update is most appreciated.
It's a great example for the rest of us, of how a thread can be successfully concluded to the benefit of all.
Many lessons in life are learned from mistakes, (at least in my little world) and hearing of those made by others is always welcomed, if just for the off-chance my brain might actually be awake and listening..

Were it me, at this point I'd be following the advice given earlier by James- ride it hard a while and then retest, before considering other courses of action. (but do retorque heads/readjust valves)
Best-
George

dellatorre6708
Posts: 62
Joined: Fri Oct 27, 2006 3:00 am

Re: poor compression

Post by dellatorre6708 »

George,
I don't have much choice at this point. I've never had such low compression in a fresh motor. Guess there's a first time for everything. Will give an update after a few hundred miles.
Frank.
NYC.
Frank,
NYC

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Discogodfather
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Re: poor compression

Post by Discogodfather »

Were the cylinders tapered correctly? I think the spec is around .02mm from top to bottom. The piston gap gets smaller at the top of the travel, like a zero tolerance, if the gap is measured at .02-.04mm"at the bottom of it's travel with the correct cylinder taper.

Did you cross hatch it before the assembly, or was it as it was given to you by the shop?
1969 r60/2, 1972 r75/5, 1973 Norton Commando, 1974 Ducati 750 GT, 1966 Honda 450 Black Bomber, 1965 Honda Superhawk, 1971 Honda CB 750

San Francisco, CA

dellatorre6708
Posts: 62
Joined: Fri Oct 27, 2006 3:00 am

Re: poor compression

Post by dellatorre6708 »

New pistons and rings, sent to millennium tech for fitting. Cross hatch was done by MT. When I received them, checked ring gap and installed. This couldn't be more by the book. I'll ride it a bit and see if things improve.
Frank,
NYC

Andrew
Posts: 59
Joined: Wed Feb 24, 2021 5:38 pm
Location: England

Re: poor compression

Post by Andrew »

Hi , I was like you when I had R67 rebored and new pistons ...very poor compression, my engineer said give a hundred miles or so to settle in, did that, very little change but now after doing 400/500 miles the compression on both cylinders is 8 Bar 120psi which is fantastic , so I agree be patient and ride it for a while then check I'm sure it will be fine just as mine is . Andrew

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Micha
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Re: poor compression

Post by Micha »

Great Info.
I also do not think it is worthwhile to treat it too gently at first.
What we want to do in the running-in process, is wear-fit between the piston ring and the honing scratches on the cylinder walls.
Do not forget that the abrasion between them is not uniform as the piston rings are made of a different material than the metal of the bore.
Proper running-in will result in a better engine and better oil carry-over sealing.
Of course just as you push the engine, you also need to know how to give it rest and coolness by driving calmly here and there.
Michael Steinmann
R51/3 1952
Engine Nr. 529466

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jwonder
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Re: poor compression

Post by jwonder »

Not to get too much into engine break-in procedures, there are many schools of thought. I am of the school of thought that you drive/ride it hard right out of the box for many reasons. When we break a racing motor in on the dyno we start it and get it warm and then start doing pulls when it’s up to temp. We then flush the oil out and check for any debris. If it’s clear we continue with hard pulls.

So, no real break in other than changing and checking oil early and often.
James Wonder
Vice President, Vintage BMW Motorcycle Owners
2022 BMW Friend Of the Marque
Long Island, New York

bellsonracing
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Joined: Wed Nov 16, 2016 11:55 pm

Re: poor compression

Post by bellsonracing »

In my limited experience, when I replaced both pistons, and got barrels bored and plated, it took closer to 1000 miles for the rings to break-in. It had low compression and hard to start initially, and got better and better.

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Discogodfather
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Re: poor compression

Post by Discogodfather »

dellatorre6708 wrote:
Tue Jun 01, 2021 7:03 am
New pistons and rings, sent to millennium tech for fitting. Cross hatch was done by MT. When I received them, checked ring gap and installed. This couldn't be more by the book. I'll ride it a bit and see if things improve.
I hear you, but the problem is this- you have to check the piston ring gap at the bottom of its stroke and then compare that measurement to the reading at the top of the stroke. The gap goes to practically zero tolerance at the TDC. Also was there any way to check and verify that the cylinder boring was actually tapered correctly?
1969 r60/2, 1972 r75/5, 1973 Norton Commando, 1974 Ducati 750 GT, 1966 Honda 450 Black Bomber, 1965 Honda Superhawk, 1971 Honda CB 750

San Francisco, CA

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