Hey,

You know when you guys say you have leak tested your engine and its held 120psi for three minutes or something - How do you leak test it???

Do you have special bungs to block the inlet and exhaust??
Do you have a special socket for the spark plug hole where a pump connects?

I need to see if my engine is leak free before i run it for the first time.
After Matt did that post about the carb insulator leaking due to overtightening i want to test mine y'see.

Cheers
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After years of moaning about immigrants now i am one...

Check this link out:

http://www.macdizzy.com/2stroke.htm

I believe somewhere on the site it tells you how to build one as well.

Hermit wrote:How do you test your engine for air leaks?]

hold it under water and see if there are any bubbles
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Hermit,

I use a "leak down tester" that is made up of plugs for the exhaust ports, as well as a blank plug for one intake opening, and a plug with a pump and pressure gauge attached to it for the other intake opening. This form of testing does not need a lot of pressure. I generally test up to about 7 psi, and make sure the engine doesn't leak for a set period of time (useally 3 minutes).

I perform a leak down test with the spark plugs in the plug holes.

You can get a leak down tester from Motion Pro (2-stroke style), or you can make your own from parts found at hardware stores, and medical supply stores (the pump is basically what is found on a blood pressure testing cuff).

Thanks
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Charles Gallant

"Taxi for Monkey!..."
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Can we hold Monkey underwater and see where the bubbles come from?
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Dave Ett wrote:Can we hold Monkey underwater and see where the bubbles come from?

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Another perfect emoticon!
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

it's not easy to test air leak in motorcycle engine.
the best way is to put out the pipes and carbs,
blank the exaust and pressurise the cranckase with an adaptater in the inlet manifold (hand made).
I use a sthill pressure pump to do that

the piston must be at BDC and the pug in
a vacum test is also important to test the cranckshaft seal

you can find the leak with a special spray
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I filled up the left crankcase with engine oil, and it slowly leaked accross to the right, so I know my seal isn't good. Seemed to be running fine before the coolant hose fell off...
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

I rebuilt my engine with new seals when i changed the crank, so they should all be good. I'm most bothered about leaks around the inlet and that carb 'boot' thing after Matt did that post about over torqued ones.

Has anyone here got a motion pro 2 stroke leak down tester??
I'm frightened of starting my expensive, 300cc ultimate motor of doom without being certain that its sealing properly!! (had lots of problems with my MZ racer in this area) Making my own tester would be a fat load of hassle that i can't be arsed with so it would be cool if someone knew if the motion pro kit works with the NSR engine or not.

Thanks Laurent, but with one piston at BDC, does it matter where the other piston is?
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After years of moaning about immigrants now i am one...

I didn't test the motion pro

you can only do the test in one cylinder, no mater where is the second piston. to track the leak, shaving foam is good too, it makes plenty of bubbles where it leaks.

with manifold in good condition and new seal every where. it must be OK
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watch my www

My leakdown tester is totally home made.

The inlet is sealed using a nylon bung I turned up in the lathe, basically copying the dimensions from the carb spigot that fits in the insulator. I then drilled a hole in the centre and fitted a tyre valve so that I can fill with air.

The exhaust is sealed using a big o ring from the hardware shop (cost about 10p) and an aluminium plate that I hacksawed and filed. This bolts on using the original exhaust mounting studs.

Now, as for the plug hole, this is where I fit my gauge. Some will say (rightly or wrongly) that you're supposed to test the plug as well for leakage, but as I'm usually pulling plugs out faster than I'm throwing fuel in I think that it's more important to check other areas such as seals and gaskets. Anyway, my gauge is a 10 quid low pressure gauge that I fixed into the gutted body of an old spark plug so that it can be screwed into the head.

Procedure for me is to pump in 10psi and then leave it. In the worse case it should probably only drop 0.5psi after a couple of minutes.

I spray soapy water over the engine and look for bubbles if it's leaking.

As far as I'm concerned, piston position will make no difference and is not important as the air will leak past the rings no matter where it is in it's stroke.

So, nearly 4 years later and at last i've leak tested my engine and it ain't looking good. Here are some pics and details of my findings for you all to see first before i go splitting the crank cases. Please tell me your thoughts;

Here is the tester - the exhausts are blanked by nothing more than a piece of rubber about 10mm thick held hard over the exhaust holes by pieces of steel plate i had left over from when i made my flash flywheel holding tool.

For the parts of the tester in the photo - 40mm PVC waste pipe fits PERFECTLY into the carb boot, i just chamfered the end to make it slip in easier. Onto this i glued a 40mm PVC cap (these are both typical plumbing parts for use under kitchen sinks for example). I hole sawed the cap and threaded in the brass fittings, some of which i dug out of the depths of the garage, others i sourced from a local diesel mechanic business. Its all PTFE'd together though i also used silicon to get the brass fitting to seal into the PVC cap.



I pumped up to 8PSI (if the gauge can be trusted), drowned the engine in soapy water and found the following leaks;

These seem to be on the joint between the crankcases, not the the base gasket.





This shows air getting past the flywheel side crank seal - though i'd have thought some leakage past this seal would be acceptable. I just wonder what is considered an acceptable size leak?



Thanks very much for any advice.

Dad reckons seeing as its always run well and started easily that i'll be wasting money if i split the cases to try to seal them up better. I'd also be replacing the outer crank seals as part of the job if i go ahead with it. The engine has done 115kms since its new crank and 300cc kit went on.
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After years of moaning about immigrants now i am one...

A leak is a leak, and looking at your pictures it's a leak that can be fixed.

Sure, it's a royal pain in the arse to go pulling the cases apart right after you've recently put them together, but a if you're doing a job then it's worth doing it properly.

A bit of advice. When I'm building a motor I'll be sure to check that the case halves are 100% flat. I've got a nice ground surface plate, but a thick plate of glass will also work well. Just sit the case on the plate and check with a 0.05mm feeler. If you get a high spot then check to see if there's any bumps or burrs that could be holding the cases apart onve assembled.

I then run over the halves gently with a fine oil stone. This will clearly show up high spots (which can be carefully stoned flat again), and also gives a good surface for the case sealant.

When the seals are fitted to the new crank, I make sure that they're greased, inside and out. This helps the seal located nicely on the crank and in it's housing. I also grease the centre o-ring to (hopefully) avoid any nicks when the cases go together. I use a specific low viscosity seal grease, but lithium based grease works well.

Next job is to thoroughly clean the cases. I wipe the sealing faces with a contact cleaner to ensure they're grease free and dry.

A good sealant is essential. Cheap goo like Threebond 1104 does the trick and is resistant to many solvents etc, but I hate the stuff. I use Threebond 1207 (Bloody expensive, but HRC use it and so should you!). Don't need a huge amount. I put a little blob on one finger and then kind of tap a little drip onto the case face and then sort of tap it around with a spare finger. It's more like colouring the surface really. I do this to both halves. Oh, and I wash my hands first so that there's no chance of contaminating the sealant with oil. It's actually the only time ever I wash my hands!

I then cheat a little and put some extra (not a lot) around key points like the edges of the transfers, up near where the case halve meets the cylinder gasket face, and also the edges where the main bearing outer races will be.

Before fitting the three dowels I give them a roll around in some grease just to stop them seizing.

Next the crank is dropped into the lower half. Be sure to pay attention to locating the outer crank seals perfectly. I even press the crank down by hand to make sure they're home and true.

Slap the top half on, pushing it down as best you can.

Fit the M8 bolts and slowly pull the cases together using a low torque at this stage. Just enugh to nip them all up starting with the centre two bolts and working outwards diagonally. Now give the crank a bit of a spin to check it's free. It might feel a little tight at this point but it'll loosen up when the cases are together properly.

Sticking with the inside to outside routine, I start at around 15Nm, then 20Nm, 22Nm and finally 25Nm. Between each sequence I'll give the bottom of the case a jolly good smack with a mallet to make sure it's all stress free and located properly.

With that bit done, fit the 4x M6 bolts and tighten to 12Nm.

Now, be a good chap and wipe off any sealant that's squeezed out. If it's done correctly there should be hardly any.

Insulators can be really annoying, as they will have bent over the last couple of decades. Some seal, most don't. The recommendation is 12Nm, but I rarely get near that. Somewhere between 7~9Nm usually does the trick with a bit of sealant.

I don't think it's essential to use sealant when fitting the cylinders, but it can't hurt. Maybe where the two halves come together on the front cylinder gasket face, and at the same point on the top cylinder gasket face. Just to be sure!

Torque the cylinder down diagonally across. I nip it down first, and then step up; 15, 20, 22, and finally 24Nm.

I never use any sealant on the head gasket. If it leaks with a new gasket then something's wrong, and you may have to lap the top of the cylinder and head. For lapping, I use a convenient plate of glass, and then mixa little fine valve grinding paste with engine oil. Don't get over excited with the grinding paste. A tiny dab will work. Slow and methodical is the best approach with lapping......"feel it cut", as my old engineering instructor used to say. It doesn't take long to get a perfect seal.

Head is torqued to 15, 20, then 22Nm. Again, across, not around.

Well, that's about it for today chaps. I might have forgotten something, but you should now have the perfect, leak free bottom end.