UK Brat Build (Engine first)


XS650 Enthusiast
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Northumberland, England
Hello again one and all!

After a few years away from the XS fray, I have decided that my next project will be another XS. The past few years I have been building race bikes and playing on the tracks and road circuits. Some of you may remember me from my UK Bobber build thread, UK Cafe racer build thread, or my last build the UK Board Tracker.

I have always like the short brat style bikes. So... the plan is to get a donor machine at some point next year after I have moved house. Simple build with the key themes around:
Lowered rear suspension
Lowered front suspension
Mild apes
18” front and rear rims
Drum rear, possible drum front
PMA, Boyer ignition
VM carbs
Full rebuild on the engine
Topped off with a fairly bright and intricate paint job

But first, the engine! With no space, a full build isn’t possible until I move house into somewhere with a double garage. However, with lockdown in the UK leaving me with little to do, an XS engine fell into my lap, so I thought let’s start the build, engine first!
The engine is in a very poor state of repair but it does turn over. The plan is the usual, to completely strip and rebuild with new gaskets, seals, bearings, bolts,nuts, etc etc. With this engine build, I’ll have a little more time to hand, so I’ll be making more custom parts to set it aside from the rest.

Things like, machined inlet tubes, oil cooler (maybe with a clear viewing panel), cam covers, sump plugs are all on the cards.

Here’s some pics of the starting point .... at least I’m saving it... the tear down begins at the weekend.





Yeah, the two screws an the right casing are interesting. I’ll know more when I pop it off but as a guess, it may have held a strip, blanking the Yamaha out?? It ok as I’ll be fitting a three hole derby cover, which covers and looks factory
Re the left case, it’s very interesting and someone has gone to a lot of trouble. They had a pulley attached to the end of the crank and judging by the huge cut out, it was driving an external alternator.
Looking forward to your build.:thumbsup: What forks etc?? Are you going to use the left side cover with the holes.:yikes: BTW you could flip that drive sprocket and use it as is.:laugh: Keep us posted and good luck with your build.
Thanks halfmile! For forks, I may stay original but lowered. I’ve thought about doing USDs again or maybe even a springer but I like the simple look I think. Shaved down and neatened up, may be polished??
The original left hand cover will be scrapped. Indeed, great minds
Very nice looking Robson 750!! I wonder if that old XS engine ran a supercharger, or maybe it was an external alternator like you thought? I am sure I have seen that drilled side cover somewhere? It would look nice if each hole were plugged with brass then ground and polished.
Well, the strip down is complete. It was as dirty on the inside as it was outside. It’s previous owners haven’t been kind to it and it has been apart before. There are some none standard nuts and 533 rods in a 447, which may have been from the factory. I know the 533 rods are longer but not sure if they stamped the casings differently.

The good news is that the top end and bores seem to be in good condition. The cam is not worn and there’s only a slight wear mark on one of the rockers, which I’ll dress with a fine oil stone

The bad news is the crank. I noticed that the gudgeon pins were slack on the small end. You can see where the coating has worn away and one is more loose than the other. There is play in the big end too. Not a lot but it’s there. Bottom end bearings are worn too, with quite a lot of movement when twisted against the crank. So, it looks like a crank rebuild is required. I have thought about a rephase but I did that on the last one and have a Boyer Ignition already plus I don’t really want to buy a cam too. Need to find someone in the UK who can do one as I don’t have a press.

Next steps ..... finish stripping the head and top casing down and then get the casings out to the blasters.

A little more work done today. Drum selector and dogs removed and the cases cleaned out. Cleaned all gasket surfaces and getting things ready to go to the blaster.

Cleaned the pistons up and cylinders. Not great news. The ring gap on the pistons is at maximum tolerance - 0.15mm. The top ring on the left pot was actually seized and need quite a bit of heat to get it removed. The left piston measured in at 74.92mm and the bore at 75.12mm. The right piston measured in at 74.89mm and the bore at 75.13mm.

That means the right bore / piston gap is 0.20mm and the left is 0.24mm. If I remember correctly, the specified gap is 0.05mm - 0.055mm, so we are way out!

Never mind, as I was looking to get the crank rebuilt, I’ll get the cylinders done too. I have noticed that Heiden tuning in Holland do a refurb service for both, so that may be a good option to take.

Next week, I’ll start on the cylinder head and disassemble that. Hoping for some better luck in there
This week starts with Santa coming early. A nice little delivery from TC Bros with some shiny trinkets - PMA, VM Carbs, Boyer ignition and an exhaust fabrication kit at a good price too. It still amazes me that an order from Ohio, US can reach me quicker than an order from the UK


While we’re at it, the crank and barrels are getting ready to take a trip to Holland for Heiden Tuning to do their thing

Some more accomplished today. The turn of the cylinder head. I don’t know how the engine had been running but it had left a good deposit of coke and soot in the chambers. Popped the valves out and checked against spec - all ok. Happy days.

A good clean ensued with my trusty wire chamber tool and wire wheel in the bench grinder. Cleaned up and dressed all the mating surfaces while I was at it. May as well get them done now, then it’s a quick tidy up when back from the powder coaters.

The valve seats were in good shape and didn’t need anything other than good lapping back in. They survived the leak down test - if they can hold a chamber of brake cleaner while I make cup of tea, without losing a drop, they pass!

I also managed to pop the core plug in to delete the starter hole.

Thinking about colour schemes. I have done plenty of silver and polished aluminium cases. So... I’m thinking of black gloss cases, cylinder head and cam cover, silver barrels contrasted with chromed left and right cases, points covers, rocker covers etc. ????? All suggestions welcome
It’s a bit of a drag waiting for parts to return, so I’ve not been able to get into much yet. However, Jerry at Heiden has shipped the crank and barrels, a replacement alternator cover is on its way, a new stainless engine bolt set is on order and hopefully the powder coating should be back this week

A few hours spare today, so I thought I’d strip down the oil pump, check out how the derby cover will fit and machine a bung for the tacho drive.

Definitely evident that the previous owner was a mechanically unsympathetic and clearly crack handed