brian h

I made a silencer the shape you describe for a Triumph TR20 Trials Cub which looks good and above all works perfectly. You need 0.062 wall alloy tube to slide over a 1.25 dia exh pipe and need access to a good Tig welder and requirea tool to created the scoops needed. The pipe that goes into the silencer has four staggered rows of scoops protruding into the gas flow, these scoops are equally spaced - four scoops in each row, offset on each alternate row and approx 1.0ins apart inside the silencer. the bottom pipe terminates open ended inside the silencer and causes the gases to swirl around into the top pipe to atmosphere. The body of the silencer began as 4.0 ins dia tube X 0.062 ins wall, squeezed to approx 1.5 ins oval, then cut along the top and bottom, fitted to the tubes with end plates and Tig welded. You would need to make a tool to created the four rows of scoops. This design works well and good both for noise and performance - these silencers are hard to find these days. Hope this helps

Hi, I restored a basket case 1955 James Commando last year and found the steering head ball races completely worn and decided that taper roller bearings were the answer, this involved turning up a upper and lower taper bearing race carrier from Mild steelf to fit in the frame headstock, then turn down the existing spindle to suit the bores of the taper bearings, its well worth the effort to modify the steering away from the ball and cones - I doubt they are available anymore. Hope this helps.

The Ossa MAR uses the dreaded Motoplat ignition,I am not familiar with the points type ignition as my Ossas have always been pointless. If you have this system, the stator coils are very finely wound with very fine gauge wire, these break down when hot and everything else stops. First you need to remove the flywheel, mark and remove the stator and examine the wiring that you can see as most of the stator is encapsulated in resin. The ignition wires from the Primary windings to the HT coil are Blue and Black, these ideally need to be repositioned to exit the flywheel case at the 1 oclock position by drilling a hole for the wires to pass out alongside the front engine log then silicon seal and take the two wires up the front RH frame tube to the HT coil under the tank. The original design with wires coming out at 6 oclock position is fine until a rock hits the bash plate and they get crushed or waterlogged! Miller Products sell the PVL Motoplat HT coil replacement for the pointless ignition - these are good, however, the next step I would suggest is to swap your ignition stator with one known to be good if you can find a friend with one that works. I rewind the trigger coil and the primary stator coil with more turns and thicker wire for each, this produces an excellant spark, there can only be three defects - ignition, carburation or mechanical!!. Hope this helps

I have rebuilt two Mk I and one Mk II Ossa MARs, the essential tools you will need is a flywheel puller, a tool to hold the flywheel,a tool to hold the clutch hub and a standard two legged puller for the front shock absorber. You need to be carefull with the gearbox shaft shims and keep them together when you strip the engine, otherwise the engine is held together with screws and a good screwdriver wil work fine, the engine is easy to assemble and there is a good book to help you. Google Ossa World and you will find the Clymer Publication: OSSA service Repair Handbook - it will tell you just about everything you want to know and web sites for parts. I use the later 80 weight Castrol Gear oil which works fine. The decompressor fits the 14mm tapped hole on the RH side of the cylinder head - the spark plug is always central, the decompressor works well as Ossa brakes are not good when wet. Hope tis helps

I have a 197 James Commando K7T similar to your FB, let me know what engine, clutch, gearbox and rear sprockets you have at present, I assume you have the 4 speed wide ratio gearbox?.

Hi Do you have a 38T or 51T clutch chain wheel. ? Brian H

Its all set to happen at the SSDT for the new Greeves - said to be fielding a three man Team Greeves - wonderfull news, and a real opportunity to prove its worth for the team who have built it and developed it so far. I remember when the first Greeves I saw in the SW Centre back in the late 1950's brought out hillarious outbursts amongst us riders, however, we changed our tune very quickly when Bert Greeves and Derry Preston Cobb got to work and engaged the top development riders of the day and produced the fine Greeves Trials bikes that won just about everything to be had, not to mention their motocross successes,road racing successes and their road going bikes. I think it is great for Trials worldwide because at last a British Trials bike is out there to take on the Spanish & Italian two / four strokes with a fine pedigree from the past - there is no reason why the Greeves name cannot be on top once more in World Trials. No, I have absolutely no association with the new Greeves organisation, however, I clearly remember the rapid development of the original Greeves motorcycles - few will disagree amongst those of us who were competing against them in 1950's - 1960's period that they were superb - cant wait to see the progress of the new Greeves.

Hi. The Ossa MAR MkI swing arm differs from the later post 1974 MkII Ossa MAR. The post 1974 Mk II has a swing arm 25mm (1.00 ins) longer, giving a longer wheelbase and the requirement for the longer of the two rear brake cables is required. I think the Ossa Explorer would be the same as the Ossa Mar Mk II requiring the longer cable. Both Cables are available in UK from Dave Renham. Hope this helps. Brian H

Ossa MAR fibreglass tanks can be successfully repaired - inclucing your cross over pipe connectors with a fibre glass resin filler, you can reinforce the pipes with a piece of glass cloth approx 25mm square fitted like a collar, laid up when the fibreglass resin is still soft, this works well, is strong and resists LL petrol. Fibreglass resin repair kits can be bought easily. The same resin / filler can be used to repair other spider cracks that may appear in the tank. The Ossa Mar Mk I and Mk II both are robust bikes, handle well and ideal for Twinshock Trials, also parts are readily available for the Engine, sprockets, brakes, etc from suppliers in the UK and USA. The Motoplat electronic ignition HT coil can be replaced by a PVL unit which works well, and replace the IRZ carburettor with a Del Orto carburettor and ensure you seal the flywheel cover to crankcase with a sealant- but provide a breather - this will keep the water out of the ignition. Hope this helps Brian H

In 1957 many looked in disbelief (I was one of them) when Bert Greeves and Derry Preston Cobb wheeled out the new Greeves Scottish Trials bike, this bike quickly caught on and started to dominate the UK Trials scene attracting the top British riders of the day, followed by riders changing to the revolutionary Greeves design - just wait a while - a few major National Trials wins notched up with this new Greeves will ensure the same success as the early Greeves Scottish Trials bikes and its derivatives - I would love to see one compete in the forthcoming SSDT - it has a great heritage. Brian H

Hi, Would be interested to know if you have a Mk I (up to 1973) or the later Mk II Ossa MAR (from 1974). Mick Andrews got the entire bike set up well, the Mk I is a shorter wheelbase with rather quick steering, whereas the later MAR Mk II has a longer wheelbase (added 25 mm on s/arm) over the Mk I. I believe this increase in wheelbase made the bike more stable and mirrored the 250 Bultaco Sherpa T, circa early 1970's for longitudinal stability. The Mk I Ossa MAR is quite different to any other bike of that period in my experience being nimble for river beds , rocky sections , tricky turns, etc with its quick steering, whereas the Mk II provided more stability being longer and gripped better in mud sections . Dependant on which model you have, you could change the overall angle by changing the length of the rear shocks. Having owned three Ossa MARs MkI and Mk II, my recommendation would be to build it up and get it running first, ride it, and see what you think of it, they are great bikes, robust and designed by a man who was a top works rider with a great Trials record, including Scottish Six Days wins and also an excellant engineer. For the period in which it was designed (early 1970's) the Ossa MAR was capable of beating them all. Hope this helps Brian H

I think the Bullet model you refer (I presume it is the 346cc Trials model) was fitted with a Lucas racing magneto, to be later upgraded by the Lucas Wader(water proof) magneto on the Trials Works Replica model from around 1958 - I owned a W.R. circa 1960. Fom your desccription, your loss of sparks when hot would indicate the magneto HT coil is likely breaking down when it gets a heat soak, these HT coils can usually be rewound successfully and become very reliable. When the magneto is running with no sparking across the contact breaker points, this is a good sign all is well that end of the magneto. Try Hitchcocks of Solihull they are the R.E. experts and would be able to give you the magneto info, etc, you require. Hope this helps. Brian H

Talk to Hitchcocks in Solihull, they will know the answer to your question, they know their Royal Enfields well and carry a lot of Redditch RE stock. Hope this helps

Does anyone know the email / phone contact for Steve Sell for Ossa MAR parts. Many Thanks Brian H NZ

OTF, I own two 1973 Mk I Ossa MAR 250,s and one 1974 Mk II Ossa MAR 250 , I am currently working out an exhaust system for the Mk II. I have an original front pipe and looking to see if I can design an expansion box , tail pipe and silencer. I see no reason why a smaller diameter tube can be used, the Mk I pipe has to go into a rectangular section behind the expansion chamber to provide tyre clearance, then open out to a full diameter to carry the silencer, the Mk I end pipe is approx 1.375 ins O/D to accept the silencer. The engine needs some form of expansion chamber - space will allow up to 3.0 ins O/D fitted with a perforated pipe inside and the cavity packed along its length with exhaust fibreglass packing to dampen the somewhat noisy exhaust note. I think the orginal silencer had a 0.875 I/D bore at the outlet. I made two silencers of 2.0ins O/D MS tube X 9.0 ins long with a 1.0 ins O/D MS centre tube X 10 ins long which I perforated drilled myself, fitted a detachable alloy end cap to allow fibreglass packing to be installed ongoing and works fine and good sound suppression. I think it is possible to fabricate the entire system as you suggest, I believe there must be an expansion chamber of sorts, you will need to make rectangular section aft of the expansion chamber to provide exhaust pipe attachment to the rear frame and to provide adequate tyre clearance, this rectangular section is 1.625 ins X 0.625 ins with approx 0.062ins wall tube. There is likely mathmatical calcs for two stroke exhaust design,etc, etc, however, the Ossa is a robust engine and pretty grunty and I think you will soon know if you have strangled the exhaust. Please excuse the use of fractions ! Hope this helps Brian H

You may have agricultural three wheeel tractors known as a Gnat in Australia, I have seen them here in NZ. I can tell you that the NZ Gnat version has a Villiers engine fitted with Wide Ratio gear cluster - suitable for your DOT project, which should fit directly into your engine if I recall. It could be worth advertising in Australia and New Zealand to see if you can find a Gnat that just might have a good Wide Ratio gear cluster suitable as a Trials gearbox. Brian H

The Great Sam used PK screws through the side rims on Ariel GOV 132 principally I believe in pursuit of weight saving and deleted the typical two rim locks on each wheel, I always assumed this mod to delete the rim locks worked well, I have no idea how the tyre beads stood up to the low tyre inflation and high loading over time - has anyone else ever tried this principle ? Brian H

Hi You can safely use 50 - 1 ratio on a MAR - works fine on Motul or Belray synthetic - no problems. Brian H

OTF A number of Ossa MAR crankcases complete seem to turn up on eBay (USA) from time to time, always usefull to have other spares - even if you have to buy a complete engine. The Ossa MAR engine is mechanically pretty tough from my experience, however, the alloy cases seem to suffer with intergranular corrosion over time and do not weld easily. I think the USA was a good market for the Ossa MAR bikes and spares in the 1970's and still the best source for parts. Try Ossa World website to start with, this web has a number of good contacts that may be able to help you. Hope this helps Brian H

Find an old worn file 9 x 1 ins, hand grind a radius on the bottom close to the headstock inner diameter, grind the file bottom edge to form a nice sharp edge, grind /remove the file tang and insert the file into the headstock, the file ground edge will fit just enough to get a bite on the old race, put on a pair of safety glasses, then give the file drift a sharp tap and it will move. You can sharpen the end of the file if required, this is a usefull tool for other jobs too. Brian H

OTF, Try Alex Snoop in NY State USA (Ossa World website) email asnoop13@optonline.net I have obtained MAR gearbox part from him recently - good to deal with, if he does not have a part on stock, I think he would know who is likely to have it. Sammy Miller also used to stock o/s MAR pistons at one time. Hope this helps Brian H

Woody, Thanks for your comprehensive reply on reed valves and mention of Harpower and Romat versions - reed valves interest me, however, as you have noted previously, the 250 Ossa MAR has plenty of grunt for todays twinshock events, they start easy and handle well and grip well in mud and do not really require any serious mods. Thanks

Woody, Any info still about on Harpower / Romat reed valves for the Mk 1 250 Ossa MAR - are they still available and if so, do you know what is involved in fitting a reed valve to the Ossa cylinder ?? Brian H