Acceptable bore on a Honda 360?

Sonreir

Sonreir

Oregon
DTT SUPPORTER
Hey all,

I'm currently in talks with my machinist to get my 360 bored out from 67mm to 70mm. He thinks that I'm pushing my luck with that much over and he wants more information before he continues.

Does anyone where have any experience with boring out a 360 to that size?

Thanks in advance,
Matt
 
Well let's back up a minute - do you have pistons to put in the holes?

Honda would have specified 'acceptable' over bore sizes and made pistons and rings to match. 3mm is a LOT of material to go up without changing out the cylinder sleeves and is either the max or beyond the max of Honda specs.

Are you trying for more displacement or just a fresh bore? When I did my XS650, I went up 0.5mm which is 2 steps up from stock. So 3mm is likely well beyond any Honda spec.
 
I don't have the pistons on hand, but I have a supplier which will custom build me a set (and provide rings) at +3mm over.
They are most definitely not on the 'acceptable' list from Honda.

The idea is to increase displacement, and not just get a fresh bore.

The sleeves are around 15mm in thickness, so we're talking about trimming a fifth from the total thickness of the sleeves.
 
Sonreir said:
I don't have the pistons on hand, but I have a supplier which will custom build me a set (and provide rings) at +3mm over.
They are most definitely not on the 'acceptable' list from Honda.

Hey! Don't worry :) I went from 71mm to 74mm pistons for cb450 without to many problem. You have to find a specialist to bore the cylinder correcly but thats the maximum ;).

The idea is to increase displacement, and not just get a fresh bore.

The sleeves are around 15mm in thickness, so we're talking about trimming a fifth from the total thickness of the sleeves.
Click to expand...
 
Sonreir said:
The sleeves are around 15mm in thickness, so we're talking about trimming a fifth from the total thickness of the sleeves.
Click to expand...

That doesn't sound right. Where are you measuring that? Typically sleeves are 4-6mm thick with the thinnest section at the bottom where they jut out of teh barrels.

According to our database, that is 74.9mm on a CB360. So if you left say 4.0mm wall thickness, the largest safe bore would be 74.9 - 6.0= 68.9mm.

70mm would only leave (74.9-70)/2 = 2.8mm which is a little thin for a race motor and way thin for a reliable street bike, but you might get away with it.

If you are spending that sort of money on custom designed pistons, why not go to a larger aluminum liner and get them plated by Millenium. Heads can be machined to a better squish angle and re-angle the valves and move them farther apart at the same time by moving the valve seats and guide bores.

Rocker geometry will change but not enough to be a major issue but port shapes will be better suited to the new intake tracts to match those new pistons and head configuration.

You might also consider a GSX400 8 valve head for a little more oomph. It has more potential and allows the use of slightly modified GSXR600 pistons and lighter Ti valves. Add one pair of a GSXR60K1 EFI with a Microquirt to drive K1 injectors and you would have a neat little custom motor putting out 45-50Hp at the rear wheel.

6 speed 360 trans is up to the task, but the lubrication might need an upgrade and re-design.

Just thinking out loud again..
 
Well I have the engine apart right now and looking at the sleeves (from the top of the cylinder jug down into it) they are definitely a lot thicker than 5mm. I, unfortunately, do not have a micrometer available to me and so I just used a ruler to measure, but I don't think I'm 10mm out...

I'll double-check again when I get home from work.
 
Actually... it looks like you're right and I did screw up somewhere...

I just took a pic I had of the jug and measured the number of pixels in the diameter of the cylinder and worked it out to 4 pixels per mm. I compared that with the number of pixels in the thickness of the sleeve and came out to just under 5mm.
 
Guess I just dial my bore back from 70mm to 69mm?

I'd love to do all the other fancy stuff which you mentioned, but I'm really stretching my budget for the cam, pistons, and springs as it is.

EFI is definitely in my thoughts as well, but I'm thinking I may wait until the next rebuild for that one.
 
its a cj360. you want a "fast" bike, then go buy yourself a used 90s 600. no point in putting that much time and money in a 360.

if you need a new bore, then overbore to the max of whats reliable. clean the ports up a bit, go for pipes and an intake, and ride it.

I once dreamed of having megacycle cam, lightweight valves, yada yada, on my 350, then realized i would be spending 3000 dollars on a motor, and it would be nowhere near as quick as a bike i could buy for the same amount of money. so my mods were done for reliability first, then power later.
1.5 oversized pistons, electronic ignition, 1969 cam (slightly larger lift), new cam chain, slider type tensioner, degreeable sprocket (to get cam dialed in perfect). Doing a small port job (mostly port matching). 1100 dollars worth of stuff, including the cost of the overbore. reality of the matter is that most of the power i gain wont be from any of that, but will come from proper tuned intake length with vm30s along with a proper tuned exhaust. and even still, it wouldnt have been a bad idea to not touch the motor at all and simply lighten the bike and improve handling (which i have already done).

if you want FAST, go with an early 600cc sportsbike. you want something else, then continue working on your 360. the only way to justify the amount we spend on these bikes is that its what WE want. Why am i not on a cbr600? simple. I would rather be on my 40 year old 350 and stand out from the crowd of poseurs that cannot use half of what the cbr is capable of.

sorry, got into a little rant there.
 
Rocan said:
the only way to justify the amount we spend on these bikes is that its what WE want.
Click to expand...

Exactly. I want a fast 360. :D

If the goal for my build could just be reduced to a straight HP to $$$ comparison then I would never even have started it in the first place. But as you so correctly point out, there's a lot more to it than just speed. We all want something that we can call our own when we're all finished. We want a bike that is truly one of a kind. Many of us accomplish this through style modifications or performance modifications, or a mixture of the two.

I, of course, prefer the last of the three options. It seems like such a shame to spend so much time and effort building a proper café racer without giving it a little extra power over a stock model. I know full well that the 360 is not well thought of, but to me that's part of the appeal: Taking something many people do not care for and turning it into something that is sought after and appreciated.
 
Sonreir said:
Guess I just dial my bore back from 70mm to 69mm?

I'd love to do all the other fancy stuff which you mentioned, but I'm really stretching my budget for the cam, pistons, and springs as it is.

EFI is definitely in my thoughts as well, but I'm thinking I may wait until the next rebuild for that one.
Click to expand...

Man, with out all the other mods you're going to end up with one POOR breathing, SLOW reving, oil starved (CB360's in general) motor.... That goes BOOOOOM!
 
Louie,

I can't tell if you're just poking fun or being serious, but I'll assume the latter. I'm inferring that you think my selection of modifications will actually hurt the performance of the bike? How will going for a longer duration cam with more lift hurt the breathing? That's exactly the problem I'm trying to alleviate.

Thanks,
Matt
 
ok sonreir, continue with your work then if your cool with the expense. you should just realize that going THAT big oversize isnt going to be worth the expense as your not going to gain much power. may see an increase in the midrange pull, but without many other costly mods, its a bit pointless.

louie has a point. grab a copy of "performance tuning in theory and practice" and youll see why.

in short, you need to match the performance of the top end to the bottom end. larger intake valve for one, reshaped combustion chamber, and some other things will need to be considered.
 
The cam will help... but the huge overbore is what I would be concerned about. And sorry, I'm not trying to be sarcastic. I guess my point being is, if you want to build a fast motor. You are really dealing with an underdog. Crazypj has a shit ton of work into his motors and is still only making 39 hp. While thats very respectable for sure. But, the same amount of work on something a little more agreeable to modification I believe would have yielded better results. He's doing it because "he" can. Meaning, he has the knowledge and experience to do it all in house.
 
@Rocan - Well, I think it's fair to point out that oversized pistons aren't the only mod on the list and I'm well aware what a waste of money it would be if they were my only consideration. I'm relying a lot on the cam to fill in the gaps in the mid and upper RPM range. To me, the 360 motor seems pretty asthmatic past 6K RPM and the longer duration and higher lift will help to cure that ailment. The pistons are to help out on the bottom end. I'm just trying to see if anyone has done it before.

There's not denying that the list of items provided by teazer would help, but as I do not have an unlimited budget, the line has to be drawn somewhere. I'm going with the cam because I believe that's where I will see the most gain for the $$$ spent. If there's something I'm overlooking though, I'm all ears.
 
Thanks for the info, Louie. I'll try to find some posts by Crazypj and see what he's been up to. :D
 
My "shopping list" was less of a serious suggestion and more of an example of where that thought process can lead a person if they don't have a firm grip on reality and your wallet.

A larger bore is possible, but I personally wouldn't get custom pistons for a street bike. If it were possible to get say Suzuki 850 pistons to fit and to get them cheaply, that might be cost effective.

It all comes down to what can be done at a reasonable cost. Extra cubes are a great start and tend to add low and mid range but add very little at the top end because the motor can't breathe deeply enough. On the street that's often a good compromise.

A larger motor with ports cleaned up can often stand a little more cam lift/timing and that props up the top end without losing too much of that just found bottom end. But now the dollars are starting to pile up.

What it all comes down to is budget and drive to try things just because you can, and experience. For someone like PJ who has been into bikes since the stone age, that's not a problem. He has the experience and probably the tools or at least he knows who to go to to get things done. He knows those beasts inside and out.
 
69mm pistons in CB360. The flange is quite big but that isn't the liner thickness.

CB3781.jpg

Stock CV carbs, unknown 2:1 pipe, no air filters, guessed jetting ***.* secondary main (didn't need anything like as big as charts suggest, f/a way too rich) Stock cam and valves, 'mild' port clean up
Dynorun5-Nov20th2008.jpg

Changed pipe for home made stepped version, lengths are not right made it as experiment, *** secondary main jet, stock primary main.
most of the info is in my build thread
Dynorun6and7Nov20th20083.jpg

I've since made air correctors nd changed jetting again but can't get dyno run done
I haven't had time or inclination to get to the 422cc conversion, I did one a few years ago but don't know what happened to it.
You have to change liners but block will bore out OK.
I would use GS1000 liners for 70mm bore (oh, I already did on the 391cc motor ;D )

I certainly wouldn't go to the time and expense of having pistons made.
GSX head swap isn't possible without more work than even I'm prepared to do, the bore spacing is different.
GS850 pistons require no more than boring cylinders, they are lighter than stock 360 pistons so give big ends an easier time
(or, 12,500 rpm 8) )
It will be reliable after I modify oiling system
Engine suffers from usual early 70's problems, the valves/ports are really too big for stock bore size, they are just about perfect for a 400 motor though ;)
 
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