Topic: CI pistons not central in bore

[disco]

Hi Guys,

I rebuilt one of my CB750 K2’s with CI 0.5mm over size pistons.

It was a smoker from the outset. I ended up dismantling as it was spewing oil down the exhaust. It was obvious that it wasn’t going to improve. I had black oil dripping from the 4-1 collector.

One thing that I’ve noticed is that all 4 pistons are not central in the bore. They all sit over to the points side (RH side) of bike. No1 & No2 are more pronounced though and there is more wet oil in the exhaust port of 1 & 2. I’m wondering if this is the cause of the smoking?

I’m thinking I’ve got a bent rod/s. Find it a bit strange that all 4 rods would be bent though. Or the pistons are manufactured incorrectly?

[HondaMan]

What piston-to-bore clearance was used? It should be a max of 0.0010" with those pistons. It would be really hard to see them "shifted" to one side if this much clearance was used.

[disco]

Hi Hondaman,

I think the machine shop used a clearance of 0.0012”.

Regardless of the clearance, wouldn’t the pressure from the piston rings just centralize the pistons in the bore? There must be another reason like either the rods are bent, or the pistons are machined slightly ‘offset’.

[Don R]

 The pistons should float on the rods with some side to side clearance, we can't tell if they do. If the rods were bent I'd think the cylinders would be scuffed. If you pull a piston, check the rings for correct orientation and check the rods for side to side centering on the pin. That's all I got.
  Hondaman could be on to something check to be sure they didn't go a decimal too big on the clearance.
 
  Sorry it didn't go well.

[disco]

Hi Don,
Thanks I’ll pull the barrels off & check the pistons tomorrow. I agree though, the pistons should float & should naturally find the centre of the bore. There’s something screwy going on.

Btw the bike rode well & made good power. It was just pi$$ing me off that I had oil leaking from the exhaust & blue smoke on start up. It didn’t seem to blow smoke when riding. The hone marks are still visible in the bore and there’s no scuffing to speak of.

I guess one way to tell will be to remove the pistons from the rods & then insert the pistons/rings on their own into the bores? If they all sit central then I know it’s related to the rod/wrist pin

[dave500]

might've been rebored out of centre slightly?

[disco]

Hi Dave,

The guy who does my machine work has been working on old Honda's forever. I really don't think that is the case. However, as I said if I throw the piston/rings into the barrels tomorrow and it all centralises itself, then to me, that points to either the Pistons or the rods as the culprit.

[BPellerine]

you could check the guides and seals as well?

[Terry in Australia]

Pistons are not completely round, they are slightly oval shaped, but as long as the rings are sealing against the bores, even if the piston-to-bore clearance is excessive, it won't blow smoke, or blow oil out the exhaust. I've had pistons with such big clearances  (over 10 thou) they rattled in their cylinders, but didn't blow smoke or oil.

There are several causes for the symptoms described. What are your valve guides like Disco? You're using a mineral oil, not a synthetic, right? You don't have a blocked breather tube from the cam cover, or between your engine and your oil tank do you? Did you do a compression test mate? Let us know how you go!

[disco]

Hi Bill/Terry,

I'll be looking more closely at the valve guides as well. This is a K1 head and it doesn't have seals on the exhaust. Maybe I'm completely barking up the wrong tree, but it just seems odd to me that the three wettest exhaust ports relate to the 3 pistons that are most offset in their cylinders.

Tezza, I'm running mineral oil. Same shop went over the head & declared the valves were A OK. Breather is brand spanking new, so def not blocked. I didn't do a compression test as yet. Bike is making good power. Just annoying that it's dropping burnt black oil from the 4-1 collector.

Hopefully tomorrow I can get the barrels off. They're not budging at present moment!

[Yamahawk]

Are you sure that it's oil dripping, and not gas from a leaking float in one of your carbs? It might look black from carbon that it picks up on the way out... or, it could be thinning the oil in the tank. Have you checked to see if your oil has a 'gas' smell?
Charlie

[rotortiller]

Just to verify the pistons rotate them 180 on the rods and install to see if the gap moved.

[madmtnmotors]

Breather is brand spanking new, so def not blocked.

Not the "air filter" but the crankcase breather vent from the valve cover. The crankcase must be vented (not blocked, plugged, or capped).

[Don R]

 Can you push the pistons over with a popsicle stick or something soft like that?

[PeWe]

Sure that clearance is not more? Like total clearance on each side which will be double up?
Rings correctly installed?
My 836 had 0.2 mm in total clearance (bore diam-piston diam at the skirt 1/2" up from front-back 90 degrees from wrist pin). Did not smoke oil at all. Old RC cast pistons not much used.
I measured bores on 4 points with a bore micrometer since they had more wear where rings have scuffed.

My forged JE pistons have 0.0025"  (0.0635 mm) as Dynoman has specified. No oil smoke when cold nor warm. Mostly full synth oil in now. 20W-50.

[disco]

Yamahawk, it's def oil. It's coming out exhaust port & also dripping from 4-1 collector under bike. New fuel tap. Tested it with fuel hose off. It doesn't pass fuel in off position. Fuel tap religiously is off when not in use.
Roto, good suggestion. I'll do that.
Mad, yep I understood the breather was meant. Its on valve cover, is new and is not kinked. Same with one off back of gearbox that goes to oil reservoir.
PeWe, the clearance is measured where you state ie just up from the skirt in a front to back direction. It does look like more, but piston is tapered. The proof will be when I measure, once I can get barrel off. It's not moving without putting up a fight!

[Terry in Australia]

No worries Disco, K0/early K1's didn't blow smoke normally, but if your valve guides have excessive clearance (and of course, no seals on that early head) that's a good source of oil to make some smoke. Also, check that the vent in your oil tank isn't blocked, have you got access to compressed air? If so, pump some air into the breather tube orifices, and make sure you can hear it passing thru. Be careful removing the barrels!

[disco]

Well as suspected the piston to bore clearance is really good. However there is definite wear showing on the side of piston closest to points.

[Yamahawk]

Do those pistons have locating pins for the rings? Is so, are they aligned with the ring gaps?
I am looking at your pics and I really don't see any significant wear there, disco. I would check to see if that lower ring (3 piece) has the gaps properly aligned with the piston groove and locating pin. And the upper one too.
Charlie

[disco]

Yep no alignment pin. The rings are free to rotate. The rings were installed correctly.

There's no appreciable wear. The cylinder still has the hone marks. The bike had probably done less than 100 miles on this engine. The pistons are a tight fit measured near skirt. I couldn't get a .0015" feeler gauge in there. I measure the bore at 61.48mm (2.420")

However, I can 'rock' the top of the pistons though. These CI Pistons do seem to have a lot of taper on them. Measured down near the skirt they are 61.45mm (2.419"). Measured up near the top ring they are 60.88mm (2.397").

Im going to get the valve guides checked tomorrow. Will update then.

[Terry in Australia]

Take your pistons and cylinder block to the shop too Disco, to get an opinion on the "taper" issue. Personally, I don't think it is an issue, but it's always good to get a second opinion from a pro.

[Keith]

My thought too!!!

[disco]

Well we have a diagnosis......3 bent rods (No’s 1, 2 & 4 ever so slightly). My machine shop guy carefully went over the pistons. There are scuff marks at the top RH side of pistons 1, 2 & 4. There is also scuff marks at bottom LH side of those same 3 pistons. Those were the same pistons that I noticed were ‘cocked’ over in their bores. So it’s showing wear at top right & lower left because the rods have a slight bend in them. Exhaust ports of 1, 2 & 4 are wet with oil. No 3 shows black dry soot. No 3 piston sat squarely in the bore.

So the entire bike is going in next week, to have the rods straightened (with engine still in frame). But he did say he saw this same problem on another CB750 recently & is confident that this is the problem. He rembembered doing valves on this ‘71 head, because he made custom oil jet holders for me. He said he’ll vacuum test it anyway.

Having the frame kit, sure made this a lot easier. Thanks Frank!

[Terry in Australia]

Well that's good to know Disco, are you happy to have the rods straightened though? I'm not sure if I'd feel that comfortable myself, I'd reckon new, or straight used rods would be better? I wonder how they got bent? I've owned heaps of CB750's over the years and while I've seen a couple of busted rods, I don't think I've seen any "slightly cocked" rods? I've got a heap of spare used rods here, that I'd trade for alcohol...........

[disco]

Hi Terry,

My machinist builds plenty of engines. He reckons all new rods undergo a straightening process that occurs because of stresses during manufacture. I don’t know how theses rods got bent. They don’t appear bent to me. I can’t see anything outwardly wrong. Maybe had something stored on them, who knows?
I have a set of 4 used rods arriving from US any day now, but that would involve me pulling engine, splitting cases etc. I’m gonna let him work his magic & see how it goes. Says he’s done it before. But if it all goes tits up, then I have another task for Xmas holidays!