Benelli 750 sei

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Re: Benelli 750 sei

Postby Lotus54 » Tue Mar 09, 2021 8:38 am

I got busy with a customer bike and my Electric Motion mods.
yes, you offered seat cutter- I have one, if it does not work I will certainly take you up on it.

I’m waiting on some parts I ordered, so I think I will start putting the bottom end together. I’ll start with pistons in the cylinder, then offer it up to the case- inserting the crank and hooking up rods. Then gearbox. That is a fairly easy way to deal with all the rings for 6 pistons. They are small and the cylinder has a taper- so just hand pressure should be fine. I scored a special deal on a new cylinder, also new pistons/rings. The original cylinder is certainly usable, but the price was good enough that I bought it. Never hurts to have spare stuff- maybe to trade for other things if nothing else.
Heck, another of these may come my way some day too (although I certainly don’t need another project). I’m trying to limit taking on more stuff and finish what I have going. Plus selling some bikes I never seem to use (I have sold three in the last year)
1974 OSSA Explorer- 78 Benelli 750 sei, 73 Benelli 250 SuperSport, 68 Benelli Dynamo trail, 76 OSSA GPII Phantom, Beta XTrainer 300, 2020 Electric-Motion Escape ‘R’
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Re: Benelli 750 sei

Postby Lotus54 » Fri Mar 19, 2021 6:09 pm

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I’ve started working on the Sei engine.
Cleaned up cylinder, threads on case studs, weighed pistons and checked ring end gaps.

Pistons are within 1/2 Pennyweight (12 grains or .77 grams). I need to check pins. With pistons this small (114 grams) I suspect if I get them as close as I can it will make a difference in smoothness.
The one I bought new was by far the smoothest bike I’ve ever ridden. By a lot. (Ok, my electric is even smoother)
1974 OSSA Explorer- 78 Benelli 750 sei, 73 Benelli 250 SuperSport, 68 Benelli Dynamo trail, 76 OSSA GPII Phantom, Beta XTrainer 300, 2020 Electric-Motion Escape ‘R’
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Re: Benelli 750 sei

Postby John H » Sat Mar 20, 2021 8:36 am

Sweet! That's going to be a real classic when it's finished.
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Re: Benelli 750 sei

Postby Lotus54 » Sun Mar 21, 2021 8:18 pm

Doing some weight checking- try to get them as even as possible.

The old scale is from the Dental Lab, weights are in Troy Pennyweight...


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1974 OSSA Explorer- 78 Benelli 750 sei, 73 Benelli 250 SuperSport, 68 Benelli Dynamo trail, 76 OSSA GPII Phantom, Beta XTrainer 300, 2020 Electric-Motion Escape ‘R’
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Re: Benelli 750 sei

Postby John H » Mon Mar 22, 2021 7:58 am

Looks like jewelry Mark. Beautiful.
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Re: Benelli 750 sei

Postby Lotus54 » Mon Mar 22, 2021 6:54 pm

Pistons/pins/rings/rods all matched and together.
Installed main bearings, popped the crank in and bottom case, measured all the main clearances. All great.
Then take it back apart, get ready to install pistons in the cylinder- then cylinder to top case. Drop crank it and connect all the rods (I measured one set to make sure I didn’t get the wrong one, but I’ll measure each one).
Then gearbox etc.

Waiting on some head stuff from Germany- but I don’t even know if they shipped yet. They are notoriously poor comminicators.
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1974 OSSA Explorer- 78 Benelli 750 sei, 73 Benelli 250 SuperSport, 68 Benelli Dynamo trail, 76 OSSA GPII Phantom, Beta XTrainer 300, 2020 Electric-Motion Escape ‘R’
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Re: Benelli 750 sei

Postby Lotus54 » Wed Mar 24, 2021 8:09 pm

Pistons in the cylinder.
Getting ready to pop the cylinder on the top case- but of course found I must not have ordered the two ‘O’ rings that go on the oil feed pins between case and cylinder. I’ll look again and make sure, but also asking around for dimensions of the ‘O’rings. I have the old ones, but it would sure be nice to know for certain the size. A leak there would be no fun and cause issues.
Oh well, I knew there would be something I forgot to get!
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1974 OSSA Explorer- 78 Benelli 750 sei, 73 Benelli 250 SuperSport, 68 Benelli Dynamo trail, 76 OSSA GPII Phantom, Beta XTrainer 300, 2020 Electric-Motion Escape ‘R’
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Re: Benelli 750 sei

Postby John H » Fri Mar 26, 2021 12:21 pm

Blame Alexa for that one! Do you dry-seat (no lubricant) the back side of your rod bearing shells?
There's two different schools of thought on this. I prefer dry-seating.
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Re: Benelli 750 sei

Postby Lotus54 » Fri Mar 26, 2021 7:30 pm

I’ve always dry-seated bearing shells.
Although I don’t clean off with any solvent or anything- just wipe off any oil that may be on there.
1974 OSSA Explorer- 78 Benelli 750 sei, 73 Benelli 250 SuperSport, 68 Benelli Dynamo trail, 76 OSSA GPII Phantom, Beta XTrainer 300, 2020 Electric-Motion Escape ‘R’
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Re: Benelli 750 sei

Postby Lotus54 » Sat Mar 27, 2021 8:27 pm

I installed gearbox etc, bolted down bottom half of case.
Still more to do (waiting on some head parts too)- but coming along really well.

All new bearings- including many that were just fine. I don’t want to have to do it again to save a few bucks on bearings.
The gearbox ones are quite good sized.
Those shaft bearings are pinned on the other side.
New primary chain and cam chain. Still need to fully seat the primary shaft.
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Last edited by Lotus54 on Sun Mar 28, 2021 11:28 am, edited 1 time in total.
1974 OSSA Explorer- 78 Benelli 750 sei, 73 Benelli 250 SuperSport, 68 Benelli Dynamo trail, 76 OSSA GPII Phantom, Beta XTrainer 300, 2020 Electric-Motion Escape ‘R’
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