Topic: Some advice on piston clearance CB400F

[madsundaysurvivor]

hello guys,
on my cb400f I have a clearance between piston and cylinder of 0.06 0.06 0.06 and 0.07mm for each hole.
According to the manual with new parts it should be between 0.02 and 0.04mm.

 How long can this clearance be achieved without large compression losses?

 Should I rectify cylinders and buy +0.25mm pistons?

Or otherwise, if I just put new std size piston rings on it, could it run a few thousand kilometers more correctly?

thank you so much

[Bodi]

I don't have the shop manual at hand. Is there a service limit shown? If under that I would not be too worried.
What happened to get the engine pulled apart? If compression was down and smoking, and valves/stems were good, suspecting the pistons/bores is reasonable.
These engines usually smoke a bit on start as there are no stem seals on the exhaust guides.
If my bike, and I wanted an update with pistons & rings & rebore, I would go with an overbore to 466cc. But that's me. A set of +0.25 pistons, rings and gaskets, if available, is going to cost probably less than a complete kit. You get a bit of a power boost but stock carb setup should be fine.
Yes this is max bore and no further rebore is possible, so that's it for those barrels. But, seriously, how many miles will this 45 year old bike be used?
Just a hone and new rings might be OK but the bores wear oval and the rings wear oval to match... so if they are worn the new rings will have a hard time sealing.

[scottly]

The service limits are shown in the right side column:
 

[bryanj]

Just doing a bore for a 400, smallest piston biggest bore gives clearance of 1.6 thou(0,04mm), biggest piston smallest bore 0.4 thou(0.01mm) so your clearances are way too big, honing will only make it worse.
Also 0.25 piston kits are not easy to find now but Cruzinimage in Japan do 0.5 and 1.0 oversizes kits at good prices.

Big problem is finding a shop who will rebore to the exacting and VERY tight clearances specified by Honda most car shops say it will sieze and do it too big, ideal clearance is 0.8 thou or 0.02mm.
Mark Paris, Hondaman, is the person to ask and tell me if my maths is ok!

[scottly]

Just doing a bore for a 400, smallest piston biggest bore gives clearance of 1.6 thou(0,04mm), biggest piston smallest bore 0.4 thou(0.01mm)

Those are the factory stock acceptable clearances for when the bike is first assembled, not the service limits. If the piston is within that range, they pass inspection and go on to the assembly line, and the same for the cylinder bores. BTW, there have been problems with the CI piston kits for the 400.

[HondaMan]

The 350F and 400F engines need very tight bores. I set them at 0.0004"-0.0006" when boring for new pistons. They run perfectly and last for a very long time that way.

If the piston-to-bore clearance in these tiny engines exceeds 0.0014" they will smoke.

[scottly]

If the piston-to-bore clearance in these tiny engines exceeds 0.0014" they will smoke.

Like Bryan said, the largest factory bore with the smallest piston gives 0.0016" clearance. That is acceptable for a brand new engine, as determined by Father Honda.

[bryanj]

I know those are as new tolerancesĺ, i was attemting to show how tight the tolerance was.
All the problems i have read about were the 350 four and seemed to be the machinist giving too big a clearance.
I will bore at exact piston size then hone a 1/2 thou clearance

[scottly]

Or otherwise, if I just put new std size piston rings on it, could it run a few thousand kilometers more correctly?

That's what I would do, unless the bores have excessive taper or out of round. Do not try to straighten the bores with a hone; you only want to break the glaze and leave a cross-hatch.

[madsundaysurvivor]

Thank you very much for the answers,

 I give you more information: I opened the engine because of a knock knock noise, it didn't smoke. It has 39,000 kilometers in theory, I thought they were main bearings and they were the rubber blocks of the primary transmission.

 Once opened, I changed the connecting rod and main bearings. The cylinders measure 51.05 and the pistons 50.99. Ovality is acceptable only 0.01mm. If I do a honing the diameter will possibly increase 0.01mm more.
My question is, do I spend the money on oversized pistons (0.25, 0.50 or 0.75 doesn't matter) or with these tolerances can it still hold up to something more?
 As you have said, the problem is finding a grinding workshop that does it well. I like to check the measurements afterwards and many times they are different from what was requested.

[scottly]

Pistons are 50.99? That's the largest stock piston. The bores are in the middle of the standard tolerance. EDIT: the bores are .04 above the largest standard size, but still less than the service limit of 51.1. Damn metric system...
 You don't need to remove very much material at all with the hone; just scratch the surface. A dingle ball hone works for this because it will follow any minor irregularities, unlike a rigid hone; just don't grind and grind away at it. You are well below the service limits, which is the point Honda says boring and piston replacement are required.

[scottly]

From my college textbook:
Deglazing Cylinders With Minor Wear
 Cylinders with minor taper and out-of-roundness can be deglazed by using a spring-loaded hone such as shown in fig.16-16. (The figure shows something similar to the common 3 stone hone)... Swab some honing oil on the walls, insert the hone, and start the hone spinning. Move the hone up and down in the cylinder (do not let stones protrude more than 1/2 inch on top or bottom) rapidly enough to produce a crosshatch finish with an angle of about 50 deg. Do not be too concerned about an exact angle. Anything from 20 to 60 deg. will suffice.
Make about 12 complete strokes with the hone, wipe the bore and inspect the walls. If a hone pattern is visible over most of the ring travel area (a narrow band at the point of heaviest wear need not be honed), consider the cylinder finished. If not visible, repeat and check again.

[madsundaysurvivor]

Thanks for the advice, I have already done it on my English motorcycles, but with diameters of 70 to 90mm.
 I have done all the measurements again, the piston rings are very worn, I have tried a set of new std rings that I had and the end gap is within tolerances for new engines (0.33mm)

[scottly]

What was the gap on the old rings? More than .7mm?

[HondaMan]

I know those are as new tolerancesĺ, i was attemting to show how tight the tolerance was.
All the problems i have read about were the 350 four and seemed to be the machinist giving too big a clearance.
I will bore at exact piston size then hone a 1/2 thou clearance

That's how the pro shop I use does mine. They bore to 0.0002" less than the piston diameter across the skirts and hone to [whatever clearance I want] after that. Their hone is impressive: it has 8 stones on it, each one is 0.125" wide with a rounded surface and an elaborate cam arrangement on its top end sets the limit to which it will expand when honing, quite the tool! They release the clamp on the top, compress by hand and set it into a fixture, dial up the desired final bore diameter on the gage and then set the nut on the top. Then it is compressed it to pull it out of the setting fixture and set into the hole, the chuck is lowered to the coupling shaft and then it is run. The fixture oscillates the hone for the crosshatch pattern, and the bore is checked every 15 seconds for dimension until done. I think that thing weighs more than my JEEP!

[madsundaysurvivor]

Yes, around 0,95 in first rings and 0,55-0,65 for seconds rings