Honda pistons fitted - very smoky exhausts

[C Clay]

I have just rebuilt my T35. It has a Mk3 engine and gearbox. Fitted new mains, big ends, cams, cam  followers, valve guides and valves etc. (great service from Dick at club spares, many thanks).
It had very worn bores and the only option was to bore out to take the Honda pistons as mentioned in the very helpful article by John Holmes. The rebuild generally went well and the bike started on the third kick, OK so far, but then after a very few seconds me and the bike was enveloped in a cloud of oily exhaust smoke that could have hidden the battleship Tirpitz. It subsided slightly after about 5 minutes but even then it was unacceptably bad.
I took off the exhausts and they are both oily inside. I removed one head and cylinder and found quite a bit of oil on top of the piston.
On examination of the rings I can see that the oil control ring has shiny and non shiny areas around its circumference, so it may be that it just takes some time to bed in, but in truth I have never had this issue before in the 46 years I have worked on motorcycle engines.
One other thing I noticed was the rings, top, middle and oil control all have a ring gap of .021". Which seems big, but is pre set by Honda and not adjustable (they are the right rings for the piston oversize, I double checked it). I read somewhere someone mentioned modified Honda pistons, not sure what was meant by that.
I would certainly like to hear from anyone who can provide some help and advice regarding this issue. If club members who have fitted the Honda pistons, successfully or not, could give me the benefit of their experiences I would be grateful.
Clinton

[oily bloke]

Hi,
Some Honda rings have an up side. This is identified by either a 'T' or a 'P' near the end if the ring or if you look at the profile of the ring the curved edge is uppermost and the square edge is lower. Some pattern rings are not marked. The curved edge lets oil past and the square edge scrapes the oil off the bore and into the oil holes to cool the small end. all the rings except the three piece type oil control rings may be profiled.  Alternatively you may have crankcase venting problems and I am sure there is someone on here that is more 'au fait' with the post war models than I.
Hope that helps
Andy

[C Clay]

Hi Andy
Thanks for your reply.
The rings, including the oil ring has "TOP" marked on the upper face so I am pretty sure I fitted it correctly.
However I will recheck the profile vs the marking in case it is marked incorrectly
Regards
Clinton

[eddie]

Clinton,
            From the fact that all rings have a .021" gap, I would check the bore sizes first. Normal practice with motorcycle engines is that the bores are 'size' (in the case of the Honda, exactly 62mm). The clearance is allowed for on the piston. I know of a couple of 'rebore specialists' who have made the mistake of adding the clearance to the bore size - with the same symptoms you have described. I have fitted Honda pistons (CB250N) to quite a few Dragonflies without any problems with oil burning. I used to do my own rebores to 1 thou undersize and then hone to final size. With regard to Honda pistons being fitted to Mark series engines - Mr Douglas never recommended going above +.030 as the early type barrels (on the Mark 3) had a known weakness just above the base flange and have been known to fracture. Late type barrels on the Mark 4&5 were of a thicker section and did not suffer in this way.

  Regards,
                Eddie.

[C Clay]

Thanks Eddie, for your prompt response.
FYI I gave the borer the pistons and he bored to suit giving the clearance stated in John Holmes maintenance guide for the Honda pistons. So I was surprised at the ring gaps too.
When I originally stripped the engine I found the bores were +.060" and very worn, so to clean up the bores are now to suit 62.75mm pistons so way over the 60.8mm of std. You are right the wall thicknesses are rather thin in places, a little worrying I must admit.
I only have a digital calliper but will measure the parts as best I can and share that with you.
I would certainly appreciate your thoughts and advice.
Regards, Clinton

[kev]

had similar experience with a g/80 matchless
 
  I wrapped the breather valve with pfte tape to limit oil leaks (as if??)
one of my wraps came adrift and ended up over the inside end of the valve..effectively reversing the valve operation...

  it smoked like I had never seen an engine smoke before.....had another RFV Honda with similar problems....the p.o. had installed the rings upside down..it mostly just blew oil out of the exhaust pipe...  and lottttts of it

I don't think ring end gap would cause the oil problem...I reckon you have breather problems...to have it that bad...try hooking up a power brake valve...

[C Clay]

Eddie,
The bore is 62.80mm and the piston is 62.70mm(fore and aft) and 62.45mm(across gudgeon pin).
With the piston in the bore I can just about get a 0.004"feeler in at fore/aft, so that confirms my measurements.
There is some initial marking on the piston fore and aft but none on the gudgeon pin side, which is what I would have expected.
I remeasure the ring gaps and got .022" on the oil control ring.
Best regards
Clinton

[C Clay]

Shuswapkev,
Sounds familiar!!
That is something I shall investigate also.i did check that the breather pipe was clear and the filter gauze clean but it fits pretty tight to one of the gears in the timing chest so it could be that it is effectively blocked.
Thanks for your input
Clinton

[C Clay]

Hi all,
I checked the breather tube, put a hose on it and could easily blow down it and could hear that the air was coming out into the crankcase via the timing chest.
One thing I did not mention earlier, on the cylinder I have removed, the head gasket blew out at the top after about 5 minutes of running. No real sign that it had leaked between the combustion chamber and the pushrod tube but maybe it could have and pulled oil in.
Clinton

[cardan]

I know nothing about post war bikes (less still about Honda pistons) but 22 thou seems a pretty big ring gap.

Radco (the late Frank Farrington) recommends "at least 0.003 per inch of bore" (0.005 on racing engines) for air-cooled vintage bikes, and ring replacement at 0.010 per inch of bore. With these numbers, 8-12 thou might be an appropriate ring gap, and 25 thou gap would point to new rings required. I wonder what Honda recommends for ring gaps in its bikes?

Leon

[Dave@NZ]

Hi,
From my days owning Honda motorcycles 0.004" piston to bore clearance = worn out ready for rebore.
From your description sounds like the oil rings are not coping, even though the top ring end gap is large this is a side issue and not the  cause.

The piston clearance is more to do with the piston and less to do with the barrel thermal expansion wise so in my opinion piston clearance recomendations should be should closer to what ever the model of Honda the pistons were for if that makes sense.

Just some info to look at.
http://www.hondatwins.net/forums/55-engine-discussion/1976-maximum-piston-cylinder-clearance.html
Standard piston to cylinder clearance is .001 to .002" Max is .004"
Ring gaps: Top and second std.. .012 to .020"... Max .031"
Oil ring std ... .008 to .016" max .031"

http://nans.1gb.ru/motomanual/Honda/Honda%20XL200%202001/07%20-%20CYLINDER%20-%20PISTON.PDF

http://www.ehow.com/list_7644539_specifications-honda-sl100-engine.html

One method of reducing the oil burn would be to machine a step under the oil ring grove and drill oil drain holes through the piston on an angle so oil can drain away from the out side of the skirt.
Dave.

[C Clay]

Thanks Dave for the great information. I am learning a lot about Honda pistons.
Regarding the piston modification, the piston comes machined with a groove and holes below the oil control ring by Honda.
My initial observations are that the ring gaps, although greater that spec are all below the max. So high but not disastrously so. I also rechecked the piston clearance at the bottom of the skirt, I originally took it at the top of the piston and it is .003" so again over spec but within max.
One thing I wish I had established, before the head gasket blew, was to identify if the smoking was from both cylinders or just one. The waffle box silencer combines both exhausts into a common chamber.
I propose to refit the cylinder, with new gaskets and run on open pipes. If both smoke badly I would assume that I have a general problem with the piston, rings and oil control ring.
Fingers crossed.
Does anyone recommend using a sealer on the head gaskets? I used grease and have a 50% success rate so far.
Once again thanks to all you guys who responded to my cry for help, if you have any thoughts or concern about my proposal please let me know.
Regards
Clinton

[eddie]

Clinton,
            The head gaskets are best fitted dry. Run the engine up to temperature - then nip up the head nuts again once it has cooled. Also recheck the nuts regularly if you have had the heads and barrels powder coated - the coating tends to squeeze out from under the nuts when the engine is hot - thus reducing the pinch on the gaskets.
  With regard to the Honda pistons - there are Honda pistons and pattern Honda pistons! The genuine pistons have a three piece oil ring that is far superior to the slotted oil ring that is fitted to pattern pistons.

  Regards,
               Eddie.

[C Clay]

Thanks Eddie for your advice on the gasket and re- torquing.
Yesterday I tightened up the other cylinder head and as you predicted the nuts needed significant  tightening, the gasket does appear to relax quite a bit.
The pistons and rings came from David Silver and were in sealed Honda packaging.
The oil ring is one of the single piece types, not my preferred version either but that is what was in the set. I will call them on monday to see if they have the 3 part ones in stock.
Anyway, one again thanks for your help.
Best regards
Clinton

[C Clay]

Update
Fitted new gasket and now it does not smoke !!!
Eddie, your points about torquing the heads pointed me in the right direction. I tried to run the nuts all the way up the threads when the head was off and found that on 2 studs the nuts bound up very severely part way down the stud. So I am sure that this was causing a loss of clamping force when I torqued up the nuts.
I ran a tap through the nuts and made sure they ran up the studs freely. On re assembly I tightened the nuts tighter than I did before and hey presto, no smoke or blown gasket this time.
I do have another few questions though.
When I pull in the clutch there is a jangling/whirring noise, a bit like a Ducati dry clutch noise, is that normal?
One of the 3 springs on the clutch release has fallen off, i removed it from the bottom of the bell housing, should I worry about this? Or will it function OK with just the remaining 2 springs?
Best regards
Clinton

[eddie]

Clinton,
            With regard to the jangling/whirring noise - the whirring is probably due to wear on the track of the release bearing. Whilst the noise is irritating, release bearings in this condition have been known to survive thousands of miles - the main priority is to make sure there is some slack in the cable with the clutch lever released - the bearing will then only be under load when the lever is pulled in. The jangling noise could be a loose retaining ring (normally held in place by the 3 return springs) being caught up by the spinning clutch plate. I would try running the engine with the clutch inspection plate off to see if there is any signs of movement of the ring - if it looks as if it is getting caught up, then the only option is to remove the flywheel to effect a repair. If left, it will probably eventually self destruct and end up in the bottom of the bellhousing - from where you may or may not be able to extract it. Provided it doesn't do any further damage, the clutch release should still work OK without it.
  Regards,
              Eddie.