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Ironhead Sportster Motorcycle Talk (1957-1985) For all those that wanna talk about Ironhead Sportster Motorcycles

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  #1  
Old 1st July 2022
Doc308 Doc308 is offline
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Default Compression test Q

What is the acceptable range of values in compression testing a '69 XLH with stock pistons?
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Old 2nd July 2022
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Best I ever managed with stock running P cams was 165.

But I test mine on starting rollers. Hard to just kick an engine over fast enough to get a "perfect" reading.

I'm happy with 150, and 140 is still ok.

I did have a 70 CH running with just 90 but it smoked - a LOT.
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Old 2nd July 2022
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You need 90 for it to fire reliably.
Also look for a difference F&R, both should be within ~10%.

On an old motor that hasn't been run, any compression is good :>)
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Old 2nd July 2022
needspeed needspeed is offline
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Compression test numbers for an ironhead vary with opinions and what book they're read from.

A clymer manual states normal reading to be 150 to 170 at sea level. It has been posted that Harley says 120 is a minimum.

If possible the test should be done with engine warm and throttle open full. Both plugs out. To be safe with electronic ignition, ground the plug wires. The valves should be adjusted so that the pushrods spin even when warm. Electric start bikes may acheive a slightly higher number than kick start.

Often overlooked is the affect of elevation on the pressure.

The chart shows correction factors to adjust for altitude. As an example, I live at 1500 ft above sea level. If my bike tested at 150 psi at sea level and was re tested at my house it would measure 144 psi. (.960 x 150 = 144) That is a significant difference but the engine condition is the same.

Conversely you can divide by the factor to go the other way. (144 / .960 = 150)

If I measured 140 at home the reading at sea level would be 145.8. Personally I would be happy with that on a stock P cam engine.

I think many get too concerned with compression readings on street ridden bikes. Personally if I had 100 or better with less that 10% variation between cylinders, and the bike was running well I would leave it alone. Most people that test and expect to see 150 will be disappointed.

But that's an opinion, others might not agree.

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Needspeed has certainly given you the drum.

The 165 PSI I got was on a freshly built race engine. I go to great lengths to bed in the rings and am a bit extreme with honing and tolerances.

At that time in Lived in Donnybrook, 3 foot above sea level. Air temperature would have been around 80 F, no idea what the barometric pressure was but most likely close to 1000.

The 90 PSI 70 CH I could not start except on starting rollers. But No one had done engine maintenance of any kind on the bike for years. From memory there was less than 5% difference between those poor sad cylinders.
That bike smoked so badly I got a letter from the EPA.

And not all compression gauges will give you the same reading. But unless they are totally RS it will be close enough for most things. I have 3 and swap them if I get a reading which seems "odd".
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Old 5th July 2022
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Thank you all for the input. I just did a test and came up with 150 and 145 psi.
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