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The 'correct' way to measure a cylinder bore and measure cylinder using piston diameter

Picture of The 'correct' way to measure a cylinder bore and measure cylinder using piston diameter
305 cylinder to measure 2.JPG

Hi All,

Purpose of this Instructable is to hopefully teach how to measure a cylinder and piston, the correct place to measure a piston and why you find the largest possible diameter.

From my activities in many on-line motorcycle groups I've seen a number of people posting things are 'worn out' or wrong parts fitted because they can rock or move top of piston in cylinder bore after cylinder head has been removed

The piston and cylinder being measured are from a 1968 Suzuki T305

Suzuki also made a T250 which looked pretty much identical and a year later a T350 so make sure you have correct specifications

Before making any decisions on parts, you will need the stock
specifications, it's hard to judge clearances between parts when you don't know what exactly they should be so get service manual or technical data manual

In this case, the standard piston size is 59.955mm~59.940" but it's also given as an inch size of 2.360"~2.359"

During operation (ie, when engine is running) the top of piston is exposed to full heat of combustion which is normally around 7~800f or higher. Aluminium has a very high expansion when heated so at running temperature the piston top expands to 'fill' the cylinder bore (top of piston is probably averaging over 300f ?) This is a two stroke (two cycle) motor, conduction removes heat from top to underside of piston and fresh charge helps cool it and prevent a 'melt down' (mostly)

Pistons are machined so the largest mass of metal has room to expand and thinner sections (which don't expand as much) are different diameters. The sides of pistons where gudgeon (piston) pin fit, having more material then the thinner sections of skirt are also smaller than the 'nominal' diameter. It may be easier to picture a piston as being barrel shaped top to bottom and kind of 'pear' shaped looking down from top. The reasons are also linked to the way piston is 'pressed' into front or back of cylinder due to the connecting rod angle and direction of rotation of crankshaft

Step 1: Measure the piston

Picture of Measure the piston

As previously stated, 'we' are looking for the largest diameter of the piston so in this case it is inverted and measuring point (usually 5~10mm from base) this particular bike uses 26mm from base of piston. Very surprisingly (in view of age and mileage of bike) the actual size of piston is in specification, 2.359 and a few ten-thousandths of an inch ( I no longer have micrometer reading to 1/10,000" but it isn't really needed - most of the time)

HorseHeaven5 months ago
Okay, I appreciate the time and effort that went into writing this up, and after reading through it a few times, I think I have a decent grasp on the basics. My question is: Since we pretty much KNOW that a bridged-exhaust-port 2-stroke cylinder is going to show excessive wear on the exhaust bridge, do we pretty much IGNORE the mid-level Y-axis numbers, which will be out of tolerance on any cylinder that has more than an hour of run time? I expect to use a glaze-breaker to identify the areas of greatest wear (where no hatch shows after a minute or so of turning) and measure the diameter at those points to see if my piston>cylinder clearance exceeds six thousandths of an inch. Since my piston was melted, I have to buy a new one, so I'm using the measurements to decide if another +1.0mm will work or if I need to move up to a +1.5mm. Barrels for 40+ year old dirt bikes are harder to get than they were when the bikes were new, and I'm a lot more mellow rider at 58 than I was at 17, so I might be willing to let the .006 slide a bit, too, rather than just boring it again.

Back in the day, we pretty much ran a given bore until the stutter-bumps on the exhaust bridge broke the rings, then bored until the bridge mostly 'cleaned up' and ordered another piston kit from Wiseco.

How would you treat the exhaust bridge wear issue? I've considered packing the port with heat-paste and wire-feed welding the bridge (and doing some careful filing/grinding) prior to honing, since the rest of the cylinder typically is within tolerance. I might even experiment with moving the ports further apart and tapering them a bit using this technique. Has this been done with any success?
crazypj (author)  HorseHeaven5 months ago
The exhaust bridge only shows excess wear when it isn't relieved properly. At the high temperatures it receives from exhaust gas it expands into cylinder. If you can see mark of bridge on piston, bridge needs more clearance. In my experience, as long as COLD measurement is less than 0.006" oversize on the bridge you will be OK. (a couple of manufacturers once gave a stock rings spec for relieving bridge, usually 0.002"~0,003") You do need to keep rpm down until motor is fully warmed up as ring breakage is possible but that is more from 'ring flutter' as it's trying to move in and out plus top and bottom of ring lands. Replacing 'stock' rings (or whatever size piston you have ) at a lower mileage/hours running will keep problems to a minimum, usually 2 sets of rings then a piston and ring set before boring is needed, (total 4 sets rings and 2 pistons in stock bore) New rings help keep piston centred in bore which can reduce wear on cylinder. Any cylinder around 54~61mm dia. with 0.006" or more clearance (actual clearance with a new piston) will break piston as it rocks and rolls around the bore.(even air cooled motors) An awful lot depends on the rpm your using. When the 4-stroke 250cc MX bikes first came out they had advisory 30 hrs on crankshaft if racing, play riding meant the lasted longer sometimes if not over-revved. All manufacturers had major issues in one area or another with 12,000 rpm single cyl motors compared to the 8~9,000 rpm two stroke being replaced
bcstoy1 year ago

Is it the idea that people pay a fee, to download the info?

crazypj (author)  bcstoy1 year ago

Nope, just not to be added to a 'Trade School' curriculum or similar. Ifr you want to link it to a site that isn't a problem

crazypj (author)  bcstoy1 year ago

Not really, Instructables isn't public/ tax money funded but relies on subscribers. With a subscription you can download 'anything' for free. I've never yet been in a position to become a paying subscriber but have had full subscription status due to my Instructables being featured. (Pretty sure I don't have 'pro' status at present, but I haven't tried downloading anything) You can bookmark link and re-read it as much as you want for free though. If you checked my profile, I was an instructor at a motorcycle school. I had many experienced but untrained colleagues, some real good who cared and others OCD about management policy. I did this as there are way too many 'technicians' who don't ave a clue how to actually measure. I took a great degree of pride in making sure 'my' classes could actually instruct instructors who were doing things wrong

It's part of the reason I was laid off when people who 'don't rock the boat' are still there even though they are totally inept in my opinion (and manufactures seem to agree, 'servicing' is now often restricted to oil and filter changes plus put some air in the tyres) less than 5% of students actually stay in the industry(2011 figures) plus, in my opinion, probably less than 1% will actually be 'good' at what they do (sadly, less than 10% of the 1% will actually make a good to decent living, no matter how good they are)

bcstoy crazypj1 year ago
Thanks, I am in the process of attempting to rebuild an engine block. But I need to know how to measure stuff and then learn how to use the lisle honing tool.
crazypj (author)  bcstoy1 year ago

The Lisle hone is almost exactly the same as Sunne which works well easy to use and VERY accurate after some practice (within 0.0001", yep, one TEN-THOUSANDTH of an inch)

It's also a 'shedload' cheaper than the AMMCO's I have plus cheaper for consumables (and is more tolerant of mis-use / accidents)

The 3 leg 'glaze busters' / brake hone's are worthless as a precision hone although work OK for intended purpose

The best way to hone by hand is with a low speed drill, preferably one you can use at around100~ 150 rpm while learning (most rigid hones have max rpm around 400~450 up to 4" bore) Slower is MUCH easier to control as you have to get the stroke (in / out of cylinder) synchronised to the rpm to get a 45 degree cross hatch. Because I'm always broke, I got a Harbor Freight low speed high torque drill (plus coupons when it was on offer.) It maxes out around 450 rpm but I've never used it 'flat out'. I started an Instructable on 'How to Hone motorcycle cylinders' but got caught up with other stuff so never got any further

bcstoy crazypj1 year ago
This is good advice. How about stone selection to get the right finish. Do you have tips on it. I have seen a couple of videos using the tool already. It does look very much like the one used my machine shops.

Thanks
crazypj (author)  bcstoy1 year ago
Hi, The AMMCO sets I have do give some idea of stone selection and speed/rpm for different sizes but doubtful the cheaper sets give enough advice?
What are you trying to do?
If you want to get cylinders to first oversize for new pistons it's best to start with a coarse grit stone, 80~100 if available. Hone to within 0.002" then switch to a 240 or 280 grit. You can either buy a 320~400 grit 'FlexHone' or use 400~600 grit for finishing, if your building a semi race motor or turbo/supercharged I would use 600 grit rigid hone as it will give a finer smoother surface but takes MUCH longer than the 'production' machine shops methods which is why it isn't used very often nowadays.
PatriceG111 year ago

crazypj do you still have a speedo elimanator for a honda cb3b360? please let me know if you do and how much?

thank you

crazypj just wondering if you found the speedo elimanator for a honda cb360. please let me know at my email address rltellier@yahoo.com

thanks

Armilite1 year ago

How about a How to Measure a 2 Stroke Cylinder properly for Making a Tuned Pipe?

2 STROKE EXHAUST CALCULATOR.jpg
crazypj (author)  Armilite1 year ago

Just realised what your asking (was in a rush this morning) You want set up with degree wheel to measure port opening and closing points, simple enough, just fit degree wheel to crank end and set to TDC=zero. You can also do it by measuring cylinder then use trigonometry but that is way more difficult as you need to take con-rod length and angle into account.

crazypj (author)  Armilite1 year ago

Hummm.... That IS a two stroke cylinder. The process is exactly the same for two or four stroke but four stroke cylinders are easier as you don't have ports. Designing two stroke expansion chambers Is relatively easy, but, you need to make several for testing plus, correctly done it will amplify exhaust note so silencing becomes difficult without destroying the power you've obtained. At one time, several (most?) manufactures actually provided information on building tuned pipes (at least I know Kawasaki and Suzuki did) In the service manual for T305 there is a diagram of pipe.A lot depends on the engine your building for and the type of use. No point having a 500rpm power band on an MX bike as it would be almost unrideable but on a drag bike it's all you need

crazypj (author) 1 year ago

Oh, and although this is under creative common licence, it's for 'home use' not commercial use

bcstoy crazypj1 year ago

NP, actually I want to learn how to do it. The nissan motor I had in a truck I bought was/is in poor shape.