I'm working on a top-end inspection and refurbishment of a 1928 101 that was last rebuilt by the expert George Hood of Garden Grove, California some time between 2002 and 2005 who stroked it to 55ci (901cc).

The aluminum pistons are cam ground, T-Slotted and 4 ring. The lowermost ring, the oil ring, was installed on the rear piston only. That land was left empty on the front piston.

I'm wondering why an oil ring on the rear piston?

Or from the other perspective, why not an oil ring on the front piston?

Maybe the result of indecision which way to go?

All still is looking fairly tight in there (rings at 50%, valves need lapping, guides now variably 003-004 over - measured for me by my nearby machinist) but as I have new rings for these pistons I may install them to close up the end gaps. Undecided what to do about the oil rings I have - I'd like to cut down on the carbon buildup cleaning chore. Whatever I do I may install intake valve seals.

Bob

Hi Bob, I guess your motor builder was experimenting , and as Manny points out, the rear cylinder gets much more oil than the front. Oil in the twenties was way stickier, thats why the bikes have priming cups, largely to de stick the gummy oil on the rings on cold days. The idea of the baffles is to create a pressure differential to draw an oil mist up the bores to lubricate the top end. I have never had a satisfactory answer as to whether modern oils mist in the same way, but presumeably not, as oil mist is not a a desirable outcome in a modern engine.

Many people leave the oil rings out of total loss motors, that will lead to decoking more frequently than using them. I would think that it is more importnt to get the oiling delivery to the motor right for your style of riding, as recirculating oil sidevalves still have no oil feed to the top end, and so are effectively the same lubrication system there, the main difference being a pressure feed to the big end, and the cooling and settlement of sediment in the oil tank, instead of it silting up the bottom of your motor.

I have seen motors with and without oil rings run well without coking up excessively, and have seen both oil excessively. The aim would be to get the correct oiling without damaging the bores, rings are a service item, it is far prefferable to rering a motor with a light  hone to break the glaze than need a rebore, or hunt for a new set of cylinders. A well bedded in set of rings without the oil control ring is going to be preferable to a badly seated set with an oil ring , I would have thought, so the bedding in process is most important. I have seen two 101 motors with oil rings fitted run up to seven thousand odd miles with no signs of adverse wear, thats as high as I have ever got personally, I have seen plenty with and without oil rings with excessive ring wear in less than a couple of thousand.

As an aside, memories of playing with brit bikes in my twenties, I used to rering Triumphs etc as per manual, breaking the glaze with some abrasive paper packed with newspaper and an up and down rotary motion, as per Triumph manuals. seemed to work fine, as long as there was no excessive scoring of the bores, whereas nowadays the default setting seems to be worn rings, need a rebore, and there goes maybe a decade of life from your cylinders.

I run 2  28 101 Scouts and a 28 Short frame Scout   all 750cc  all 3 have Sport Scout Pistons   with an oil ring only on the Rear Cylinder   but with a 30 thou  gap . All 3 use a small amout of oil  and are used almost every week  with other friends on  our almost weekly  runs  .  I happened to change the cylinder heads on one of  the 101s over the weekend  from ALLY  Modified  38 Sport Scout heads to original Mainly as the compression was  just to high   due to an old Racers   handiwork.  It took a while to get the  pump  to a point where when i returned home  i usually have around 25 to 30ml to drain off  after around 100kms  . Im happy with that situation  ,We genarally sit on around  40 to 45 MPH    Back  to removing the heads  the front had some Carbon  build up   and the back  just a slight amount, The bores were still very clean  and unmarked .  .About every 3rd ride I drain the sumps  and replenish with  fresh oil via the hand pump. As for riding  i have never used the hand pump on a ride  for many years !    Its all a personal choice I think and  getting the balance correct   for oil  usage . 

Regards Phil  

Tim Raindle wrote:

Thats really good info Phil. Interesting enough for me to want to try it. How often are you doing a decoke ? What rings are you using ???

well  I took the heads off  last weekend    after about  10000 km    It was  not real bad  but   noticable    , The heads i had on the bike  were modified Sport Scout   to fit  101 bolt pattern  but had also been shaved    conciderably   with WLA  flywheels  the compression was close to 6.8 to 1     Making the bike a bit of a beast to kick over  But  plenty of power  .  My leg strength   was getting a bit tired   so have gone with standard heads  with relief for the pop up on the pistons 

Philip , Sorry to change the subject a little , you have WLA flywheels , as mine does , can you tell me what ignition timing you have please , mine runs Indian 101 rods not WLA , also has OHV conversion so suggestions on timing will be appreciated as am having running problems , starts and ticks over great but very erratic on the road ! Maybe as Tim says - 99%of magneto problems are carb orientated ?? have fun , Ken