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Not many non UIM, APBA, CBF racers know these Documents exist. They are used by the race site inspector to verify that the outboard in this case is legal. They are used by the racers as well because it is the racers responsibility that their outboard is legal. I found them a great tool when working on even non race outboards.
https://www.uim.sport/Documents/Homolog ... 202022.pdf

https://www.uim.sport/Documents/Homolog ... 202025.pdf

https://www.uim.sport/Homologation1.aspx

Now some educational history on the Yamaha 70CES and a Document to back up what I am talking about.
There were two UIM homologation sheets for the 70CES.
The first version was the blue color model with the two piece cylinder head. It could have been issued in 1988? #00398 I believe was the number. Now there were early models dating back to 1984 so it is possible there was an earlier document to the #00398.
https://www.nettivene.com/en/moottori/yamaha/900296
In 1990 the last UIM document was issued. That document was revised in 1992 for the second (Yamaha 4D grey) version which had the one piece cylinder head. That document is #00435.
https://www.youtube.com/watch?v=j4VXLN0-Cnw
The 00435 has had revisions from time to time year to year and does include drawings of the difference in porting between the 1990 and 1992 versions.
Both T850 and S850(F4) used the same 00435 document.
The production was 1200 units according to the 00435 document.
Most of the other spec information has now been deleted and the T850 is a HS class. Which is a history class. S850 is no longer a class.
Notice on the port drawings the 1990 model is a bit different to the 1992. And each cylinder is also a bit different cylinder to cylinder. Now that is different to most outboards.

https://rfem.es/media/redactor/435%20-% ... inas-4.pdf

Why did each cylinder have unique chamfering and why were there two or possibly more versions different from each other?
I think the reasoning is simple. The basic ideal behind the 70CES was copied ideas from the OMC Johnson/stinger/ Evinrude /Hustler. Yamaha wanted to build an equal or better version. The problem was their basic 70hp model called the 6H3 model was an excellent recreation and commercial outboard, but, wasn’t nearly as good as the OMC to be a good platform to build a performance version off of. The crank was very heavy, the engine porting small, the carburetors were small , the ignition system was not designed for high rpm applications. AND they had to use it!
I think the reason there are several versions was Yamaha being limited under the race rules of what they could do to the motor. There was no such thing as a model year for these 70C motors . Changes were made when Yamaha wanted to make them.
The gearcase wasn’t a problem. They were using it on both the twin and triple 50 to 60 hp. Yamaha had race success history in the early 1980’s with the 60D model of the twin cylinder and it used the 1:71 gearcase and a megaphone style tuner.
So based on what they knew and parts they had, the 70C came out in 1983-4. It was a custom parts add on motor. Special cylinder head, special megaphone tuner, plastic reeds, 90 hp main jet in the carburetors, modified base plate, 2 ring pistons. The part number for each add on part were prefix 6J2 . Everything else is a stock 6H3 part.
The actual porting of the motor is not much different than the stock 6H3. They messed with the chamfering to get the best performance out of each cylinder. So there are 3 different sleeves in each motor. From a manufacturing cost point of view . That wouldn’t increase the cost and they had no other option.

The 70CES was a motor built to specifications. It’s the sum of all it’s parts that make it a 70CES. Deviate from using the correct parts and from matching the specifications and it’s not a 70CES..
Those motors are Mod70 Yamahas.
And yes a Mod70 Yamaha can be built to be superior to the 70CES. I think many people try to modify some parts on the 70CES and likely we’re disappointed in the result. It was mention on another thread that it’s difficult to get major performance gains from a 70 hp Yamaha. You can’t just do a few changes and expect it to work out. The engine builders I know of paid their dues trying to build a super Mod70 Yamaha. Most of the Mod70 Yamahas were built to race in APBA SLT Mod class.

56 cubic inch APBA SST 60 Document:
https://www.apba.org/documents/15461589 ... 022020.pdf

What was very interesting about the SST 60 is that the engine sleeve porting are not that much different from the production 56 cubic inch Bridgeport motor. The biggest difference was the quality of the actual sleeve porting itself. The production model sleeves ports are very rough and there is a noticeable difference comparing different sleeves from different years and even motors from the same year. The oval port sleeve is much better quality and more consistent in porting sleeve to sleeve compared to the production Bridgeport sleeve.
The transfer port and the boost ports are almost exactly the same size on all three sleeves.
The exhaust port on the SST 60 was actually smaller in size than many of the production Bridgeport exhaust ports. But the SST 60 exhaust port is raised a few thousand more than the production sleeve and SST 60 sleeve is very consistent in accuracy porting sleeve to sleeve.
The SST 60 has a smaller cc cylinder head which increases the compression. The rest of the internal parts are production model parts.

The .440 pocket depth is important. When you add the head gasket the total volume of the head is 28.5 cc which translates into a full stroke compression ratio of approximately 10 to 1. That compression ratio is about as high as you should go if your planning to run pump gas premium and not want to run higher octane race type fuel.
Go smaller on the head cc with a modified powerhead and you will get lower compression PSI readings but your compression ratio will be much higher. That’s a problem most don’t understand with a 2 stroke. You can have a compression reading of 160 but the compression ratio is 12.5 to 1 based on the cc measurements.

I had a viewer email me about the different chamfering on the 70CES porting cylinder to cylinder. He had noticed it as well but thought it was done by previous racer prior to him purchasing it.
Here is another thread about 70CES and Mod 70 Yamahas. Carl who replied to my questions was very knowledgeable about the motor and F4 class.
https://www.boatracingfacts.com/forums/ ... aha-70-ces

[quote="Hounddog"] The gearcase wasn’t a problem. They were using it on both the twin and triple 50 to 60 hp. Yamaha had race success history in the early 1980’s with the 60D model of the twin cylinder and it used the 1:71 gearcase and a megaphone style tuner.

Matt was Racing in T750 running a 55 hp twin cylinder Yamaha. He became very interested in the various other versions of that model and the differences between each version. Matt as able to get a copy of the UIM Document for the 60D model. Matt feel free to correct me if I am incorrect in any of these facts.
There are 3 different displacement size models.
There are 2 different blocks.
The 60D and 55B have the same block but different sleeves. The 60D has a smaller bore to reduce the displacement to 45 cubic inch.
The second block 6F0 is a different block, different head, different exhaust, different ignition and is approximately 49 cubic inch.
The crankshaft is the same one for each model.
Same carburetor but different jetting on each model. The 60D maybe? Larger.
The horsepower is 48 (50?), 55 & 60 .
The gearcase has different gear ratios depending on the model. The 60D had 2 different ratios.
The engine porting is the same on all models.
The early 60 hp was crankshaft rated not prop shaft rated.
The 60D was the only model with the megaphone exhaust pipe.
https://i.ebayimg.com/00/s/NzY4WDEwMjQ= ... F/$_86.JPG
Most of the 60D powerheads have now been bored to use the 55B piston with larger displacement.
The best version performance wise is the 55B version.
The compression on these motors is a bit different on certain models. The 48 hp could have less, but the others are in the 150 range.

The OMC T750 information.
The Pdf for SST 45 which is the ENGINE specification document
T750 does have the option of different pistons and other minor changes.
https://www.apba.org/documents/11433501 ... ations.pdf
https://www.apba.org/sites/all/files/do ... 022515.pdf

This is an interesting fact finding thread about the T750 OMC.
http://www.hpbc.ca/viewtopic.php?f=27&t=4073&start=0

ARE there any classes without detailed specification documents? It just seems like such a waste of time and money especially in grass roots racing when you just want to race with your buddies a few weekends in the summer and have fun. You are not interested in moving up to a higher level or a national title.

One problem is the entries have a different view of fun racing. I have not met many racers who raced for fun didn’t want to be at least competitive. Not being competitive takes the fun out of racing. That’s why those entries quit.

The simplest rules should make you competitive provided the other entries want to follow the rules. Those entries are there too Win. Entries with that mindsets end up competing against each other so races get fewer and fewer entries.

That being said.....simple engine rule STOCK.
Example:
https://www.apba.org/classes.php?id=126 ... table-menu

The other suggestion which I have made to help increase the boat count in T class is different motor classifications within the same heat. And you reward them . You could base those groups on engine or maybe by speed categories similar to what we see in the wildcat races in Australia.
T has a big advantage because there are many T boats in Ontario and we know the speeds of each combination of boat and motor. You could setup the categories based on what combos showed up to race that weekend . If you wanted to make it a bit more interesting have each racer in the category put in a few dollars and the winner of that category gets the prize money. The more entries a category has the more prize money. Lake Racers will notice that and come out to run in that category.

Many racers in the lower speed categories will want to move up and challenge the top runners while top runners may decide to drop down to a lower category.

Your racing at the competition level you are comfortable with and that makes it FUN!

ARE PEOPLE HAVING A PROBLEM POSTING?
A former T850 racer e-mailed me these OPBRA rules. He said he couldn’t post on the site. The rules are spelled out for all their classes including T750 and T850.
https://ontariopowerboatracingassociati ... lement.pdf