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Prm Gearbox Oil Level Check

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I'm hoping that someone may know the correct answer to what at first glance seems like a simple question, but is really bugging me at the moment. I have a PRM gearbox and yesterday whilst doing an engine service checked the oil level of the gearbox. On the top of the gearbox is a nut that you undo and attached to it, see picture, is a dipstick. So my question is, what is the correct method for working out the correct level of the oil in the gearbox? Bear with me whilst I explain my predicament, on what seems like a very simple task.

I have undone the nut and removed it along with the dipstick and wiped it clean. If I then rest it back on the gearbox without screwing it back in, then the oil reaches line 5 and to my mind needs topping up as it should be somewhere between line 3 and 4. As can be seen from the top of the picture, there is about 10mm of thread between line 1 and 2. Now if I screw the nut all the way down, and then unscrew it and check the level again, it is very close to line 3 which is probably about ideal.

So my question is, is the dipstick calibrated for dipping with the dipstick screwed all the way back into the gearbox? or is it calibrated for just resting back in place without screwing in? Having to screw it all the way back in, and then unscrew it again seems a pain compared to just resting it on top and pulling it back out again, but I don't know which method the dipstick is calibrated for? and it makes the big difference to being low on oil, or being at the correct level.

I guess a second question is, what is the consequences of over filling the gearbox with oil? If I add oil now, only to find that the reading should be taken with the dipstick screwed down, then I will have overfilled it, but is this preferable to potentially running light on oil? 



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I have just found the below on the PRM website;


A: To check the oil level, the dipstick must be fully screwed down and a level can then be taken. DO NOT over fill your gearboxes as this is detrimental to their performance. If a mechanical unit is overfilled, then this will cause the clutch to wear out and the performance will continue to deteriorate.

I was assuming before i found this that it was the case that it should be fully screwed in to obtain the correct reading

Hope that helps


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The idea is to unscrew the dipstick when it is cold or when you first go on board lift it out to find level if correct re screw in, no need to wipe with rag and less likely to get dirt in gear box  and cold /thicker oil will show up better. John

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Thanks John, Shall bear that in mind for the future. The gearbox oil was changed about two years ago and still very new looking so hard to see on the dipstick. Add to that I had taken the boat out for a short run to warm the engine prior to extracting the oil and changing the filter and oil on the engine, so the gearbox would also have been warmer and the oil thinner.

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Yes by all means warm a box/engine up before draining oil this will flush any dirt ect out with the oil and is best practice, cold oil will give a lower level then warm/hot oil, which expands with increase in temperature,  mechanical devices are built/designed by engineers for repair/service by engineers, it all goes wrong when DIY novices get involved,BUT!! we all start somewhere .John

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