Jump to content

1974 Montesa Cota 247


AaronLister
 Share

Recommended Posts

I took the job of restoring my uncles montesa cota 247. The project has been on going for a few years now as I haven’t always had the time or money to get stuff done. But now I’m getting somewhere, all parts are polished, chromed or powder coated. I have the task of the engine now, once this is rebuilt I can begin to build the bike again! 

But the problem lies with the crankshaft mainly, when I dismantled the engine and started to take parts out/off I noticed the crankshaft is broke on the flywheel side. Each side of the woodruff key has broke off. The broken parts were still there just not fixed on. So my question is what can be done with this? I’m a welder/fabricator so I can weld this up and get it turned back flush on a lathe if that’s possible? I can’t really afford to be buying a new crankshaft, plus the fact the one I’ve seen comes from America. I have attached a picture of the crankshaft, the broken end is visible any suggestions would be great thanks! 

Also, the piston that was in measures 71.80mm, what is the next oversize to this please? 

646C35C2-50A6-4730-9A5E-B46CF07549A6.png

Link to comment
Share on other sites

 

The main thing is to check the bid end and connecting rod little end for wear, play and visible signs of other damage. Welding up the crankshaft should not pose too many problems provided that there are no inclusions and you allow air cooling naturally. If you are going with the stick method then select rod material type with care flash grinding will indicate type of similar metal by spark colour. You have little choice with mig. Penetration could be a strength problem. Maybe you could find a good condition crank half on eBay etc. Not sure from pic but if it’s the drive side you are repairing there will be a fair amount of torque loading on the shaft. If it’s the generator side then there will be a lot of inertia going through that side of the shaft. Add up the cost of new big end bearing and conrod kit and you maybe better getting another engine and using both to make one good engine with your original cases. Hope there’s some useful info for you.?

  • Like 1
  • Thanks 1
Link to comment
Share on other sites

 
1 hour ago, section swept said:

The main thing is to check the bid end and connecting rod little end for wear, play and visible signs of other damage. Welding up the crankshaft should not pose too many problems provided that there are no inclusions and you allow air cooling naturally. If you are going with the stick method then select rod material type with care flash grinding will indicate type of similar metal by spark colour. You have little choice with mig. Penetration could be a strength problem. Maybe you could find a good condition crank half on eBay etc. Not sure from pic but if it’s the drive side you are repairing there will be a fair amount of torque loading on the shaft. If it’s the generator side then there will be a lot of inertia going through that side of the shaft. Add up the cost of new big end bearing and conrod kit and you maybe better getting another engine and using both to make one good engine with your original cases. Hope there’s some useful info for you.?

Not a real easy repair,welding @ that point will pull and the crank won't be straight ,it may be rather difficult to correct this to "zero'' runout,,if you factor in welding ,straightening (if possible),machining after welding,then cutting another keyway you are paying a specialist a lot of labour.

I would think your flywheel is damaged also,IMO I would be searching for the parts secondhand....good luck.

  • Thanks 1
Link to comment
Share on other sites

 
1 hour ago, section swept said:

The main thing is to check the bid end and connecting rod little end for wear, play and visible signs of other damage. Welding up the crankshaft should not pose too many problems provided that there are no inclusions and you allow air cooling naturally. If you are going with the stick method then select rod material type with care flash grinding will indicate type of similar metal by spark colour. You have little choice with mig. Penetration could be a strength problem. Maybe you could find a good condition crank half on eBay etc. Not sure from pic but if it’s the drive side you are repairing there will be a fair amount of torque loading on the shaft. If it’s the generator side then there will be a lot of inertia going through that side of the shaft. Add up the cost of new big end bearing and conrod kit and you maybe better getting another engine and using both to make one good engine with your original cases. Hope there’s some useful info for you.?

I was thinking about tig welding it? So it could maybe fuse together and burn in well? Rather than stick welding as that won’t be efficient really. There is no play or wear in the con rod, big end, small end and all the bearings in the engine are so smooth. These will be changed anyway because there’s no point doing half a job. 

Thanks for the information, I might look for another engine and do as you said about making 1 good engine out of 2. 

I only started on this forum today, is there anywhere or anyone local to Wakefield, West Yorkshire that I could contact if I get stuck or need parts sourcing if anyone knows? Thanks guys ??

Link to comment
Share on other sites

 

Aaron you could try InMotion which is also known as Bultaco uk. Google will supply contact details. They are very good and supply parts etc for all the Spanish makes plus. Others on this forum will recommend different suppliers and contacts. 

  • Thanks 1
Link to comment
Share on other sites

 
 

I have a similar crank shaft in my 348. Not quite as bad. Only one side of the key slot had flaked off.I silver soldered mine back on. Ive only run it for two weekends since. I used loctite bearing fit on reassembly since the shaft is a loose fit in the flywheel and I imgained the damage is because the shaft has somewhere to expand into.

I managed to find a replacement half crank on ebay, one day i'll get around to fitting that.

There have been stories of people simply doing with out the extra flywheel and I saw one that had the flywheel welded onto the shaft although that makes oil seal changes a bore!

 

Link to comment
Share on other sites

 
On 06/03/2018 at 12:55 AM, AaronLister said:

I’m a welder/fabricator so I can weld this up and get it turned back flush on a lathe if that’s possible

This type repair has been done since welders were invented, so it's indeed possible. However the cost will be extreme unless you can do it yourself, and in this application it will be difficult to get it 100% true. I've seen cranks on eBay that would cost less and a better result.

I'm almost finished a complete rebuild on a basket case Version Ulf Karlssen 247, so appreciate what you've been going through, also appreciate how much it costs to repair/restore these things.

  • Thanks 1
Link to comment
Share on other sites

 
On 06/03/2018 at 10:30 PM, oldaz said:

This type repair has been done since welders were invented, so it's indeed possible. However the cost will be extreme unless you can do it yourself, and in this application it will be difficult to get it 100% true. I've seen cranks on eBay that would cost less and a better result.

I'm almost finished a complete rebuild on a basket case Version Ulf Karlssen 247, so appreciate what you've been going through, also appreciate how much it costs to repair/restore these things.

Thanks, the cost hasn’t been very much at the minute but that’s because the welding, caroming, shot blasting and powder coating has been done through work. The polishing cost me £250 for all parts expect the engine block as it wasn’t fully stripped at the time. But I thought about welding it as to keep costs down and as it’s something I can do myself. I’ll have to pay a company to turn the weld down on a lathe though as I don’t have access to one. 

The engine rebuild company i’ll be using said they can bore the sleeve out. But they need to know what size the next oversize is. It’s 71.80mm standard so will it be 72.30mm? I’ve seen oversize kits at that size 

Link to comment
Share on other sites

 
7 hours ago, AaronLister said:

The engine rebuild company i’ll be using said they can bore the sleeve out. But they need to know what size the next oversize is. It’s 71.80mm standard so will it be 72.30mm? I’ve seen oversize kits at that size

Mine has +30 thou stamped on the piston, so I guess it's like car engines where oversize usually goes up in 10 thou increments. I think maybe a better option is to see what size piston you can actually get before you bore it out.

I used to repair drive shafts by welding and it was often difficult to get them 100% true to centre - in that industry/application the shafts were in slow rotation so being a few thou out didn't matter, but in an engine turning a few 1000 rpm they need to be 100%.

  • Thanks 1
Link to comment
Share on other sites

 
9 hours ago, AaronLister said:

Thanks, the cost hasn’t been very much at the minute but that’s because the welding, caroming, shot blasting and powder coating has been done through work. The polishing cost me £250 for all parts expect the engine block as it wasn’t fully stripped at the time. But I thought about welding it as to keep costs down and as it’s something I can do myself. I’ll have to pay a company to turn the weld down on a lathe though as I don’t have access to one. 

The engine rebuild company i’ll be using said they can bore the sleeve out. But they need to know what size the next oversize is. It’s 71.80mm standard so will it be 72.30mm? I’ve seen oversize kits at that size 

If you use someone like inmotion as an example, than 72.00 mm would be it. They supply the 'italkit' brand pistons I believe. With a 72 mm italkit kit the piston is going to be something like 71.95 mm + 0.05 mm clearance = 72.00 mm bore size.

https://www.inmotiontrials.com/product/cota-247-piston-kit-complete/

  • Thanks 1
Link to comment
Share on other sites

 
 

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
 Share

×
  • Create New...