What Compression Tester to Get

"The orifice in this test unit and the gauges on it aren't suitable for Rotax. Toss the test unit. "

Alternatively fill the orifice with HD epoxy - using a 1mm drill bit make a 1mm hole.  The 3 mm specification is unlikly to be met, however as I keep emphasising this is a comparative test ie the more you do over time (each 100 hr service) the more relevant the information. This is the same principal used for testing/analysis of the crankcase oil.

Sure you can compare your individual test readings with a Rotax standard but this is always going to be subject to any variations delivered by technique & the test equipment.

Reducing the variables is key, testing by the same equipment is more important that equipment accuracy.😈

Sean

I think you misunderstand the purpose of the orifice requirements.  The readings as given in the Rotax manual are so you can compare your engine to factory callout standards.  Rotax did the verifications using standard aircraft specification leak down test equipment.  The main variable is they use metric and the aviation standards were outlined decades ago by industry.  For example the orifice in a standard aviation tester is .040 inch by .120 inch.  The metric call out is basically the same, 1 mm by 3 mm.  The use of 80 psi air pressure does not translate well into a nice European number, where they tend to use BAR pressure rather than psi, Rotax calls out 6 BAR which is much higher than the normal 80 psi used in other engines.  The significance of the orifice by diameter AND length is what controls the bleed.  Larger orifice or shorter length will change the bleed.   The significance of the call out in the manual is to use norms that other aviation maintenance providers have or can obtain.  The manual expressly states that standard practice for maintenance methods from AC 43-13B are acceptable.

While you are correct in that this is a comparative test for some reason you do not think it is worthwhile to use the tooling called out so your numbers will be "comparable" to the Rotax values.  I am a bit lost on the logic that you want to "reduce variables" but start with different equipment that would negate comparison to what is in the manual. 

Cheers

RW,

I understand and agree, somewhat,  with what you have  written.

My position is simply;

Conforming with Rotax specifications, for this test instrument, is a costly exercise for the home builder/mechanic. Costly in this context covers acquisition and very low usage (cost : effect).

As this is primarily a comparative test ie the longer the history, the better the analysis, it seem to me to be more important that the same technique and test instrument is used at every leak down, recording the results for future comparison, than conforming exactingly with Rotax tool specifications

Purchasing a cheap leak down tester, modifying it (to come close to Rotax specifications) and then using the same tool for all tests (100 hrs) just seems to make sense for the owner/maintainer, who is just looking after the one engine.

The cost:effective alternative, is for owner maintainers to band together (Club) and purchase expensive equipment as a collective, reducing the cost /individual. Unfortunatly this is not always possible.

I do not agree with your Euro (bar) US (psi) comments - its easy to do the conversion. Here, in Au, most air pressure tools are dual scale, making the matter even simpler.

Note:

I am not apposing or advocating against the Rotax advice on this test - just commenting on real world economics.😈

Engine differential compression testing is greatly aided with this 12 mm spark plug hole to air fitting extension. The hole is deeply buried and the usual adapters supplied in kits are either too fat to fit or so short, tightening is a challenge.

https://www.aircraftspruce.com/catalog/topages/atscompressiontester.php