Help gsx400 flywheel on gsxr250r gj73a and other problems Now Running with Video :)

Removing an alternator center boss is actually quite easy. Just involves drilling the heads off the rivets and carefully driving them out.
But redoing the rivets isn't quite so easy...I've never had to do it myself. I was asked some time back who here could do it and I pointed the guy at a place here which reconditions vintage and veteran car diffs.They are frequently riveted together - with some MF big rivets - so i knew this mob could do it. Didn't hear back so I assumed it was OK.
In the UK, I'd look at someone who specialises in Vintage car engineering. They'd hopefully be able to judge which approach is easier - machining or swapping centers.
The apparently simpler method of using screws or bolts isn't simple either. A lack of room - and a thin flange on the boss don't help. I have anchored starter clutches to the boss flange with larger screws which are then TIG welded in place. Not really applicable to an alternator rotor IMO.

 

Got the PM but it wont allow enough characters to respond - I went with a bespoke one made to order - £600 or so gasp as no other options or so I thought - I did the same with swapping the centres initially (GF 250 and gs500/550)- find a proper old school engineering shop though - or GregT's suggestion of a vintage specialist would get the thumbs up - it needs to be spot on as you'll get vibration issues. If I were to do it again - in fact do you have the ID of the original flywheel (with/without magnets) and the depth of a magnet - I cant get to the unit at the moment due to the lockdown so cant get my stuff - but if you get those it should be easy enough to find a matching one piece magnet ring with an adapter ring insert - that's essentially what the one I had made has in it - you'll get to keep the original flywheel - and you can fix it in yourself to keep costs down..hope this helps m8

 

Shippings a drawback lol!!! Try places like BB Bikeshop Ltd too pal..they wont know a lot about the cobra but might have a kokusan conversion kit for something similar m8..

 

GP9125 is a Gf250/gsx 400 flywheel and your correct, change the middle and will go straight on physically but weighs different ..original on the bike was gp9147 as far as I'm aware but possibly the cobra has an updated part

 

Weight yes - its a flywheel so go with as close to standard weight as possible if your looking to bolt on and go with standard fuelling and advance..run ok - yes but the work needs to be done right..vibration will be an issue if its not done perfect..the buzz you'll feel is the throw out on the crank oscillating at a set frequency if not done right (1st engine about 1000 miles, pick up on the end shell closest to the flywheel and the furthest away)..first time it was revved over about 14k..I got about 4000 miles on the second though but same issue to a lesser extent..redlined several times but then the crank endfloat went awry..I don't think the 'true as possible worked' for me as the guys I used couldnt get 'true enough'. I believe its do-able though. My theory is that the nine 'rivets' are weighted or balanced to do a basic balance of the flywheel when its made and positioned accordingly..shoot me down somebody but just going of what reckon! You thought about getting an easier available engine from an across or bandit and bolting your head on? GSF250 Bandit Engine Motor 5k Miles Suzuki 1989-1994 831 | eBay..easy swap and no issues with the flywheel..will need to swop the sump over and factor in a head gasket??? just thinking out of the box without spending a furtune pal....

 

It should be quite feasible to get the rotor running true to less than a 'thou.

It's also possible to get it dynamically balanced on it's own. Plenty of specialist balancers in the UK.

Stock are balanced as a separate unit. Look at what you've got and you should see the occasional hole on the OD.
This is where it's been drilled to balance it. i certainly wouldn't do all this work and then not get it balanced.

 

gregt -stock are balanced as a seperate unit usually but you wont see any holes on the outside o/d of the 73a flywheel (kokusan 9147)..there balanced through the use of the selection and weight of the rivet material itself closer to the centre. I cant say definitively it should make any difference how they are balanced (closer to the centre or on the OD - my head says OD is more accurate ) but suffice to say I had two 'dynamically balanced' flywheels that ran crank bearings as they resonate/oscillate at certain revs - the second had holes drilled into the backplate too - only my experience with this engine. The ones I had fitted with genuine flywheels suffered no issues until the magnets broke up and did much the same..

No difference in performance changing the bottom end - same crank bar the thread on the flywheel and the flywheel itself weather you go bandit 250/across/katana- only the sump and sprocket cover - so get one complete if you go this route..and check out the flywheel first!! Done it several times. Biggest issue (bar the flywheel) is finding a good donor motor - there are two holes on the side of the head which lead to water ingress to the bores in any climate where it rains. Nine in ten gj73a gsxr 250 (cobra 250) will have knackered bores due to this - thats why yours kept indoors should be good...if you see rust residue coming from the two holes in either side of the block by the cylinder head mating surface then walk away as your looking at a rebore minimum (chinese pistons only as new o/s are gold dust) and more likely getting cylinders made and a new piston set to suit. All the differences are head/cams/carbs..ideal would be a known good motor from a 73a or early 74a which will slot straight in - with good compression and a borescope to look at the cylinders lol..blocks are no longer available..

If it were mine i'd keep the original engine and wait for the bits to come up but they'll be expensive..the last flywheel I saw was 90,000yen on yahoo jpn but thats about £600 plus postage and customs..thats why I suggested the bottom end fix - good news is that when I last checked that engine I sent a link too is still up for grabs..!!!! Let us know how you get on fella..got my fingers crossed for yus m8

 

Bore/flywheel issues are the only known faults and that's why there are so few around..hugely expensive to fix so most are just laid up..they sold massive amounts of these back in the day in Japan but the 73a gsxr/cobra is rare now only for the above reasons..highest compression of all with 12.5:1 !!!! don't give up!!!!!!!!!!

 

Draw a line at the head gasket - transfer everything above that line from your engine onto it - pistons/rings/crank/block are identical - the difference is the head/cams/carbs. Parts list you will need is a head gasket and four exhaust gaskets (exhausts may be reuseable). Head gasket is around £50 and exhaust gaskets around £10 all in. If the bores are good on one and bad on the other then swap to suit - will need a base gasket if you want to swap the blocks/pistons (about £20)..i'd allow a full weekend to do it but you could probably recoup the costs of the parts needed by selling the unwanted head - they go for around £100-150. If you need to take the block off (rust etc and swap out the pistons) then chamfer the base of the cylinder barrels before sliding it back over the pistons - you can do it without but lots who have done it before break a ring unless its done as described - you will not be able to get a piston ring compressor in as there's no space. Murdo's idea is good - and has been proven to work - there was a guy on here (Boydie) who successfully ran a flywheel for a few seasons racing that had its magnets initially come off - he made a template and reglued them (epoxy) into place - had no issues with it