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Quick compression test question.

  • Thread starter Thread starter bexabarr
  • Start date Start date
B

bexabarr

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I want to test for compression but my carbs are off the bike, what effects, if any, will this have on my results? Thanks in advance.
 
Other than the fact that your engine won't be warmed up....

Other than the fact that your engine won't be warmed up....

no other real issues.
 
The compression numbers you get will be low and may look terrible depending on the age and condition of the motor.
Basically, you can check if it has compression but the amount wont be true until the motor is warmed up.
 
Follow-up question:

A local friend and m/c mechanic told me that he did NOT like to do comp. tests on a "too-warmed-up-or-hot" m/c... Something to do with alum. m/c engines and steel spark plug threads.

Do you all have any more info on what he's talking about ??

Thanks
 
GregM. said:
Follow-up question:

A local friend and m/c mechanic told me that he did NOT like to do comp. tests on a "too-warmed-up-or-hot" m/c... Something to do with alum. m/c engines and steel spark plug threads.

Do you all have any more info on what he's talking about ??

Thanks
Click to expand...
He doesn't like the idea of threading a compression tester or spark plug into a hot head for fear of damaging threads.
 
Damaging threads on the cylinder head... or the spark plug ??

Is this a valid fear?

If so, then what about doing a comp. test on a warm engine... how warm is good for the test but NOT too hot for those threads.

What about those "plug chops" - I've read about here... Aren't you doing those with a fully warmed engine?

Sorry, not trying to be too picky here, or worried. Just want to learn from you all, and avoid damage. Like I've heard many of you warn other rookies (like me) to be VERY careful during oil changes, to not put the filter cap back on TOO tight (cause you can easily damage those parts).
Thanks
 
GregM. said:
Damaging threads on the cylinder head... or the spark plug ??

Is this a valid fear?

If so, then what about doing a comp. test on a warm engine... how warm is good for the test but NOT too hot for those threads.

What about those "plug chops" - I've read about here... Aren't you doing those with a fully warmed engine?

Sorry, not trying to be too picky here, or worried. Just want to learn from you all, and avoid damage. Like I've heard many of you warn other rookies (like me) to be VERY careful during oil changes, to not put the filter cap back on TOO tight (cause you can easily damage those parts).
Thanks
Click to expand...
Damaging the threads in the head. It's not a real concern as long as you're careful.
 
NEVER start a spark plug with a ratchet and socket. Only use your fingers to start the thread. There is plenty of room for this since I can do it and I have very large hands.
 
I'm going to climb up on my soapbox again and begin preaching about the wonders of anti-seize. But to address the original subject first, any compression check will be more accurate if the engine is warm or hot. The cylinders, rings, pistons and valves will all be expanded to their normal operating size and you will get more relevant information. You can do a cold check, just realize that it won't be as accurate. Possibly MUCH less accurate.
The soapbox sermon begins now... you can never go wrong by judicious use of anti-seize on a variety of locations on a motorcycle.... especially considering you're talking about lots of steel and aluminum parts. ANY time dissimilar metals are next to one another, especially when heated or wet, a galvanic, chemical reaction begins that causes corrosion at the points of contact. ie... steel spark plugs threads in an aluminum head, steel exhaust studs and nuts against an aluminum head, even stainless studs and bolts. When a few drops of a quality anti-seize paste is applied to those parts before they are assembled, it prevents the corrosive reaction, and it lubricates the actual metal to metal contact so that threads don't gall and stick. I prefer a copper based anti-seize, but there are others as well. I don't assemble ANYTHING (metal to metal) on any vehicle I own or work on (Aircraft primarily... lots of aluminum) without at least one drop of anti-seize on every fastener. Every time I install them.
Lots of folks assume that a drop of oil on threads will prevent this problem, and it can in the short term. In the long term, it can actually aggravate it. under high temperatures and pressure, (non-circulated) oil or grease can break down and bake out the solvents, and what is left is a caked-in carbon that can act like an adhesive.... the opposite of what you desire.
Long story short, as often as you pull your spark plugs out, place a drop of anti-seize on the first few threads and always start them by hand or with a piece of flexible rubber tubing pressed over the end of the plug. Trying to start a plug with a socket and extension on it is a great way to cross-thread it and damage the soft threads in a head. With that spark plug threading easily into and out of that aluminum head, you'll be comfortable doing those hot compression checks or plug chops\\:D/

A word of caution... anti-seize is a copper or graphite powder in a liquid base, so if you get it on the electrode tips of the spark plug, it WILL short it to ground as effectively as hammering the electrodes together. Instant dead cylinder.:oops:
 
You don't need them very tight....

You don't need them very tight....

Loosen the plugs while they're cold, use anti-seize compound and put them back in, just snug them up. Don't need to be reefed in just to ride a few miles to do a plug chop or to warm up the engine for a compression check.
When you are done tighten them normally which really isn't very tight anyway.
 
Thanks you all for the helpful tips... AND explanation.

Love the detail and education you offered DaveD.
 
DaveDanger said:
The soapbox sermon begins now... you can never go wrong by judicious use of anti-seize on a variety of locations on a motorcycle.... especially considering you're talking about lots of steel and aluminum parts.
Click to expand...

I'm going to argue this...I put some on my metal slintered brake pads where they contact the rotor...it didn't help the bike to stop at all! In fact, it got worse!:shock:

Hap;-)
 
Hap Call said:
I'm going to argue this...I put some on my metal slintered brake pads where they contact the rotor...it didn't help the bike to stop at all! In fact, it got worse!:shock:

Hap;-)
Click to expand...
C'mon guy!!!!!
:lol: :lol:
 
BUT!!! Hapcall, you can and should use anti-seize on your brakes... just not on the pads themselves. The calipers do usually have a pair of studs which slide in and out thru a bushing on the caliper mount. Over time those studs can develop a tendency to bind in those bushings and they place uneven pressure on the discs when you apply brakes. The caliper doesn't move like it should to allow the pads to grip both sides of the disc. If you place a drop or two of anti-seize on those slide studs... your brakes will move much more smoothly... facilitating maximum braking without uneven wear of frictional components! \\:D/
 
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