Author Topic: 1966 Sport 90 piston/cylinder swap?  (Read 2686 times)

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Offline superspin1

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1966 Sport 90 piston/cylinder swap?
« on: March 12, 2016, 08:22:42 PM »
My 1966 Sport 90 has a bad piston/cylinder. The Sport 90 had aluminum cylinders with chrome plated bores I believe. Is it possible to swap a cast iron 90 cylinder/piston assy like from a Mountain 90 onto the Sport 90 engine? Or, maybe even a TMX 100 piston/cylinder? Just looking at options here and wondering if other 90 Sport owners had run into this situation and what they did? Any help/information on the subject would be greatly appreciated. Thanks!

Offline mqtsteve

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Re: 1966 Sport 90 piston/cylinder swap?
« Reply #1 on: March 13, 2016, 07:18:11 AM »
The aluminum cylinder bore from the 90 sport can be re-chrome plated.  There are several companies that do plating.  There's probably one near you (there are other threads about platers).  You could put the cast iron cylinder, piston and rings on it.  If you do, you should also change the cylinder studs, head studs and head gasket.  Download a parts manual to figure out the parts needed.  The 100 TMX parts will NOT fit the BS90.  The parts should be available from Richard(site owner) or on eBay.  Good luck with your project, Steve
Steve S.
4/1966 BS90 Sport(o.i.), 5/1966 BS90 Sport(o.i.),
1965 BS90 Trail, 1965 BS90 Mountain

Offline BS Mechanic

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Re: 1966 Sport 90 piston/cylinder swap?
« Reply #2 on: March 13, 2016, 09:51:04 PM »
I did the aluminum to cast iron swap on a 90 I bought cheap because it's cylinder was bad.  This was about 1969, and it was a cheap solution.  As I remember, the cast iron cylinder was about $15!  No one had any idea where to get the aluminum cylinder fixed, and a new one was off the chart for a poor college student at that time.

It was an easy swap, but not necessarily the best solution in todays world. 

I would definitely check with one of the cylinder repair places first to see if it can be fixed.

Offline disc_valve

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Re: 1966 Sport 90 piston/cylinder swap?
« Reply #3 on: March 14, 2016, 07:08:23 AM »

the cast iron cylinder is a straight swap for the alloy one, and if you buy a used cylinder it will come with the right head studs anyway, I'm not sure that you need to change the cylinder base studs, though. I built a spare engine for my 90 staring with a knackered bottom end from a 90 Sport, end fitted a 90 Deluxe top end onto it. It seemed to fit OK. One extra thing you will need to change, though, is the Exhaust Clamp nut - the alloy cylinder has an internal thread, whilst the cast-iron  one has an external thread. Also, the Exhaust pipes are different part numbers - I'm not sure what the difference is, though. It may be a different bend angle or maybe slight difference in length. I guess that's a case of try it and see.


Online OldSwartout

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Re: 1966 Sport 90 piston/cylinder swap?
« Reply #4 on: March 14, 2016, 08:49:13 AM »
Make certain you use the piston for the cast iron cylinder as you said. There will be an issue if you use the piston from the alloy cylinder in the cast iron cylinder.  The pistons have a different coefficient of expansion and the one for the aluminum cylinder may tighten up in the cast iron cylinder at operating temperature unless you build in slightly more clearance than recommended. There is about .0015 in. difference in cylinder bore growth between the two cylinders from room temp. to operating temp.
« Last Edit: March 14, 2016, 08:51:03 AM by OldSwartout »
Karl Swartout
Mooresville, IN
BS175 Roadracer

Offline superspin1

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Re: 1966 Sport 90 piston/cylinder swap?
« Reply #5 on: March 14, 2016, 11:59:11 PM »
Thanks all, I have a line on a Mountain 90 cylinder but condition is unknown at this time. I don't think the original aluminum cylinder can be saved due to the deep 360 degree gouging. I'm thinking at this point a usable cast iron cylinder/piston is going to be the least expensive way to go. I'm hoping I can utilize my stock muffler and get away with just swapping the head pipes between the two? I will know more once I get the cast iron cylinder in hand. I really appreciate all your help and input (and I will probably have more questions as I proceed)...:)

Offline slawsonb

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Re: 1966 Sport 90 piston/cylinder swap?
« Reply #6 on: March 15, 2016, 11:30:02 AM »
I have had cylinders repaired (welded) to fix large chunks missing around the exhaust ports of a 350 cylinder. They weld them up, machine and hone, and replate (in this case with nicasil). Pretty severe damage can be fixed. If you send pics to one of the shops that specialize in these fixes, they can give you a quick idea of your cylinder's repairability.



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