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Old 02-02-2014, 03:55 PM   #1
lairdog
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Default Low Compression in one cylinder -fresh rebore

So My 93ci S&S panhead barrels were bored out to .20 over about 2,500
miles ago, & I did a compression check because the motor is getting difficult
to start. Front cylinder shows 110 rear cylinder shows 65

normally I need to give it all I got to kick the beast to life, But
with the low compression I can turn the motor with my hand!

I re torqued the heads after 50 miles & then again at 1,000 miles.
one of the head bolts on the rear barrel was definitely loose (maybe
40ft lbs). & was torqued to 70ft lbs.
now after 2,500 miles, I re check the torque on the head bolts, that
one bolt that was loose before was again loose like it was at 1,000 miles.
I am thinking the threaded insert may have slipped in the casting causing
the low compression, but the front cylinder even though it is considerably higher compression than the rear, (110) It should be closer to 130, so
I am wondering if the rings need more time to seat.
One guy told me that the rings on His panhead did not seat until 4,000 miles.

I also checked push rod adjustment each time that I did the compression check & They are seat correctly with no tightness at lower cam lobe position. The valve guides & valves are in excellent condition & well within spec.

the rings are S&S 94-1222X & were installed following S&S instruction sheet. 1 ring looks like hard chrome & the other looks like a molly.

also the oil consumption is much higher than normal, so it appears to be a piston ring issue rather than a bad head seal or cracked head etc.

I don't really want to pull this motor out again if it is just an issue of needed more time for the piston rings to seat properly.
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Old 02-02-2014, 04:12 PM   #2
Rob Axel
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Default Re: Low Compression in one cylinder -fresh rebore

do a leakdown test....it will let you know where you are loosing the pressure.
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Old 02-02-2014, 04:37 PM   #3
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Default Re: Low Compression in one cylinder -fresh rebore

First check ya pushrods for tappet clearence........easiest thing to check and if tight ya valves may be leaking...
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Old 02-02-2014, 07:23 PM   #4
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Default Re: Low Compression in one cylinder -fresh rebore

When you say you checked the pushrod adjustment does this mean that you un-adjusted them? Very easy to loosen the valve adjustment on the two with low compression and do a quick re-check.

Solid or hydraulic?

Next a leak down or just tear down and inspect.
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Old 02-02-2014, 07:25 PM   #5
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Default Re: Low Compression in one cylinder -fresh rebore

First off don't think the head bolt insert has moved, Odds are it would spin before you reached torque. And if you were leaking at the head gasket, it would be obvious. Secondly (and sadly) what you describe sounds very close to my tale of woe. Scorched a piston or pistons. Leak down will make it obvious. Either a vacuum leak, too lean jetting or heavy throttle hand, or all of the above
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Old 02-02-2014, 08:22 PM   #6
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Default Re: Low Compression in one cylinder -fresh rebore

S&S rings are really hard and not of the best quality in my experience,if the cylinders were not properly honed with a good stone hone and not a ball hone it will wipe them out quicker than shit,found that out on a shovelhead and it took three sets to get it,went with weisco rings and all was good
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Old 02-02-2014, 10:36 PM   #7
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Default Re: Low Compression in one cylinder -fresh rebore

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Originally Posted by cdan View Post
S&S rings are really hard and not of the best quality in my experience,if the cylinders were not properly honed with a good stone hone and not a ball hone it will wipe them out quicker than shit,found that out on a shovelhead and it took three sets to get it,went with weisco rings and all was good

Eh....not that I'm doubting your experience, but the last two S&S Sidewinder kits I used had Wiseco supplied Piston and ring sets. So I'm thinking (and I may be wrong) that they are one in the same.
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Old 02-03-2014, 12:49 AM   #8
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Default Re: Low Compression in one cylinder -fresh rebore

tappets are Jims hydros (powerglide) adjusted out 5 wrench flats, spin freely
by hand after each p-rod leaks down. no tightness no looseness set correctly.
has to be the damn rings.
the shop that did the boring & honing has a great reputation for doing
quality work & all they do is Harleys, & they supplied the S&S rings.

I squirted some 60 weight into each chamber & re checked compression,
& compression shot up on the rear barrel from 65 to 90.
Front stayed the same at 110.
I will try & borrow a leak down tool to confirm that the problem is rings.
if it is I will probably just pull the motor.
If the compression was 80 or 90 I would just ride it, but 65 is way low.
I was real careful installing the rings too, making sure that all gaps were
staggered, & the correct ring on top, etc.

I'm wondering if S&S or possibly the mechanic gave me the incorrect rings because
the last time I did a bore job on the motor the upper & middle (S&S) Rings were identical.
This set I have now, they are different. Both look to be cast iron, but one looks like it is molly coated, & the other is not.
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Old 02-03-2014, 01:48 AM   #9
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Default Re: Low Compression in one cylinder -fresh rebore

With that drastic a drop in compression, its going to be more than just rings in my experience. Rings not seated will cause excessive oil consumption, but by themselves will not cause an appreciable drop in compression. At least not to this degree.
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Old 02-03-2014, 06:32 AM   #10
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Default Re: Low Compression in one cylinder -fresh rebore

...any chance you may have forgot to stagger the ring gaps?....Question, would not staggering the gaps cause "this" much compression loss??
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Old 02-03-2014, 06:51 AM   #11
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Default Re: Low Compression in one cylinder -fresh rebore

We can guess all day, but a leak down test will tell you the facts. Much easier than pulling the motor, and tells you more about the problem, than inspecting the parts. You can make one, if you have a few things laying around. You need a air regulator and a fitting(weld a hose quick connector male end to a spark plug housing that has been gutted.) to go in the spark plug hole. Helps to have a piston stop, but not necessary if you do it in gear with rear wheel on floor. Start out with about 20 or 30 lbs of pressure, and listen. The leakage will tell you if the rings, ex valve or int valve is the culprit, even the head gasket, if that turns out to be the bad guy in the motor. It is not a perfect way, but much easier than trying to find a needle in a haystack.
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Old 02-03-2014, 07:16 PM   #12
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Default Re: Low Compression in one cylinder -fresh rebore

And make sure your front wheel is against a wall... Ask me how I know..
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Old 02-03-2014, 09:55 PM   #13
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Default Re: Low Compression in one cylinder -fresh rebore

Quote:
Originally Posted by gasketscraper View Post
Eh....not that I'm doubting your experience, but the last two S&S Sidewinder kits I used had Wiseco supplied Piston and ring sets. So I'm thinking (and I may be wrong) that they are one in the same.
you may be right,the last set I messed with was in the early 90s and they were an S&S brand,hope my two cents didn't throw you
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Old 02-04-2014, 03:22 AM   #14
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Default Re: Low Compression in one cylinder -fresh rebore

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Originally Posted by shovithead View Post
We can guess all day, but a leak down test will tell you the facts. Much easier than pulling the motor, and tells you more about the problem, than inspecting the parts. You can make one, if you have a few things laying around. You need a air regulator and a fitting(weld a hose quick connector male end to a spark plug housing that has been gutted.) to go in the spark plug hole. Helps to have a piston stop, but not necessary if you do it in gear with rear wheel on floor. Start out with about 20 or 30 lbs of pressure, and listen. The leakage will tell you if the rings, ex valve or int valve is the culprit, even the head gasket, if that turns out to be the bad guy in the motor. It is not a perfect way, but much easier than trying to find a needle in a haystack.
in a good motor how long should the air hold, before it leaks down?
I am going to try & test it tomorrow (tues).
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Old 02-04-2014, 06:15 AM   #15
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Default Re: Low Compression in one cylinder -fresh rebore

Your not looking at how l long it holds air but how much... The regulator will tell you the psi going into the chamber and the post gauge will tell you the psi of what is holding
Say for instance your compressor is set at 100 psi and that is the a mount you feed I to the spark plug hole... And the guage at the hole reads 70 psi... That cylinder is loosing 30 psi or 30% .. If you hear air leaking from the crankcase you have a piston or ring issue.. If is in the exhaust , it is a exhaust valve, ,..... From the carbs, a intake valve..
Check you tube there are a lot of videos that can explain it better... And making the tool isn't that hard... I think harbor freight has one for 30 bucks??
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Old 02-04-2014, 06:31 AM   #16
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Default Re: Low Compression in one cylinder -fresh rebore

Quote:
Originally Posted by lairdog View Post
in a good motor how long should the air hold, before it leaks down?
I am going to try & test it tomorrow (tues).
You are not looking for how long it last, but listening for leakage. Listen to the carb intake, for intake valve leakage, exhaust port for exhaust leakage, on both heads, crank vent for rings leaking, and soak the head gasket area with soapy water, to look for bubbles. A slight sound, is just that, a slight leakage. A wind tunnel sound, is a major leak. Hearing a slight leak, is usually normal, unless the rings, exh/intake, are all leaking on the same head. Piston has to be at TDC, with both valves closed, to do this test. Rear wheel on the ground, in 4th gear, or the compression will spin the motor and open a valve. Or a piston stop.
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Old 02-04-2014, 06:49 AM   #17
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Default Re: Low Compression in one cylinder -fresh rebore

This is one of many videos.....also
http://www.harborfreight.com/cylinde...ter-94190.html
you can make on from simple plumbing parts...
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