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View Full Version : 650 to 750 pre-unit head sawp



charlie500
11-23-2009, 11:12 PM
i have a pre unit 650 And im wanting to put a 750 head on it i have done it with a unit but never a pre unit is there anything that i should look out for or is it as eazy as the unit swap it works great on the unit i hope it works on pre unit or it will be one big paper wate when im done i guess that would be forking wonderful let me know if u guys can help thanks and keep on slicen and dicen

TRUMPNUT
11-23-2009, 11:31 PM
Pre unit head bolt pattern is smaller so you have to use a complete unit top end . If you use a 67 or later make sure you plug the ex cam follower oil hole so it don't leak .

KT
11-23-2009, 11:35 PM
not positive but I think you have to swap the pre unit jugs for unit jugs push tubes so on ....I have a 57 I am going to do this to already have the unit jug head and such ...It is suppose to be a damn near bolt on deal from what I have been told ...I am sure like most things it wont be that simple but then again the pre unit is a basic engine design.... so shit maybe it is simple ...

charlie500
11-24-2009, 12:02 AM
thanks i hope its that easy it would be nice to have somthing go as planned for a change

TRUMPNUT
11-24-2009, 8:00 PM
It's bolt on from cylinder up . Just make sure the head / rocker box / push rod tube / cam follower holder are all matching style . They changed them a few times in the 60's and if you mismatch you get worse leaks then normal . And if you go to a 750 match your CLC so it responds right .

Torch
11-25-2009, 12:00 PM
It's bolt on from cylinder up . Just make sure the head / rocker box / push rod tube / cam follower holder are all matching style . They changed them a few times in the 60's and if you mismatch you get worse leaks then normal . And if you go to a 750 match your CLC so it responds right .

If you use the short rod 10 bolt Triumph 750 it is a little work to do on a pre-unit, the spread on the cylinders is wider at the base, you will have to hog out the cases to fit. the distance in between each cylinder is wider to allow for the 10th bolt, but it can be done.
If you use a long rod 9 bolt 750 aftermarket kit it is just a bolt on deal.

KT
11-25-2009, 1:17 PM
wish I had my stuff closer to going together I would do a how to on this subject as I did mine....

TRUMPNUT
11-25-2009, 5:26 PM
If you use the short rod 10 bolt Triumph 750 it is a little work to do on a pre-unit, the spread on the cylinders is wider at the base, you will have to hog out the cases to fit. the distance in between each cylinder is wider to allow for the 10th bolt, but it can be done.
If you use a long rod 9 bolt 750 aftermarket kit it is just a bolt on deal.

I was talking about using a 9 bolt set up in genral . I Know building a short rod motor is a big differance then just installing a unit top end . It is a major to build a 10 pre unit . The piston pin size is different on stock 750 then 650 so you can't end jus bore it out . Then you have to run the cams to match the short rods and the crank and so on . Then you have to account for the metric years cranks and BLAHBLAH BLAH .

GreaserMike
11-30-2009, 3:42 PM
I was talking about using a 9 bolt set up in genral . I Know building a short rod motor is a big differance then just installing a unit top end . It is a major to build a 10 pre unit . The piston pin size is different on stock 750 then 650 so you can't end jus bore it out . Then you have to run the cams to match the short rods and the crank and so on . Then you have to account for the metric years cranks and BLAHBLAH BLAH .

Using a 9-bolt setup is way easier. I'd recommend using the one piece crank as well.

The motor in my 1950 project is a 1973 750 in 1954 cases. (I hate taking the easy road) Not only do you have to machine out the cases for the barrel to fit, but it needs to be clearanced for the rods as well. Also, The 750 crank needs to be machined down considerably or your case halves wont fit together. (I also lopped off the end of the crank for the stator as I'll be running an Alton generator and belt drive)

You can see how tightly everything fits here:

http://i47.photobucket.com/albums/f171/greaser_mike/1950/DSC01936.jpg

http://i47.photobucket.com/albums/f171/greaser_mike/1950/DSC02205.jpg