blue smoke from exhaust

[gpobantam]

Afternoon folks.

I have just acquired a 1950 MarkIV Sports. It has been standing awhile during which time there was no oil dripping on the floor.I did over 80 miles, gentle riding, no real problem although slight clutch judder. I then did 150 miles after which time it started blowing blue smoke out of nearside silencer. I thought it must be broken piston rings. I have removed head and barrel. No broken rings. Removed one ring and put it inside the barrel - result with gap of 2 thou. No marks on cylinder which is showing 2.398 inches bore, each of piston rings is shiny all way round no sign of blueing/oil transfer. Piston is marked 0 (or 6) 155 on crown. I have checked valves each is uniformly shiny at joints. Everything appears ( to me an amateur mechanic) to be O.K. I then thought it may be an engine breather being blocked causing excess pressure in crank but there does not appear to be a breather system on this engine.
Any suggestions before I put it back together ?
Thanks, Mick

[eddie]

Hi Mick,
            The standard size for the bore should be 60.8mm (2.394"), so it would pay to check that your bores are not tapered as they could have been honed to remove the wear ridge - and then oversize rings gapped to suit. This would account for small (2 thou) gap - the reccommended gap is about 6 thou. A tapered bore would also account for the smoke, as the rings moving in and out would then tend to pump oil.
          Regarding the number on the crown - it could be 61.55 which would mean it was once 30thou oversize (but may subsequently have been turned down to fit it in the standard bore). Repro pistons made by either Hepworths or JP are marked in this way to denote the size of the bore they are intended to fit  (the running clearance is already allowed for on the piston, so the bores should be actual size).  JP pistons have 1/16" wide compression rings whereas Hepworths fitted 2mm wide rings.
        Originally your engine would have had an open breather exitting centrally at the bottom of the timing cover - over the years, owners have experimented with modified systems using non-return valves in the oil filler cap, so it is possible the original port may have been blocked at some time. If all else is in good order, the standard breather system works well.
        Hope this helps,
                            Regards,
                                          Eddie.

[gpobantam]

Good morning Eddie.

Many, many thanks for your reply.

I have checked the ring gap at various places in the bore and it seems pretty parallel. The biggest ring gap I can register is 2.4 thou and the smallest 2 thou. These measurements have been taken at the top and bottom of the bore and three places in between. Accepting the fact I might be less accurate than a professional I do not think I am far out. There is no noticeable "wear ridge" at the extreme end of the bore.I have also checked it with a pair of calipers with similar results.
The ring width is 2mm.
At this stage I have not disturbed the timing cover to check the breather.

The oil filler cap is standard. ( I am not familiar with oil return valves being fitted to filler caps. I have one in the pipe line adjacent to the oil tank on my Norton.

We have the first dry morning for ages, here in Devon,but I do not think it will last so another few hours will be spent in the shed. My wife is away at the moment so I will not need to listen for the cry "Your dinners ready, it will go cold"

Cheers Mick

[eddie]

Mick,
        From the fact your pistons have 2mm wide rings, they are probably Hepworth pistons - in which case they should have 'PHC' cast on the inside. These pistons were the first attempt by the LDMCC to get repro pistons made. Some engines fitted with these pistons did smoke - and some badly! - whereas others ran cleanly - it didn't make any sense! Later batches had drain holes drilled in the bottom of the oil ring groove - this reduced the incidents of smoking considerably.
     Regarding the breather - there is no need to remove the timing cover to check it. The breather port exits at the bottom of the timing cover (behind the central vertical rib). You should be able to check that it is clear by inserting a length of wire into the port - the wire should go in about 5" before coming up against any obstruction. This is an open port, so it is normal to get a pumping action with the engine ticking over.
          Regards,
                        Eddie.

[gpobantam]

Thanks again Eddy.

I did prod a piece of wire in the hole yesterday and it did go about 5" so all seems well there. I will look at the inside of the piston , at the moment there seems no reason for the smoke. If the piston is stamped and obviously an early batch without the drain holes, I will either see if my mate can modify it or if not get a new one.
There is, crossing my fingers, nothing untoward with the offside pot so I will just hope it is not going to be affected.

Thanks again.
Cheers Mick

[gpobantam]

Morning Eddy.
Sorry to bother you again.

The piston is now off - Yes it is stamped PHC , it is also stamped A102. There is a series of 10 holes( 5 either side) drilled from the bottom of the piston up into the ridge below the oil scraper groove. There is another series of 8 holes - 4 either side- drilled through into the oil scraper groove allowing one to see right through from the inside of the piston.

I am still baffled

Cheers Mick