How can I tell piston size??

This probably sounds stupid....but....I think my piston is in need of some new rings. I'm not 100% on this yet, but if I do need new rings, how can I tell what size my piston is?? It's a minarelli v1 engine. Do I have to tear down the engine to get to the piston and check it or is there any other way to get to the piston without completely disassembling the engine? I checked the mopedriders site and looked at the service manual for this engine, but its confusing to me. There are so many rings and sizes. Like the 25mph version has one size, then there is oversize!! I feel totally lost!!! This moped isn't running, so I don't even know how fast it goes or anything. I'm going to try and start it Wed, once I get a fuel valve and some new bulbs for it. Can anyone help?

Re: How can I tell piston size??

Steamboat Aka J. R. Stevens /

y Angie, Don't guess about needing rings,this can be determined by a compression test. If you don't have a compression gauge you should take your bike to a local motorcycle or lawn mower shop. To check piston size you must remove the head and measure the distance across the inside of the cylinder(bore). At the same time you can check the cylinder for imperfections and de-carbon the head and ports.Jim.

Re: How can I tell piston size??

I wish I could. I don't have a compression checker or a truck. And I really don't see that moped fitting into my Camaro lol. Can I get a compression checker at a local hardware store or anything?? The appropriate compression is 120 psi right?? But to remove the head, does this mean the whole engine has to be disassembled? If so, I might just have to find a way to take it to the local moped shop.

Re: How can I tell piston size??

Steamboat Aka J. R. Stevens /

Hey Angie, some people will say a moped is easy to work on but what they mean is, moped parts are easily accessible but one must have tools and an understanding of moped mechanics/electrical systems to repair them. I admire your spunk and willingness to learn but it may be time to let the experienced people take over. Jim.

Re: How can I tell piston size??

Jim,

I understand what you are saying, but people keep telling me, "If you're going to own one, you need to know how to work on it." I can do some things, but no, I'm not an expert on mopeds. Even experienced people had to start somewhere. I already know that if I can't get it to run, the local shop probably can. I'm very eager to learn about how they work and how to work on them. Sometimes people don't understand me, and sometimes I just don't understand people. I've seen quite a few mopeds in the 20 years I've been on this earth. I used to just tinker with them even though most of them weren't complete. I liked to take apart whatever was there and just obsorb all the details and intricate parts of them. I've seen so many mopeds when I was growing up because my grandpa owned a junk yard, and he used to get in about 3 or 4 a month, sometimes more. Some of the mopeds would even start and run!! People just took and dumped them there because they wanted some room in the garage!! It's a shame now that I think about it, I could have had a HUGE collection of them by now lol!! The best moped I think I saw down there was a Puch Maxi. It was mint and ran nicely, but the man who owned it passed away, so the family just dropped it off and left it.

Re: How can I tell piston size??

Angie,

There are two basic types of compression gauge. One has a hose, or set of hoses (with different size fittings) that screws into the spark plug hole. You then attach the gauge to the other end of the hose, and crank the motor.

The other style is just a gauge, with a tapered rubber tip that you hold firmly in the plug hole, while the engine is cranked.

If you don't have a gauge, you can hold your thumb firmly on top of the plug hole. When the engine is cranked over, the compression should easily force your thumb off the hole. If it forces your thumb off, there should be enough compression to run.

The size of the piston should be marked on the top; you may see things like "STD", "0.25" or a code of letters/numbers. About piston & ring sizes, this gets a little complex, but I'll try to lay it out clearly.

There are two 'sizing' issues that you commonly deal with - stock vs. overbore size, and 'grading' size. There are also two types of cylinder bores - iron, and plated. When you have an iron bore, you are normally talking about stock vs. overbore sizing, and with a plated bore, you are talking about grading. Here's how it works:

When new, iron bores are precision machined to accept the standard piston size. Let's say that the stock size is 39mm. If the bore wears out, or becomes damaged, it can be overbored - usually twice. We call these overbores the first and second oversize. As an example, the first oversize might be 39.25mm, and the second 39.50mm. So, for this one iron bore, there are three different piston and ring sets that might be used - stock, first and second overbore.

With a plated bore, things are much different. When new, the aluminum bore of a plated cylinder is precision machined to something slightly larger than the stock piston size - lets use 39mm again. The bore will be machined to something like 39.20mm, and then it is plated with a hard material, like industrial chrome. The plating adds material to the cylinder wall, thus making the bore smaller, with the target being 39.00mm.

Trouble is, the plating process can't be controlled with total precision, so the final size of the plated bore is usually something other than 39.00 - it's slightly larger or smaller. This is where grading comes in - pistons are made in a series of very close sizes called grades; there may be as many as eight grades for a given bore. 38.8, 38.9, 39.0, 39.1 etc. When the plating process is done, a technician measures the actual bore size, and stamps the correct piston grade to be used on the top of the cylinder. During assembly, a technician reads the cylinder grade and selects the correct piston & ringset for it. Oversize pistons are not available for plated bores - as it's not practical for a machine shop to bore the cylinder, have it replated, and then go through the grading process.

To complicate things just a little more, sometimes a plated cylinder can be machined out large enough to accept a cast-iron sleeve, which is then pressed in. Once the cylinder is 'sleeved', it is then treated like an iron cylinder - it has a stock piston size, and perhaps one or two oversizes. I've never heard of this being done on mopeds, but it's common on certain motorcycles (Moto Guzzi comes to mind, but there are others).

More recently (last 20 years or so) manufacturers have gained much better control over the plating process, and some no longer need to use the grading system. They can just produce the exact size they need, every time.

Re: How can I tell piston size??

Angie:

If the 'ped is not running why do you think it needs rings?

You do not need to remove the whole engine to measure the bore, just remove the 4 bolts that hold the head to the cylinder.

I am currently working on a V1 that has been sitting for 10-15 years. The rings look OK but the crankshaft oil seals are dried out causing air to be sucked into the engine along the crankshafts instead of thru the carb.

Good Luck

Re: How can I tell piston size??

I'm going on the mileage. This moped is just shy of 8,000 miles. If I do get it to run, all will reveal itself in time. I was told that putting your thumb over the plug hole is not too accurate because 60psi can blow my thumb off, but is not enough compression to make the moped run. I checked the compression with my thumb and it does blow it off, but I'm not sure if it's enough for the moped to start and run. Also the choke on the carb is not there, so I don't know whether or not the choke is stuck on or off. Also, Legendre, thank you for your post. It was pretty simple terminology and I did understand what you meant. I am just not sure how to tell if it's plated or iron. Thanks all who have posted, you have been really great.

Re: How can I tell piston size??

Angie,

If the head is off, and you have access to the cylinder, just use a magnet :)

attracted = iron, not = plated.

Re: How can I tell piston size??

my rings are so shot that when i stick my finger in there to test for compression and have someone PEDAL crank it there's NO response.

Re: How can I tell piston size??

Wow, that's very bad rings Max. Mine does have compression, and it feels strong, but I don't know exactly how many psi it's got. My dad is bringing over a compression gauge tomorrow, so I'll find out then. Everyone keep their fingers crossed for me :) I'll let everyone know the results tomorrow.

Re: How can I tell piston size??

Steamboat Aka J. R. Stevens /

Hey Max, You may have other problems, If that piston is moving at all, you should have some compression. When you pedal can you see any part of the engine rotate? Jim

Re: How can I tell piston size??

Not to change the subject, but does anyone know if they make a performance exhaust for the minarelli v1 engine? Ya know, just in case I ever feel the need for a bit more speed lol.

Re: How can I tell piston size??

Leon Swarmer /

Angie, pulling the head off isn't that big a deal. The biggest danger is trying to torque down the head.. everyone wants to overtorque them..then bolts or nuts snap and there ie serious trouble.

YOUr 8,000 miles might be on the odometer, but it is quite possible that some previous owner has replaced the top end...(cylinder and piston) who kknows it may have been kitted and you have a hot rod...don't go looking for mods until you get it running as it is.

Leon

Re: How can I tell piston size??

Fede Berkelaar /

Check this out and if you are interested email me

http://speedkits.8k.com

Re: How can I tell piston size??

Your almost their Angie!

Re: How can I tell piston size??

Also it is a 50cc. The cylinder jug should have 49cc engraved in to it some where. The top speed of that bike was 35mph and that was true b/c I was in my mom's van behind him on a highway. It was staying up with the Tomos Targa with a bi-turbo. So it is a quick little bike.

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