Compression Tester for Flatheads with Limited Accessability

Production K Models

Compression Tester for Flatheads with Limited Accessability

Postby wz507 » Sun Dec 04, 2022 9:41 pm

In a recent thread here on the forum a comment was made about the difficulty of attaching a compression tester to a flathead engine due to space constraints between the cylinder head and gas tank. Thought this adaptor might be of interest as it is compact (about 2.75” length) and allows easy compression tests to be performed on a KHK. I suspect it would likely have broader applicability on flathead hogs than just the KHK.

The base is a sparkplug with the porcelain knocked out. A brass pipe coupling was silver-soldered to the base and a brass street elbow and quick disconnect added to the coupling. Inside the sparkplug base a length of valve stem with internal Schrader threads was soldered in place and a Schrader valve installed. That’s about all there is too it and it works well in this application with limited space/access.

LR valve body.jpg
LR valve body.jpg (101.02 KiB) Viewed 648 times

LR Schrader valve.jpg
LR Schrader valve.jpg (113.11 KiB) Viewed 648 times

LR compact compression tester.jpg
LR compact compression tester.jpg (136.89 KiB) Viewed 648 times
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Re: Compression Tester for Flatheads with Limited Accessabil

Postby Coolbreeze » Mon Dec 05, 2022 12:35 am

This setup is simple and slick. Making one of these might be a winter's project for me.
'56 KHK
'60 XLCH
'48 Chief
'42 Indian 841
'64 BMW R69S
'84 Moto Guzzi V65Sp
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Re: Compression Tester for Flatheads with Limited Accessabil

Postby chuckthebeatertruck » Mon Dec 05, 2022 6:13 pm

Just don’t be shocked when the gauge shows far less compression than you thought. . .
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Re: Compression Tester for Flatheads with Limited Accessabil

Postby Rubone » Tue Dec 06, 2022 12:14 am

wz507 wrote:In a recent thread here on the forum a comment was made about the difficulty of attaching a compression tester to a flathead engine due to space constraints between the cylinder head and gas tank. Thought this adaptor might be of interest as it is compact (about 2.75” length) and allows easy compression tests to be performed on a KHK. I suspect it would likely have broader applicability on flathead hogs than just the KHK.

The base is a sparkplug with the porcelain knocked out. A brass pipe coupling was silver-soldered to the base and a brass street elbow and quick disconnect added to the coupling. Inside the sparkplug base a length of valve stem with internal Schrader threads was soldered in place and a Schrader valve installed. That’s about all there is too it and it works well in this application with limited space/access.

LR valve body.jpg

LR Schrader valve.jpg

LR compact compression tester.jpg


Keep in mind that earlier SV engines all used 18MM plugs, so other than Ks that will only fit post '55 Servi-Cars although building one with an 18MM core wouldn't be too tough..
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Re: Compression Tester for Flatheads with Limited Accessabil

Postby wz507 » Tue Dec 06, 2022 6:46 pm

Rubone wrote:Keep in mind that earlier SV engines all used 18MM plugs, so other than Ks that will only fit post '55 Servi-Cars although building one with an 18MM core wouldn't be too tough..

Should be easy enough to adapt to the larger thread. Perhaps the hex on the adaptor would require thinning to keep the overall height appropriate for the application, or in the alternative the sparkplug based could be relieved above the top thread to allow deeper thread-in to control overall height.

sparkplug adaptor.jpg
sparkplug adaptor.jpg (50.55 KiB) Viewed 493 times
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Re: Compression Tester for Flatheads with Limited Accessabil

Postby wz507 » Tue Jan 31, 2023 3:21 pm

One other consideration for anybody considering making a related gauge is the type of Schrader valve employed in it. All Schrader valves are not created equal and compression testers employ a special Schrader valve with very low spring force on the valve core assembly. For instance, a K model might typically have 100 psi compression, but when a compression tester fit with a standard automotive Schrader valve core is employed to measure compression of such an engine, it might show 0 psi compression, because 100 psi is too low a pressure to unseat the valve core and allow air to enter the compression gauge. Special Schrader valves for compression testers have extremely weak springs on the valve, and thus allow very low pressure air to enter the gauge. I've tried several different automotive Schrader valves in my adaptor and recorded readings of anywhere between 0 and 100 psi on the same engine. With the correct Schrader valve, the compression was 110 psi every time.
Schrader valve for compression tester1.jpg
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