Survey piston to cylinder wall clearance

[jmkjr72]

yes the hm engine is mostly the go to engine now
the other big 2 stroke out there is the morini
kymco runs there own engine

those specs are the very simular to what is used in the lml 150 2 stroke engine and vespa engine

[bigkahuna427]

Well I had two choices.
A cylinder with a bit of pitting that had a clearance of between .002 and .003 inches.
OR, A new cylinder that was over .010 inches.
What would you do?

[jmkjr72]

i would proably hone the pitting out
then get a +1 piston for it

[bigkahuna427]

Done, but the piston I had was reasonable it is and together and running now with no problems. I am scratching my head at anyone who would say more then .010 inches is within any tolerance. Am I wrong?

[jmkjr72]

i just went thru a buch more service manuals tomos vespa kymco and the all show pretty much the same as what was quoted before
0.1 mm or 0.0039 in being the service limit
so yeah they are out of there minds

[bigkahuna427]

I don't think anyone is going to find anything in writing for any engine to support a clearance that is in excess of .010 inches. I think they just have a bunch of cylinders with the same clearance and haven't had anyone complain. I wonder if they have really sold many as these engines aren't very popular having no variator.

Of course these scooters I have I wouldn't have even bothered with if it wasn't cheap to get them going and it gave me something to do while I was down here. I think I really don't know how to take a vacation.

[fugaziiv]

Hey guys,

Matt from PartsForScooters here, some of you know me from other forums, or have purchased from us before (and I thank you for that) and I thought that it was time to weigh in on this one.

Full disclosure time.

The part in question is here:

www.partsforscooters.com/169-185_TB60__Big__Bore__Kit

And I am also the guy that bigkahuna427 is now emailing back and forth. Here is the deal:

After contacting our manufacturer, they stated that the part is within spec for that particular engine, so I went and measured our entire stock. Our stock matched the spec in question, as in quite a large clearance. So I went ahead and contacted some of our dealers who sell multiples of these kits (mostly moped specific customers), and while they agreed that they would also like to see the kit with a tighter clearance, they also stated that to the best of their knowledge that this spec was fine due to the fact that this was such a low power design and has issues with cooling. Were it to be a Minarelli or Morini it would be a whole different story.

Now, the facts for me here are that we have a customer who has specs that match our manufacturer and our established customers, and our stock, and this is literally the only complaint that we have ever received regarding this particular spec for this particular engine kit. Keep in mind that over the last two years we have sold 100's of this kit. We've had it for much longer than that, but that's as far back into our sales data that I went.

As much as he nor I enjoy the spec itself, based on the historical data at hand it is difficult for me to consider the part defective. I'm not in the business to start fights, or make people unhappy, but this is where I am with this one. Since bigkahuna427 decided to post email content in a public forum, I felt I had to come out and comment.

Matt

[2strokd]

Glad you did Matt. Most retailers tuck tail and hide from this sorta stuff.


Off topic a bit: You guys gonna make TnG this year??

[fugaziiv]

2strokd - Man we are going to try, but we have some tight scheduling conflicts this year right about that time, and we are growing like crazy so I may not be able to free up to make it.

I'd hate to miss it though...

Matt

[90GTVert]

Thanks for joining up and giving us more details Matt. It's at least nice to hear you looked into the issue. I think a lot of sellers would either have no idea what Noah meant (too many sell and don't know about parts) or just wouldn't care.

[fugaziiv]

90GTVert - Thanks! I've been prowling the forum for awhile now anyway, so I'm probably overdue to comment.

Lucky for us we've built a few engines in house over the years (we are particularly proud of our QMB drag scooter) and because we actually do the importing from overseas and many of the parts that we carry we are the first to get here in the US, we have to be very particular about the specifications of most of our product. You wouldn't believe the volume of parts that we reject on any given shipment from China because they fall out of spec. It is actually rather disturbing.

As far as the issue at hand, I have to agree with Noah as much as I would have questioned the clearance as well, but after looking into it I'm satisfied with what I've found. It isn't as perfect as I'd like it to be, but it does seem to fit the particular application consistently.

Matt

[bigkahuna427]

Hi Matt,

It was not my intent to even say who this was I was dealing with even if asked. I am not the kind of guy who is out to make a public rant, so that was not my intention. I actually found specs for the engine the design was based on but couldn't find any for the chinese version. The specs for that engine were nearly identical to specs posted earlier as in around .002 to .003 inches and a service limit of .005 inches which is exactly what I would have expected. To be at more than .010 inches (probably more like .014) is alarming and the piston is obviously very loose in the bore. I would have actually been fine at even .005 or .006 and would have just put the engine together and called it a day. I do not believe I am splitting hairs here. It is a substantial difference between what is reasonable and what I have. None the less I have written this off. I have spent enough time on this and $50 is not that important to me.