750 Head on 850 Barrel

[Stephen Hill]

I am looking for some details on how to modify a 750 head to fit an 850 barrel.

A search found a few posts saying that:
-a couple of people (including Ken Canaga ) have done this
-you might as well change to 850 barrels
-you should probably just find an 850 engine
-a new Fullauto head will solve the problem

Does anybody know what specific modifications need to be made to the 750 head?

 

[SteveBorland]

The holes for the 4 through bolts (on the 850 cases) need to be plugged and redrilled on the 750 head. That’s all Steve Maney did on my 750 head to convert it for use on an 850.
I can send a photo tomorrow showing this if you want.

 

[Stephen Hill]

Hi Steve,
A photo would be great. It would also be helpful if you could tell me what diameter the plug (or dowel) is. I am guessing that the plug is larger at the lower level than the upper level. This would create a shoulder in the head which would keep the dowel from sliding up inside the head as it clamps on the barrel. But just a guess on my part.

 

[Fast Eddie]

Hi Steve,
A photo would be great. It would also be helpful if you could tell me what diameter the plug (or dowel) is. I am guessing that the plug is larger at the lower level than the upper level. This would create a shoulder in the head which would keep the dowel from sliding up inside the head as it clamps on the barrel. But just a guess on my part.

Maney puts a large steel (or SS?) insert in the head to provide a really solid base to take the torque without the bolt heads crushing and sinking into the head. It’s a nice job.

 

[Yorkie]

Buy an used 850 head off me!

 

[baz]

I am looking for some details on how to modify a 750 head to fit an 850 barrel.

A search found a few posts saying that:
-a couple of people (including Ken Canaga ) have done this
-you might as well change to 850 barrels
-you should probably just find an 850 engine
-a new Fullauto head will solve the problem

Does anybody know what specific modifications need to be made to the 750 head?

Do you mean change to 750 barrels?
I did this on a mates 850 when he was having trouble with valve guides coming loose etc

 

[Stephen Hill]

Nope. I want to modify a 750 head to fit on 850 barrels..
I have a couple of spare 750 heads. And I am looking for some detailed info about the mod.
In particular, is the insert/plug/dowel loading against an internal shoulder? I kind of doubt that it is just pressed into a through bore. The way to tell for certain is to measure the diameter of the top and the bottom of the insert.

 

[baz]

Years ago there was a feature on here how commoz did it
I cannot find it
But I seem to remember he counterbored the head on the underside to take a dowl with an offset head
That was pressed in then the head was resurfaced
Then drilled
As said it was a while ago and my memory isn't brilliant

 

[Schwany]

Ask yourself if factory 850 heads have steel inserts in them for those 4 bolts next to the spark plugs.

I have a 750 head on 750/850/larger Molnar barrels with 750 iron sleeves in them. The 4 holes in my 750 head were already drilled out for 3/8 bolts. No steel inserts in them. Nifty idea, but I doubt it is necessary. I have those torqued down to 30 ft lbs. Thick SS washers under the bolt heads.

Only thing I would do is drill out those 4 holes next to the spark plugs in the head for 3/8 bolts and put 3 thread inserts (helicoil or whatever) in the head for the 3/8 studs that are in the head. All that work was done on my 750 head 30 or more years ago. It works fine on the Molnar barrels that use the 850 through bolts. Also worked on 750 iron barrels threaded for 3/8 bolts next to the spark plugs previously.

 

[Stephen Hill]

My understanding is the inserts were used on a 750 head because the 4 outside bolt holes need to be moved further apart to fit an 850 barrel. I also read that the just oversizing the 4 bolt holes in the 750 head will achieve the same thing. Hard to reconcile the two approaches.

 

[baz]

Ask yourself if factory 850 heads have steel inserts in them for those 4 bolts next to the spark plugs.

I have a 750 head on 750/850/larger Molnar barrels with 750 iron sleeves in them. The 4 holes in my 750 head were already drilled out for 3/8 bolts. No steel inserts in them. Nifty idea, but I doubt it is necessary. I have those torqued down to 30 ft lbs. Thick SS washers under the bolt heads.

Only thing I would do is drill out those 4 holes next to the spark plugs in the head for 3/8 bolts and put 3 thread inserts (helicoil or whatever) in the head for the 3/8 studs that are in the head. All that work was done on my 750 head 30 or more years ago. It works fine on the Molnar barrels that use the 850 through bolts. Also worked on 750 iron barrels threaded for 3/8 bolts next to the spark plugs previously.

It's not enlarging the holes to 3/8 it's moving the 3/8 holes to suit the 850 head bolt pattern

 

[Schwany]

It's not enlarging the holes to 3/8 it's moving the 3/8 holes to suit the 850 head bolt pattern

I didn't have to move them. I may have made them larger than 3/8ths though. It was a while ago. Molnar is using the same barrels for 750's and 850's. Why didn't I have to move the holes? I guess he moved the holes in the barrels for 750's. I misinterpreted his description of the barrels.

Stephen you should have received 50 or more PMs by now telling you not to listen to me. Best not listen to me then. There are no likes in listening to me.

 

[Schwany]

My understanding is the inserts were used on a 750 head because the 4 outside bolt holes need to be moved further apart to fit an 850 barrel. I also read that the just oversizing the 4 bolt holes in the 750 head will achieve the same thing. Hard to reconcile the two approaches.

As I mentioned don't listen to me. I can't keep the crayons inside the lines of my coloring book.

 

[jseng1]

My understanding is the inserts were used on a 750 head because the 4 outside bolt holes need to be moved further apart to fit an 850 barrel. I also read that the just oversizing the 4 bolt holes in the 750 head will achieve the same thing. Hard to reconcile the two approaches.

You can't just enlarge the holes - you have to move them or elongate them in the outward direction only to fit the 850 cylinders (use a template). There will not be much material left to clamp the head gasket but there will be enough. That's the easier way to do it. It works and I did it long ago on my race bike before I had a mill. Its better to put in a threaded insert or an insert with a lip but its a lot more work. The 750 head is a good way to go espcially if you do some porting and start with an early 28mm head and port upward to get a better radius leading down to the valve seat. These "steep downdraft heads" will get you the most power.

 

[lcrken]

I am looking for some details on how to modify a 750 head to fit an 850 barrel.

A search found a few posts saying that:
-a couple of people (including Ken Canaga ) have done this
-you might as well change to 850 barrels
-you should probably just find an 850 engine
-a new Fullauto head will solve the problem

Does anybody know what specific modifications need to be made to the 750 head?

Sometime in the past, I posted pictures of the procedure, but I'm unable to find that old post. This is a set of pictures I edited from the original files, to come up with something that showed the basic process, but without a lot of the intermediate steps, so I could fit it into a single post here. I think it is pretty self-explanatory, but if you have any questions, please ask, and I will post any other needed pictures.

Step 1.

Step 2

Stop 3

Step 4

Step 5

Ken

 

[Fast Eddie]

Ken, is that the same procedure Steve does ?

 

[lcrken]

Here are some more details on the conversion, this one on the head I did for the infamous Hobot.

This one shows drilling the offset bolt holes, using an end mill because a drill would not be rigid enough to drill along the edge of the existing holes.

Next step is making a counterbore to hold the insert.

And then using a boring bar to size the holes more precisely.

This shows the finished counterbores and inserts.

After they are pressed in, the head surface is fly-cut to get an even surface.

Ken

 

[lcrken]

Ken, is that the same procedure Steve does ?

Yes.

 

[lcrken]

Also, the combustion chamber counterbore needs to be enlarged to the 850 (or 920) bore size.

And, last picture, cutting the top counterbore for the hardened steel washers.

Ken

 

[lcrken]

Yes.

And Steve has done lots of them. I've only done two. One of the main reasons both of us liked converting the 750 heads was that it meant a big valve conversion would start out with the smaller 750 diameter guide, so the finished head would have a smaller diameter guide than one starting from the 850's larger diamter guide. The 750 heads were also more readily available at the time.

Ken