Top end issues

Paul Marx said:
Yes. Electronica is the last series of SFC. Head and liners are specific. Pistons are specific with a low deck, height from gudgeon pin to the top of the piston, meaning that used in a standard engine, they give a compression ratio of near 5/1. Compression ratio is not the pressure measured in the combustion chamber.
The picture of your engine with the head off and the crank at TDC clearly shows the top of the piston too low prompting the Electronica piston remark.

Paul
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But racing pistons have just gotta be good... no?:rolleyes:

This nonsense has been repeated over and over, results have always been disappointing to disastrous.

Simply a case of backyard bodgers not doing their homework. At least it keeps the spares supplies going! :LOL:

piet
 
Oehlin, your pistons are standard SF750 pistons, identical to the ones in my photo. However some of the damage seems quite deep. You could clean it up and it would probably run fine, but you risk the piston breaking and damaging the bores. Safer to buy a new pair of pistons.
 
Started reassembling the top today after tidying up the damaged piston top.
The engine was on the TDC and the cam gear marking aligned with the marking on the top.
This meant that the left valves were unloaded and the right exhaust valve was loaded.
When I now set the distance on all the valves, both cylinders exhaust valves go into the top and the engine can not be turned around.
If I set the valve clearance on the exhaust valves to about 3 mm, the engine goes around.
What have I done wrong ?
Everything actually worked before I had the problems.
 
With absolutely no malice or criticism, I strongly recommend you have a professional with experience in assembling the top end of a motor do this work for you - even better a friend with an SF who's done one before and can go through it with you. Sometimes you save yourself money and time having it done properly. Too many things creating problems for you with this.

Nat, no way are these pistons the same as the ones in your pic - these have no flat area (swish band) that I can see, which is why anyone who knows what an Elettronica piston looks like thinks they are Elettronica pistons. I recently sold a pair of NOS ones to someone with an Elettronica - domed, no flat, no swish.

I could post pics of SF and 74 SFC pistons in profile to show how obvious the flat areas are but couldn't be stuffed doing the resize.
 
piranha-bro2 said:
With absolutely no malice or criticism, I strongly recommend you have a professional with experience in assembling the top end of a motor do this work for you - even better a friend with an SF who's done one before and can go through it with you. Sometimes you save yourself money and time having it done properly. Too many things creating problems for you with this.

Nat, no way are these pistons the same as the ones in your pic - these have no flat area (swish band) that I can see, which is why anyone who knows what an Elettronica piston looks like thinks they are Elettronica pistons. I recently sold a pair of NOS ones to someone with an Elettronica - domed, no flat, no swish.

I could post pics of SF and 74 SFC pistons in profile to show how obvious the flat areas are but couldn't be stuffed doing the resize.
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Please send me the pictures to oe36sail@hotmail.com
Thanks for the suggestion to hire a professional but I ask to learn more. However, I have disassembled and assembled several engines before so I have some experience.
 
This marking on the camshaft is the only one I can find.
I
 

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sfcpiet said:
But racing pistons have just gotta be good... no?:rolleyes:

This nonsense has been repeated over and over, results have always been disappointing to disastrous.

Simply a case of backyard bodgers not doing their homework. At least it keeps the spares supplies going! :LOL:

piet
Click to expand...
Racing riders are better than racing pistons.

Paul
 
Sent you an email with pics Good work sorting the cam timing - your motor has definitely been a part before if the mark is on the 'wrong' side - you'll have to hope the work done previously was good work - or strip the whole motor and check everything!
 
piranha-bro2 said:
you'll have to hope the work done previously was good work - or strip the whole motor and check everything!
Click to expand...
If it's got the wrong pistons and the cam sprocket in backwards, it sounds like it was thrown together by someone who didn't have a clue. If it was my engine, I'd be going for the "strip the whole motor and check everything" option. Or at least strip the top end and install the correct pistons.

Not being familiar with 750 twins, the timing mark on the wrong side worries me. If its on the wrong way is the timing mark still correct?

Hey oehlin, I'm wondering why you're reassembling it with those pistons, when the experts are telling you they're not suitable for your engine.
 
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The cam sprocket can go either way around as far as I remember, it is just that the mark on the head is on the RHS, so it is easier to see both marks at the same time. Ohlin, if you want your bike to go as it should you should get some correct pistons. There would be very few motorbikes from 1974 that have never been taken apart before.
 
Ha I wish, bought some pistons around 1981 and they sat on the shelf for 35+ years then when I needed some for a rebuild they didn’t fit! so still on the shelf
 
Comment from onlooker..... dependent on mechanical "compression" the oem rods would be my concern.... j
 
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