Bargain Motorcycle Shopping – What Does No Compression Really Mean?

So you have been shopping for the vintage/antique/rare motorcycle of your dreams, and the hunt has been dragging on.  Days turn into weeks, and weeks turn into months.  Then it happens!  You come across your dream bike on Craigslist, but the listing says that the engine has “no compression.”  What does no compression really mean?

Maybe you have heard the term before, but don’t know exactly what it means.  In particular, the question you are asking yourself, is whether it is still worth taking the plunge and buying the bike?  Well, of course, provided the price is right and you understand what you are getting into (this is the bike of your dreams we are talking about!).

The bottom-line is that anytime you buy a non-running motorcycle, you should do a compression test before forking over (no pun intended) the cash.  The seller might say it is a “simple electrical problem” or the “carburetor just needs to be cleaned.”  The truth of the matter is if the bike is non-running, the problems (and associated cost to repair) could run a lot deeper.

When I bought “Cal,” he ran, but barely.  He had a visibly leaking front head gasket (very common on old Shovelheads), so I already new the front cylinder compression was weak.

Anyways, let’s get back to the matter at hand of what does no compression really means for a motorcycle engine (and the costs associated with each of the possibilities).motorcycle no compression

Think of an engine like one of those bicycle tire hand pumps.  As you pull up and down on the handle, air squirts out the nozzle end.  This only works, however, if the plunger on the inside of the pump has a tight seal.

The same goes for a motorcycle engine.  As the piston moves up and down and valves open and close, there needs to be a tight seal all around during combustion – otherwise, compression will read low.  Therefore, you have probably guessed it, the most common culprits for low compression are the rings, valves (intake or exhaust), and head gaskets (like in Cal’s case).  The head gasket is probably the easiest to deal with of the three, but all three generally aren’t so bad.  They simply mean that the top-end has to come off and some relatively basic to moderate complexity engine work needs to be performed.  The parts themselves are also relatively inexpensive, which is a huge plus.

The thing that worries me is that “no compression” is often used to generically refer to any engine with “low compression” and vice versa; and these two things are dramatically different from a cost standpoint.

As engine components wear out, you will experience low compression and have to perform the engine services described above (rings, valves, gaskets).  Again, these are inexpensive parts that someone with time and a little patience can replace themselves.

However, “no compression” means there has been more of a catastrophic engine failure – broken camshaft, hole in a piston, broken piston connecting rod, dropped valve.  These types of failures generally require a full engine overhaul, including pulling the engine from the frame and potentially separating the bottom end to retrieve fragments of parts.

This is the difference between approximately $1k and $4k if you are having a shop do the work for you; or the difference between an afternoon and an entire weekend if you are doing the work yourself.

So, what does no compression really mean?  Well, first figure out if it is “no compression” or “low compression.”

Then, if it is no compression, make sure the price is right to warrant the extended time and effort it is going to take to give the engine a full overhaul.  If I have learned anything from working on old bikes, it is that patience is a prized commodity.

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One Response

  1. Justin Hartway August 8, 2018 Reply

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