My 750F2 project.

[Cb750r]

I have never liked the floppy passenger pegs, and would have removed them ages ago except for the fact it’s the one bike my wife likes riding on as a pillion and this is the bike we will go for mellow cafe cruises on together.

So I undertook the task of making them less floppy, by making a Delrin washer that’s thicker so in the upright position it’s snug. I also made Delrin bungs for the end of the peg so when in the “down” position the Delrin plug butts up against the bracket at 90 degrees. The added benefit is that the Delrin should be a lot more gentle on the brackets paint so folding them up and down won’t be sanding the powder coat down.

[SteveD CB500K0]

Had to Google that…


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[Cb750r]

Had to Google that…


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Same material a lot of modern bikes frame sliders or similar bike protection is made out of. It’s a very easy to machine plastic. I think non trademark name is Acetal plastic.

Ive made a variety of sliders and rear stand bobbins, bushings spacers step washers etc with the material.

[Cb750r]

Well I went to speak with the shop that did the cylinder boring, they put on a grand show, measuring a cold cylinder and moving around so quickly as to not really show me anything but say all the right words to convince me they have done no wrong.

I got sick of this and purchased my own dial bore gauge and measured all 4 cylinders. And my machinist is right the very top of all the bores are on target… but sadly that’s as good as the story gets.

All cylinders are barrelled. All of them are the most narrow at the top of the bore and tighten up a bit right at the bottom. Unfortunately that leaves them at the most wide at points where piston velocity and side thrust are at the peaks if I’m not mistaken.

While I’m not a machinist I did my best to take accurate measurements and can see how easy it would be half a thousandth out. However I can consistently repeat measuring the inconsistency in the final bore.

I think these cylinders will make a great decoration and will have to get another set machined by a motorsports specialist. I’ve attached my measurements below.  Note I re measured #1 it does barrel as large as 0035, but comes back down to 0030. Have it written down that 0035 was at bottom but it’s not.

[Johnny4428]

Not qualified to comment on these figures, thankfully I had mine done by one of the very knowledgeable and competent qualified guys on this forum. Hope you get it sorted.

[McCabe-Thiele (Ted)]

I'm certainly no machinist - it did make me scratch my head as to  how you end up with a barreled shaped bore when using a boring bar of the right size range?

Bryanj will know.

[K2-K6]

With no idea what their machinery is like, it's speculative to assess from me.

Usually associated with feed rate in general machining, the tool has to be buried into the material it's cutting, with a wedge shape of the base material being sheared out of its structure as the tool is forced through it in removal.
If the entrance feed was cautious to preserve face of bore, speeded up through mid part, then slowed for exit, this could produce a barrel effect. Basically with higher feed it has more aggressive cut width on each revolution, with the tool being "pulled" further into the wall as it travels with that difference in feed. Its a bit like stripping wallpaper, when you get too much angle on the blade and it's goes "oink~twang" taking a neat little triangle of your plaster with it 

You'll get it on a lathe too, with a longer tool extension being a little flexible if reaching a long way in. On that you try to offset that little wedge of removed material by grinding a back rake on top of tool face to minimise that wedge in size, the angle of attack effectively.

Drill bits in lathe headstock do it too, you have to go at it really slowly to stop the drill nose "walking" around in a circle if biting too hard, especially as it's likely there's imbalance across the drill cut blades from one side to another.

It shouldn't happen obviously from a good machine shop, often you'll see experienced machinists going about something in a certain way that may look odd, but experience in that process gives them ways to mitigate shortcomings in process.

[Bryanj]

If those are sizes in thou over the size of the piston its been bored too big anyway

[Cb750r]

.0025" is what JE calls for for these pistons.

From what I understand this is likely to bad honing setup/ technique. Either way these sleeves are toast as 65mm is max bore on stock sleeves.
 
I may try these on a cheap set of 65mm pistons in these if I build up all my leftover engine parts into a second engine some day. But these cylinders sure aren’t going together with all the expensive bits I’ve assembled for this build.

[royhall]

0.0025" is way too much for a Honda. At that it would burn oil for fun. Maximum on those is 0.001" and sometimes less than that. Plus they may not all be the same. Pistons should be allocated to a bore then measured for each bore. Fairly likely that the pistons are not all the same size. If you should get a slightly undersize piston on a 0.0025" bore you will hear the slap. As for the barrel effect, from my time as a machinist that is usually too high a feed rate. Rushing.

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[Nurse Julie]

Agree Roy, that piston to bore clearance is massively over the top, like you say, 1 thou inch maximum on most aftermarket and down to 0.6-0.8 thou inch on genuine Honda pistons.
At 0.0025" that's almost old British bike spec where they use a lollypop stick to measure the clearance 😂😂😂

[Cb750r]

These are 2618 forged pistons, which require more clearance than other piston materials.
Also according to the measurements required which is .5” up the skirt this is closer to. .0020 measurd conventionally at the base of the skirt
. I’m not really debating the spec set out by the piston manufacturer. I am more concerned about the lack of workmanship and my Un usable cylinders.

[Bryanj]

Roy, i disagree about piston sizes. If using genuine Honda, or even aftermarket i personaly have never found a measurable difference from quoted size, as to the big bore ones i dont know

[royhall]

That's fair enough, if your pistons are all the same size then the bores will be as well. But it would be a poor job not to check and just trust them. I've had two sets of aftermarket pistons that have been slightly different, they were machined to suit. Never had a difference on genuine though.

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[Bryanj]

Years back we bought our pistons from a man on a van and his were all Art who i believe made the o.e. honda