Topic: Oversize piston ring help CB400F

[Johnson]

Hi everyone,

I have a 76' CB400/4 and am currently attempting to do some needed work on the engine, I hope that someone may be able to give some experienced advice as I am somewhat of a novice with these engines and the like.

I'll try not to go into the full history of my bike and will try to keep it simple in the aim to not annoy anyone. 

The bike was seemingly running well apart from hard to start when cold, and also some blue smoke on start up which I assumed was the piston rings. So I brought the engine into a motorcycle workshop to see if I needed a rebore on the barrels or just a hone and new rings- (I've since given the engine and job to the workshop as sadly I don't have the time or tools to do what is needed). The guy measured one of the barrels and told me that it is 51.5mm, and so I would need to source and purchase 0.50mm piston rings to be fitted by him, and then also they will hone the barrels.

My main question is this;

My pistons are labelled Honda on the front and measure 51mm, I assume then that they are standard pistons for the 400f, therefore would I not need to buy 51.5mm pistons to go with the 0.50 oversize rings? Or would the 0.50 rings be fine on these pistons?

I understand this may be a stupid question but I'd prefer to do this once so need to ask for advice.

I would have liked to do a 466cc kit large bore but I asked the shop for a price to bore out and it is way too expensive for me right now. So I am aiming to just do the rings if I can.

Any advice would be greatly appreciated!

J

[Powderman]

The bore size, piston diameter, and rings must all match. If he's indicated you need 0.50 rings, that's referred to as second over (0.25 is first, 0.50 is second, 0.75 is third, and 1.00mm is 4th).

You'll need 4 new pistons and 4 new sets of rings, plus the shop will have to bore and hone all 4 cylinders.

You can, purchase the Cruzinimage 466 kit and get all the pieces you need, then the shop will just have to bore the cylinders to them. Costs the same whether it's 0.50 or 2.0mm of boring.

I'm behind this idea. But not necessarily cheaper. When I had mine done, if it was the next size over bore ,they would have charged $30 a hole, going to the 466 meant multiple passes to get to the final bore and the price jumped to $50 a hole. Still for $80 extra it's a no brainer to go with the 466 kit.

[Johnson]

Thanks for getting back...

When I brought in the barrels/cylinders to be checked the guy measured just one of the holes and then said it was 51.5 in size. He then stated quite quickly that I would need 51.5 rings. When I asked would I not need the pistons he stated that I would already have the oversize pistons. I then checked one of the pistons myself at home and found it to be 51mm. I went back into the shop and asked if he had found the oversize rings yet (as he said he can pick them up), I wanted to make sure we both knew what size everything was before ordering parts. I told him that the pistons seem to be 51mm, but he did not seem to be fussed with what I was saying and he still is ordering the 0.50 rings.

This is why I wanted to ask, if I just don't know enough then that is fine and I will leave it to the experts, but if someone is going to carry out work incorrectly or so that it won't last then I have to ask elsewhere.

I was quoted €200 to hone the cylinders if I got the 466 kit.

I'm just caught in between trusting someone with the engine and not knowing enough facts to understand if something is incorrect. 

Thanks again

[Johnson]

Cheers, I am in confusion also and that's why I had to try to find out exact details on this stuff before trying to correct someone who seems to have been doing this for years.

He seemed to say/think that I will already have the 51.5 pistons fitted if my cylinders were 51.5. I wasn't sure if this was true so I measured a piston and it was 51mm and marked Honda-I would then assume it was the original.

The thing is the head gasket was changed at some point as the previous person to work on the engine had put it on incorrectly, and this would lead me to think that possibly they rebored it to 0.50 but maybe just fitted oversize rings on standard pistons in the past, therefore the guy now is not getting a complete picture of the measurements/making too many assumptions and then not really listening to what I say or think.

While I understand this viewpoint if someone feels they have been doing things for years, it doesn't really help me when i'm paying and putting trust in them.

I can't tell at this time then if any of the work done in the past was correct, I think not as when I took the head off and the cylinders, there was no oil restrictors fitted and added to the gasket being the wrong way around I would think whoever done work on it in the past may have been a bit forgetful or just incompetent- to put it nicely.

I think I will have to go back into the shop and tell him to measure the pistons and see where we go from there. I didn't really want to spend 200 on a rebore, 120 for the kit and then whatever else if there is a safer,easier and possibly cheaper option.

I will update once I get in there.

Cheers!

[Johnson]

Yeah, I originally took the engine off to retap/helicoil the head exhaust stud holes, as the last owner put nasty incorrect bolts in. I knew the engine would need a bit of work so took the head off to check the valves and get them cleaned/lapped etc, that is when I then saw that the pistons were in a bit of a state so thought to get new rings/pistons and or a rebore.

I originally gave the head to the shop just to get the valves done and machine the exhaust studs, which has turned out to be me bringing in the rest of the engine to attempt to just get it sorted without lots of hassle for myself (doesn't seem to be going that way )

It is a motorcycle shop that has a workshop at the back so I assumed by their advertisement and there talk that they knew/know what they are doing.

Again, I guess I will have to wait until next week now to see what happens (Easter holiday season here at present)

J