Looks like I may need to do this.

Previous posts regarding my engine problems have pointed to a compression test being a good idea and an 8mm adapter being required for the tiny plugs on a VFR400

I've googled and e-bayed but nobody seems to go down to 8mm

Any suggestions?

Or am I misunderstanding the plug sizing?

Your not misunderstanding.

The NC30 and NC35 along with the RVF750 all use stupid 8mm plugs (hence the price of them)

The NC21/24 and most other bikes use 10mm plugs.

The last time I compression tested one of these engines I turned a piece of brass bar down on the lathe to fit as I couldn't find one

You can also get the compression testers that use a tapered rubber plug that you just shove onto the hole.. these work but there a bit flakey.

If I was in the UK i'd turn you down some brass bar... do you know anyone handy on a lathe? Even paying a firm to do it shouldn't cost much

Your not misunderstanding.

The NC30 and NC35 along with the RVF750 all use stupid 8mm plugs (hence the price of them)

The NC21/24 and most other bikes use 10mm plugs.

The last time I compression tested one of these engines I turned a 10mm compression adapter down.

You can also get the compression testers that use a tapered rubber plug that you just shove onto the hole.. these work but there a bit flakey.

If I was in the UK i'd turn you down some brass bar... do you know anyone handy on a lathe? Even paying a firm to do it shouldn't cost much

Have a word with our resident machinist, Kayla, i cant recomender her enough shes really quick and its reasonable priced,

i can remember the pit of the 8mm spark plug thread but your other option is to buy a short bolt and drill a hole straight through in the center, then tap a 5mm (m5 thread) and stick a brass spiggot for you to attach the compression tester hose, remeber not to use a long bolt as if it goes to deep it will smash into the piston!

porndoguk wrote:Have a word with our resident machinist, Kayla, i cant recomender her enough shes really quick and its reasonable priced,

i can remember the pit of the 8mm spark plug thread but your other option is to buy a short bolt and drill a hole straight through in the center, then tap a 5mm (m5 thread) and stick a brass spiggot for you to attach the compression tester hose, remeber not to use a long bolt as if it goes to deep it will smash into the piston!

I have a mate who works in a machine shop I will ask him if he could do something, I may be able to borrow a tester but can't get that until Friday at the earliest and don't know what thread size it will have.

Delays, delays. Plus MORE cash

One question (again) I forgot:

Engine temp?

Should I run it, smoke and all, to get it warm prior to testing the compressions - Haynes says do it this way but if I do have an engine problem isn't this going to cause yet more grief?

Mind you I suppose if its lunched anyway what difference will it make!

you dont have the engine running for compression testing,

turn the engine over on the starter, with the kill switch OFF and remove all the plugs to make it turn over better.

do it cold as the metal will shrink and youll get a more true reading with the engine warm the metal will expand and mask any leaks etc,

Compression test.

This is the best way to test for your situation.

Engine cold and remove all 4 plugs.

Test each cylinder by cranking the engine over a few times until the needle on the meter will go no higher (most meters have a non-return valve so the needle wont drop)

Record all 4 cylinder compressions.

Check them in the haynes / honda book.. all 4 cylinders should have very similar compression.

If you find a cylinder(s) that has lower compression that the book or lower than the others or both... put 1 tsp of oil in the cylinder thru the plug hole and repeat the test..

If the compression jumps up then you have knackered / worn piston rings.

If the compression stays the same then you have leaking / badly adjusted valves.

porndoguk wrote:you dont have the engine running for compression testing,

turn the engine over on the starter, with the kill switch OFF and remove all the plugs to make it turn over better.

do it cold as the metal will shrink and youll get a more true reading with the engine warm the metal will expand and mask any leaks etc,

I know it doesn't need to run and to use the kill switch etc I was just questionning the method in Haynes which stated you had to get the engine to operating temp first, I guess the cyclinder compression should be good warm or cold

well it would be better warmer because as metal heats up it swell/expandes so any small gaps that were there cold maynot be there when warm, hence why me and martin have both said test it cold, the haynes isnt always true, there are editing mistakes that a hawk eyed enthusiast will pick up, it also tell you to test the valve clearances on an NC30 wrong too!