Hi all, with renewed enthusiasm, I’m going to attempt another leak test on my DT400 twin shock.
The last time I did this (over 12 months ago, so the details are a bit fuzzy), I tried various methods to try to seal the exhaust port, but with limited success. 
The exhaust port is circular only at the very outlet of the casting, so a rubber bung/plug probably won’t work? (I must admit that I didn’t try that method).
I made a thin metal blanking plate then put the exhaust back on (including an exhaust gasket), but then I remember it was very difficult to get the plate centralised and also very difficult to check (with soapy water spray) if the joint was fully sealed or not.
So, then I made a thick metal plate (including an exhaust gasket) - and fitted it without the exhaust, but I remember that the plate bent before I could get it to seal.
Before I have another attempt, fail, then hide the bike away for another 12 months….., has anyone done this with success and if so, what was your method please?
I guess that last method was my best attempt. Maybe I should try again, but with an o-ring or make a gasket so that it compresses easier than an actual exhaust gasket?
Edit.. I've just found a 56mm x 6mm neoprene rubber 'blank' washer on ebay (the 'recess' for the exhaust gasket on the cylinder block is just less than 5mm). So, unless anyone can advise otherwise, I'm probably going to try that, along with a large flat metal plate with two holes in it for the exhaust bolts, some sealant? and not too much graunching of the nuts!
Many Thanks

I had the same problem wanting to do the test on a DT360A (similar output port configuration) with an old MotionPro test set after a full rebuild. Was never really convinced I had it truly sealed there. Finally shrugged my shoulders and started it up. It's been ok, but I would be real interested in your solution to that setup problem.

i just performed the leak down on my R5 and DT400. Best to remove exhaust at the head and I used 1/4" plywood with some 1/4" closed cell foam. The wood is easy to shape, make it like the exhaust collar and slide on with the nuts. Sounds half assed but it works and very easy to do. Foam like the underlayment for pergo flooring would work if you can find a small piece.

You can get a 2'x2' of the plywood at home depot.

I am always thinking a new to me cycle won't have leaks and now perform a compression test and leak down on all of them.