Bonneville America Forums Forums Technical Forums General Tech 790 or 865 cc ?

Is there any obvious differences to ascertain if an engine is a 790 or a 865cc? when in doubt consult a higher authority.

I may be mistaken, but i believe all of the 865cc motors have black cases. Then again, so did the 790cc speedmasters.

OK, Blk. case, check, need a little more, anyone? Looking at a complete motor that has been passed around, supposed to be from a '05 Speedy, just need to be informed so I'm not surprised (disappointed) later, thanx

an 05 SM should be an 865.

The key word here is passed around, believe it or not, I have experienced that some people will tell you anything to close a sale, an intelligent decision takes before hand knowledge and that is why I am seeking it here, "THE TRUE SOURCE of ALL THINGS" B.A. and Spdmstr. Seller thinks (but can't guarantee) it came from a '05 Spdmstr. if that clears up why I seek this info.

If you can indeed look at it in person then get the serial # and call a dealer. They might be able to tell you, assuming it has never been apart. If it has been apart then all bets are off.

Sound advice, will do, thanks

The serial number idea is a giod one. The main differebces between the twi are the bore and cams. The 790s have stronger cams that are desired by 865 owners as a performance mod. The 865 has a bigger bore with milder cams. I had a Speedmaster w/865 before my 06 TBA and I could go 190 miles before hitting reserve which was much better mileage. The other dufferences are in the ECM with a higher rev limit for the 865 ad probably a little different timing map. But a motor that has been passed around may be risky unless you get an unbelievable deal. At worst you get some good spare parts off a not runnung engine. At best you get a good running unhurt engine.

Doesn't the 865 have different carby inlet manifolds.

I am sure it has an additional heat spacer.

pull one spark plug and put a volumn tester on it. You can probably borrow one from a mechanic friend. An 865 will puff 432 and a 790 will puff 396. usually 05 and earlier is a 790 and 06 and later are 865s. I perfer the 790 myself they produce a greater torque to horse power ratio so its easier to hot rod it. That is because the cam as mentioned by Dr Pc above. Plus if you decide to go big bore the less expensive 904 is a really nice step up.

It is more than likely a 790 if it is in fact from an 05. thats not really a problem though. you should pull the plugs and look at them and get an endoscope and look inside to see the cylinder walls and check the valve clearances. to make sure its a healthy motor. I think its an endoscope any way there is a small led on the end of a fiberoptic tube that you can slide in through the sparkplug hole and have a look see.

An 05 from a Speedie will be 865cc and the Americas were still 790cc until 2007, as previously mentioned a pre 07 Speedie engine will be black and a pre 07 America will be gray. So if it's black it's from a Speedmaster and then you will need to know the year because 2003-2004 were 790 and 05 and up are 865.

My 06 america has black jugs. In a comparison photo I saw on bikez.com the 05 ba and 05 sm both had black jugs. I don't think there are any definates on color of engine parts between the 790 and 865 the ser nun is prob the only way.

They definitlely use different CDI units.

OK, I ran the # s with T. of Tacoma, it is a Spdmstr 865 from Feb. of '05, I am now the proud owner of a spare engine at a price I could live with, thanks to one and all for the info, you folks are amazing, next step, fab up a engine stand.

Well, pass out the cigars! :-)

I'm smoking a CAO Brazilia right now, matter of fact.

Gota watch the cases on the early 05 865 engines,they had'nt made the upgrade to the stronger cases yet.Ask me how i know.

I got a good price on this mill, in fact I stressed to the seller, I was really in the hunt for a 790cc, but yes Marty, please share your observations as to any inherent case weaknesses, I have planned a build for this motor, but not if it won't hold up to increased stress factors.

The casting where the intermediate gear pin for the starter rides is weak.

Thanks Dog, so will a increase in compression stress this pin mount during the starting cycle?

Probably not enough to do any harm but if it were to backfire on starting then those dogs could break out of the cases. Compression increase shouldn't make it more likely to backfire but could make that backfire a little stronger if it did happen.Tune the carbs right and there should be no issue, and no throttle on starting. There is a member who fixed the cases after they broke and Triumph has been fixing them under good will. Probably not going to do a good will repair on an engine bought used.

I vaguely remember getting launched off my '67 500 Daytona with a backfire in the middle of a kick stroke when I was 14 years old, a violent force indeed. Those decompression levers on the bars were there for a good reason. God, would I like to have that bike still. Of course I am no longer 125 lbs. and indestructible like then.

I never had one of those old twins kick back on me and I ran 10.5:1 too. I did all my own tuning though too.They also started on the first kick maybe 2 or 3 if it was dead cold.

Just one word of advice to those like myself who are a bit concerned about backfiring doing damage to the cases. Someone on here suggested when starting from cold to only pull the choke 3/4's of the way out.

For me it was a ripper of a tip as mine hasn't backfired since

Well at least you have a set of cams to send out if you want a re-ground set. Keep the 790's!!!

plus 1 on that tip.It works for me too.