Piston/cylinder measurement questions

[Tippmann98]

I got a digital caliper to take some measurements on the piston and cylinder from an '84 200x. I measured the cylinder 5 different times on the top and 5 different times on the bottom and all my readings were between 64.87 - 64.94. Stock size is 64.99 - 65.00. I think it's safe to say my cylinder has not been bored out and my readings were slightly off due to user error or the caliper not being perfect BUT when I measure my piston I'm way off. I measured it numerous times and all the readings were within a few hundreths of 64.50. Stock size is 64.96 - 64.98 according to my service manual. The piston has 965 stamped on the top and after doing a little research I found that this means its the OEM standard piston. Could my piston really have that much wear on it?

Before anyone tells me, I know I should let a professional measure it, bore it out, and order my piston and rings for me but what fun is that? I like doing work on my toys myself because that's the only way you learn and it's a lot more rewarding when you fix something yourself. Thanks for all the help fellas!

[79fordblake]

Its better to just let a pro meaure it for you. Measuring at just the ends of the cylinder isn't the best. If you want good results you have to buy good tools and they are very expensive. It was great having access to all this stuff when I was in diesel school. Now I have to let others do it for me since I don't have the tools anymore.

[just ben]

order the piston and rings before having it bored other wise you may end up eating the cost of a piston kit or having to be rebored.

[RubberSalt]

Location of piston measuring is critical. They are not perfectly round and they are smaller up top. Normally measured at the wrist pin. Cylinders - you need a dial bore gauge, take TONs of measurements and average them out.

I've noticed with blaster/YT pistons, the pistons can 'appear' to be 20 thousands smaller up top.

[zzmegad]

http://www.amazon.com/FOWLER-Dial-Bo.../dp/B0006J3LL0

you also need a micrometer to set the bore gauge to size and thats a pretty cheap bore gauge...

I also remember measuring taper in my piston when it was new,

also stick a ring in the cylinder and check the gap


edit: ebay is a decent place to find used inspection tools, stick to brands like starrett, mitutoyo, brown & sharp, and fowler, those will usually last forever. SPI is decent cheaper stuff. Theres tons of stuff out there tho that would probably work fine for occasional measuring. make sure you get some standards too.

[ctbale]

I am with you Tipp, its more fun to do it on your own, I just did my 83 200x. I did it like you, I figured out I was standard bore, and decided to go 3 over, my cyl was pretty bad. I ordered a 3rd over kit, gave the piston and the cyl to the guy who does my bores, and everything is good. I can always figure out what bore its on with a digital caliper (a good one) then I always go 2 over, but this one was the exception.

[WilliamJ]

Measuring internal bores with a digital caliper (if you are talking about a vernier type) is really hard. You really need a digital bore gauge and then you need to practice using it.

If you want to know the gap you can usually persuade a small feeler gauge to flex enough to slide it between the piston skirt and the bore. Bear in mind the piston skirt is oval and is bigger at 90 degrees to the wrist pin. Measure at the bottom, middle and top of the bore in both left to right and fore and aft directions. This will give you a picture of the wear shape of the bore. It won't tell you exactly what the size is but if you have a micrometer to measure the piston it will be very close. You have a max clearance of 4 thou, max taper of 4 thou and a max out-of-round of 4 thou. If you can cram anything bigger than 4 thou down the side of the piston then either piston, or bore, or both are worn because it is hard to get a 4 thou feeler into a curved 4 thou gap! With car you can get to about 1 thou accuracy with this method which is enough to decide what rebore size to go for.

[just ben]

if you are planing to bore the cylinder you really dont need to measure anything.get your self an over sized piston drop off at the engine shop they will do everything. cost me 70.00 to bore a 2 stroke 4 stroke is 60.00. but the cylinderwall to piston clearance is .005" or .125mm

[zzmegad]

well someone needs to measure it to decide what size piston to order, I agree you probably only need a set of calipers to decide what piston to order

[Tippmann98]

I was not planning on boring the cylinder. I'm mostly interested in getting it to stop smoking so I can resell it and make a little profit. I agree if I was planning on keeping it around for awhile I would definitely bore it and do it up right.

[just ben]

I was not planning on boring the cylinder. I'm mostly interested in getting it to stop smoking so I can resell it and make a little profit. I agree if I was planning on keeping it around for awhile I would definitely bore it and do it up right.

sorry I misread your previous post. deglaze the cylinder and a set of rings will probably get it to stop smokin. but i really dont think it would cost much more to do it right and have a clear conscience

[ctbale]

When I do the hone and rings deal, (assuming the piston is ok) I ALWAYS buy the next size bigger ring set and file em to .006" end gap, you buy a set of standard rings on a standard bore with normal wear after honing it and your end gap will be over .020

[WilliamJ]

No you can't do that and expect it to work properly. The rings must be right for the piston and the piston must fit the bore. The ring gap is NOT the issue. It sounds as though there is a lot of wear. The rings will flop in the ring grooves and the piston will slop in the bore. Did you try sticking the feeler gauge down the between the fat part of the piston skirt and the bore?

You need to take it to someone with the right tools to measure it - and get them to show you how it should be measured. You do not have the tools - a caliper is just not good enough.

[atc007]

Whew!!!! To properly measure a piston,you ALWAYS measure the skirt,IN side to Ex side. To properly measure a cylinder,you measure IT,6 places,X<Y<Z,Top,middle,bottom. W a snap gauge. You don't have those. That's all this dummy knows after 23 years : ). The good news is ,,you don't need ANY of that! All you need is to switch to Green tractors,hone that cylinder,with a straight hone,not a flex. New rings will stop the smoke,,however,the intake side of those pistons get weak. To do it right put new Piston and rings in. The fresh piston will give you a little more plug,it will be almost as new given the dimensions you have given . It will run beautifully. You should also do a valve job,,clean up,new seals. If you don't have a spring compressor,any decent shop will throw them back on there for a few bucks. Do you know how to remove them w a towel, and socket? Also ,it will still smoke like a pig until the smoke burns out of the exhaust! Usually a couple hours. Done deal.

[Tippmann98]

atc007 - It's nice to see someone else as cheap as myself. That's the route I decided to take before you even posted it. Nothin runs like a Deere. . . but nothin stinks like a John!