A crank case for Sherlock

After getting the engine out of the frame, decided to swap in the new engine, after re-rigging a new stator and rotor and matching up the connections to the ignition coil.

Two engines

Engine in and also went back to one of the original exhausts, as the mounts had broken on the Circle F I had. Made up a new bracket for the rear using great meccano strip metal which I’d found near Abergavenny station (not off the track) whilst trying to fix the Polo exhaust.

After that, decided to strip the original engine and see why the selector was stuck. Removing the clutch side and selector mechanism, demonstrated that the drum was indeed still stuck. Oh well, time to split the cases.

Doesn’t take long to split the cases, (with the soundtrack was Saint Etienne and The Seahorses) and the problem was revealed, with some pieces of metal lying underneath the selector drum.

Something inside the engine

Removing the metal, it look like the inner race of bearing, but a check of the the cases and all the bearings were intact. Strange. Time to call Mr Holmes. Cleaned out the cases and continued to search.. Hmm, no problems.

Mystery solved

Going to the Parts book and searching through the Transmission diagram didn’t turn up anything and check, all the bits that were meant to there, were there. But checking the balancer shaft diagram, which had been removed from the engine, showed a bearing with a detached inner face. Probably root cause, which hadn’t damaged the gears, but broken up and ended up in the bottom of the case.

Further work to be done, but the cases are back together, with clutch side and top end to be rebuilt.

It’s been a while…since the last article

So as well as this site, there is now
ClassicMXWales the new site for the Welsh Championship
Bwlch Blog for the non biking stuff
Bwlch Offroad Parts

so something else to content on.


The Yamaha SC500 has seen some development over the last couple of months, and I’ve managed to build a new engine (also built an engine for someone else). New engine has Boyesen reeds, a recently acquired piston, as well as new bearings and seals through.

It’s also had a make-over, with a plastic tank (like the one on the 250) and new plastics. In the grand scheme of things, very much a straightforward exercise and the engine started and ran from the off.

Currently working getting the 250 ready and trying to solve the power problem with the bike, with a new piston and rebore, as well as a carb overhaul. The TT500 hasn’t seen much effort and the exhaust and valve stem seals are still waiting for workshop time.

6 weeks till the start of the season (for me) and still lots to do. Including work.

A few things SC500

I was on the hunt for some brochures or articles on Yamaha’s opinion splitting dirt bike.

Firstly, found this poster

SC500 or MX500

Where does the MX500 come from ? Never seen it referred to as that, so quite interested in why, potentially for local marketing reasons ?

The article for the Dirt Bike review from 1973 is online . Nicely written and some good pictures. The top quality comment “[the SC500] has all the delicacy of a belch at a funeral” is priceless.

Also located another SC500 rider (mad fool) with some great pictures of him racing in the US. The pictures and gallery is on photobucket

Also, there are few threads on the Circa74 forums on the SC500, including from me

Working on two fronts

Spent some of the weekend working on two of the bikes; the SC500 and its long awaited engine rebuild and the timing on the Husqvarna WR250.


Going back to April, I’d damaged the cases on the SC500 at the Llanthony Clyro meeting, following a failure of one of the dog gears. Lack of oil seeming to be the reason for the failure. The cases had been repaired and I’d reassembled the bottom, but decided at getting another bottom end.

With the acquisition of the replacement from California Connections / AP Motorcycles , I’d started looking at the rebuild. The original crank had a damaged rotor nut thread when I had it, and getting it out of the crankcase didn’t improve matters too much either. Therefore used the crank that came with the new cases.

Though I’d rebuilt the engine originally, getting the crank out of the cases turned into an epic and needed the use of plenty of heat, and my oven, in order to get it down. The balancer shaft had already been removed from the replacement case and therefore I’m guessing it had been raced at somepoint in the past.

Rebuild went okay, but left with a couple of issues that need resolving:
• the rotor for the PVL ignition was catching on the stator and though there is some room for adjustment, I couldn’t get it to run true and it therefore goes solid when you tighten the stator plate.
• the clutch basket is excellent hasn’t got the wear the original has, though it seems a little reluctant to get into place.

Husky WR250

I’d done a brief look at the bike after picking it up and the main issue was the lack of a working ignition. It may have been DK who’d look to put a Motoplat ignition coil on the bike with the Femsa stator and rotor in order to make it work, but that wasn’t going to the solution.

Some research had shown that for WR’s of the period, with the larger crank ends, that PowerDynamo ignition was the way to go. They may not provide greater performance, but as experience as shown, there’s nothing like a modern electronic ignition to transform a bike. The PowerDynamo kit – has the signification advantage in that it supports lighting kit. The WR250 RT was a road-legal bike, and using their AC kit, getting a rear light and headlamp in place isn’t going to be an issue. They do an DC kits also, but don’t have a battery and unlikely to fit indicators.

The ignition got delivered 3 days after ordering, and although the 20 euro delivery charge to the UK wasn’t cheap it worked.

Installation was pretty straightforward, though the LH thread rotor nut supplied wasn’t correct (maybe this is for the RT model only ?), so reused the existing nut and a couple of washers. The block connects aren’t the best and the PVL kits are better quality. That said, after resoldering some new connectors on, the bike actually fired first kick (see below on other items)

Timing the ignition wasn’t too bad, but Husky timing information (from both the Clymer and Haynes manuals I’d acquired) is in degrees BTDC, rather than mm’s of piston travel. Therefore borrow Tamara’s protactor, and chalked up the measurements on flywheel. It’s clockwise rotation, so the firing mark is 22 degrees anticlockwise of TDC, ie to the right of the TDC mark. Therefore, got the piston to TDC and then aligned the flywheel, before tightening the nut and then checking it to ensure the crank hadn’t moved.

Working out the Timing

The Mikuni carb had been left with petrol in it, so after a quick look, decided to bolt on the matching carb from the Yamaha MX250. A strip and clean on the agenda, if I can battle my way through the smell of stale petrol, my favourite. Though the original carb was a Bing, I’m not going to be loosing sleep on its orginality.

Also ended up using the plastic tank from the Yam, as the whilst externally in good external condition, there was some rust inside the Husky tank, maybe the difference of a bike from Colorado rather than one from California. I doesn’t look too bad, but will need an application of brick cleaner and maybe some stones rattled around inside it as part of the clean up. Petrol tap needs de-gumming.

A running Husqvarna

The engine ran okay, but a bit noisy and I’ll plan on doing the bearings and seals once I’ve finished the 500 engine currently on the bench. However, optimism abounds once you actually get the thing running, however briefly.

In terms of other parts, the bike needs:
– lighting. This is going to be non-trivial to get the original Husky units, but I’ll probably go on the hunt for some period replacements. There are some good aftermarket D250/400 headlamp repo’s which will fit nicely and will work with the 12v output.
– cables, need a front brake cable, for the full width hub that’s fitted and a new throttle cable.
– brake pads; not essential but to be sourced
– seals and bearings; the engine is going to need a rebuild, but it’s in the queue along with the SC500 and another MX250 engine that needs doing.
– tank sealant, will probably find something to line the tank.

SC500, bit of a gearbox problem

After the SC500 locked up at the beginning of 2nd race at Clyro, I’d initially assumed that it might be the clutch problem I’d had at the end of last season, however it clearly wasn’t as the absence of neutral confirmed. Somehow I persuaded some of the guys to heave the bike back to the trailer whilst I was out on the 250 race.

Oh, what are these bits ?

Back in Bwlch and time for a quick look on Sunday night (along with looking at the MX250 problem that had appeared), as I knew I was on the road for 10 days. Removing the clutch cover revealed that a loose shim from somewhere, along with what looked liked the remains of a needle roller bearning. More pins/needles were found as I removed the clutch, but limited time meant that splitting the cases wasn’t going to be an option.

That evening got me thinking about spares and parts; unlike the 250, I’ve not really got any parts for the SC500, though the cycle parts are interchangeable with the 250. I’d missed some parts on ebay earlier in the year (part of the off-loaded Barry Townend collection). Engine spares I have none.

Whilst out and about this week, I found the time on the train journey between Utrecht and Frankfurt to have a quick look on the online spares catalogues that North West Vintage Cycle Parts and the only needle bearings are mounted into the crankcase and it maybe that these have failed, causing the problem. The good news, that the bearing is a common part with the MX250/360 (good old Yamaha) so may have a relatively quick fix. I’ve also scanned the parts book in attempt to preserve it. The North West pages has the advantages of direct ordering and a list of the superceeded part numbers.

It’s a week or so befire I’m back to the workshop, but it gives me time to mentally prepare for the worst scenario and a knackered shaft.

SC500, finished project ?

Work (and some pleasure) had limited the amount of shed-time on the SC500. Both myself and Gavin had had problems getting the bike to run on the ignition components we’d had from Rex Caunt Racing . Debbs had sent to some replacement coils to the other Malcolm Herbert in Hereford, but something more fundamental was going on here. Speaking to Rex, moved to the designed for TY ignition and so spent a morning with him and his workshop fitting a new charging coil and getting it set up correctly.

The bike ran and managed to fill the corridor outside the workshop with smoke and back in Herefordshire managed to complete the bike in the 2 days I had before the first race of the year at Clyro.

Finishing touches

So, to run through the work on the bike and what I’d done with it during the 5 month period of restoration;

|Bars|Renthal Desert Bars, sourced from LG Racing in Pontrilas|
|Controls|Domino levers and a Gasser throttle, the latter from Motolink |
|Wheels|these are original and didn’t need respoking, just tightening|
|Tyres|Golden Tyre set, 100/100 R18’s on the rear, not too big in terms of profile|
|Plastics|Though the side panels are original and good condition, the mudguards were not so good. Replacement grey (rather than silver) from DC Plastics |
|Carb|The original Mikuni VM38 was badly worn, so decided to replace. Could have got a replacement from Allens Performance , but went for Dell’Orto equivalent from Eurocarb -> http://www.eurocarb.co.uk |
|Engine spares| Seals from Motolink, with the bearings sourced locally in Hereford. New piston and ring (original bore) came from North West Vintage Cycle Parts -> http://www.nwvintagecycleparts.com/ |
|Ignition | From Rex Caunt Racing, see above and other articles|
|Exhaust|Original unit, though considering Circle F replacement at some point|
|Rear Shocks|These are 340mm units from [Rock Shocks ->
www.rockshocks.co.uk ] |

In terms of work on the bike, the frame has been cleaned up and resprayed in Hammerite Satin, along with the swing arm and bash plate. The forks were in good condition and unlike the MX250 project they didn’t need rechroming.

Completed and running (for now)

Smaller version of this image

The engine rebuild was straightforward and I removed the balancer shaft during the rebuild (this doesn’t seem to have much of a negative impact with engine vibration). The gasket set was found on ebay. The main problem is a pretty screwed flywheel / rotor nut, which was a bit mangled and needs replacement. The barrel was on the original bore, though the piston in the bike had a chunk missing out of it. New piston and ring along with a honed barrel did the trick on the rebuild.

And does it go ? I can’t get it out of 2nd gear in my small field.

SC500 Ignition and Electrics

I’d spoken to Rex and Debb before Christmas and they’d kindly put the relevant electronics in the post. Seems the technology had moved on (a lot) since 1973.

The charging coil now doubles up as the trigger, so its a case of just of connected one end of the coil to the earth on the stator plate and connect the other to the spade connector on the ignition coil. I’d got the grinder out and ensure a good earth for the coil, and got it to fit to one of the existing mount points (this will need further tidying when I get a spark).

Ignition system in bike

Using the meter, I check for continuity from one of the charging coil connectors to earth and then from the other to the ignition coil. All ok. I then checked for earth continuity from the ignition coil to the engine, which was also good.

As the charging coil is also the trigger, its rotational location under the flywheel is going to be important and need to work on this, as currently not getting a spark. There is voltage from charging coil to ignition coil but need to see what is expected.

Ignition Coil

Charging coil / trigger

The one connector on the coil had come away slightly from the coil, so I’d passed a cable tie round to hold it in place. There’s not a lot of room under the flywheel and I may have knocked it after the initial soldering of the wires.

More research and information and hopefully we’ll get a spark. Electrics are not my forte, but on a bike like this they should be pretty straightforward.

SC500 Parts Book

You can find them here and hopefully you’ll find the useful. Its a good, full size A4 book, and hopefully scanning will make it less easy to damage it.

Yamaha SC500 Photo's

SC500 Engine Strip

Top End

Firstly, as has been seen already, the piston has a chunk out of the skirt which means its not perfect. The bore isn’t too bad, and it’s not been rebored. The head does need cleaning and the barrel need honing, and have already acquired a new piston and ring [[From North West Vintage Cycle Parts, for $99.00]].

Unlike the MX250, the SC500 has a larger external rotor (and uses a standard puller). I’d had this apart already on the hunt for a spark and it was clear then that the seals were knackered and this is what prompted a complete engine strip and the need to break the cases.


The clutch doesn’t too bad and should be okay for future use. The SC500 has a balancer shaft and its removal is straightforward, though installation needs to be done carefully in order to align the balancer correctly. The 1968-77 250-500cc Clymer manual is pretty good and gives you information on the different variants and options .

The kickstart and gear selector mechanisms are the same as the MX250 and fairly straightforward to strip down (and fairly easy to put back together). You can’t detach the kickstart spring and remove the whole assembly as one, and you’ll need to remove two circlips from the shaft, and then using the kickstart lever move the shaft round anti-clockwise until it disengages from the spring.

I’ve a template made up for the crankcase bolts and this is useful if even to ensure that you’ve all the bolts removed before you try and split them. Again, split the crankcases is the same as the 250, using the two threads for the crankshaft seal retainers. Very easy split.


What was obvious straight away was that the main bearings and the primary drive bearing had a lot of corrosion, so now only were they difficult to remove from the cases (lots of heat, cold day), but will need replacing. The primary drive bearing is retained in position by a large circlip and I have a feeling finding a replacement bearing won’t be straightforward.

The seals are also slightly different from the 250; I think the rotor side is maybe the same, but not clutch side.

Big end seems okay, so no need to split and rebuild, I hope. More to come, but at now at the stage of knowing what needs to be done. Some engineering decisions to be made, whether to balance the crank (and leave the balancer out), nicosil line the barrel with the new piston etc and also what to do about the electronic ignition.

Starting work on the SC500

I’d bought the SC500 back in July whilst starting the trip round for the National Road Rally. The guy at Governor’s Bridge / Huggy’s Speed Shop had mentioned there was no spark from the bike and after a quick faff with the CDI units from the 250 bikes (which are different) I’d not managed to get the bike to fire either.

Rather trying to solve this on its own, time for the complete strip down and to also have a look at the engine. The frame / chassis / cycle parts on the bike are similar (well the same) to that on the MX250, so some interchange will be possible. Frame is essentially the same, which is slightly worrying in terms of performance.

Engine, the external view

The bike looks like its been untouched for a while, with the 1976 California off-road certificate in place of the rear mudguard. The autolube system was still in place, and was first to be removed in the strip down. The fuel tank internals and the state of the carb seem good, so all promising (so far).

Out with the engine and plonked onto the bench (which was clear of the MX250 engine rebuild) and then stripped the rest of the bike. Wheels and spokes good, and fork chrome isn’t pitted and is serviceable. The frame has some rust spots, but not too bad so a strip and rub down will do the trick.

The plastics are also okay and I’ll not look to replace them. They’ve a nice patina so will make a nice look for a 37 year old dirt bike. Once the strip was complete I cleaned and then sanded the frame, before spraying with black Hammerite Satin. This has worked really well on the MX250 . However, tried spraying with the compressor and there’s a bit of overspray to cure and finish a little more neatly.

Now for a quick look at the engine. Removing the cylinder head is a surprise if you’ve been working on 250cc bikes; at 94mm diameter its big. A quick check without getting out the vernier is positive, with the bore and rings looking in good shape. The only problem is that there is lump out of the piston skirt, but not sure if this is recent.

The 'wow' factor

The crankseals are gone (hence the draft when cranking the engine over) and though they can be done without splitting the cases, I’ll probably go the whole hog and go for the full strip.

On the work list:

|Frame|respray and tidy up|
|Wheels|spokes and rims in not too bad a shape, go a rebuild may not be necessary|
|Forks| as well as the yokes seem pretty good. Will replace the steering head bearings, around £20.00 off ebay|
|Shocks|like the 250’s they are not good. Went for NJB shocks before and may do the same here, but longer body, probably 340mm. Had thought about Rock Shocks but need to check prices and performance.|
|Seat|Fine, covering is good|
|Ignition| couldn’t get this sorted before the strip and needs to potentially get a replacement stator, as have spare CDI and ignition coil|
|Exhaust| the main pipe is fine, though the California required spark arrestor may impact power. Got some pictures and prices from Circle F exhausts and will have a think about getting one. Not sure about what the performance increase might be|
|Airbox| in good shape, though the carb rubber doesn’t seem right (too short) so may need to find or make a replacement. |

On the engine
|Autolube| Will remove it and go to premix; could leave it on the bike with an empty tank|
|Crank| seems fine, as do the main bearings|
|Piston| has this missing bit. Its the original piston, so may go for a (relatively) cheaper same size piston and ring. |
|Barrel| is in very good condition and seems not too loose |

The strip will reveal any other problems and will debate the removal of the balancer in terms of power and vibration.

More to come and some shed time in the next couple of months. Also will have an ebay trawl. SC500 spares are slightly more pricey than say MX250 and MX360, but availability is there and still relatively reasonable.