750 Head on 850 Barrel

When you have done all of this stuff to the head, how do you know when you have gained ? If you change the torque characteristic, the bike can behave differently in various circunstances. It is normal to change gearing to suit the circuit - you then have another variable. To my mind, the whole thing depends on optimisation. Sometimes what you thimk might good, can be a long way from that. If you stuff the head, reversing the situation can be expensive.
Somebody once claimed that Barry Sheene used to convince himself that he had imnproved his bike when he actually had not. A lot of things can be very subjrctive
What really surprises me is my 850 motor is as good as it is, when it is so close to standard. When I built the bike, I decided I would try the motor with as little modification as possible. Even then tuning it is still a task.
When I have raced the bike, it has always been on only two very similar circuits. So any gains have been detectable. If I took it anywhere else, it might be hopeless.

Stopwatch revels all.

Stephen Hill said:

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.

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Using a 73mm bore (750) head on a 77mm (850) bore through bolt barrel was one of Steve Maney's preferred methods when preparing race heads. More reliable supply of usable 750 heads was one of the reasons.

Go to his website http://stevemaney.com/products.html and scroll down to CYLINDER HEAD WORK and enlarge the picture. On the right you will see a 750 with inserts to align with a through bolt 77mm barrel.

lcrken said:

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.

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Step 2

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Stop 3

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Step 4

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Step 5

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Ken

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What sort of interference fit did you use for the dowls into the head?

A huge thank you to lcrken for a detailed post! I have a lathe, a mill, and a few spare 750 heads. With this info I am sure I can get the job done. And good to have the info available on this site.

baz said:

What sort of interference fit did you use for the dowls into the head?

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To be honest, it's been so long ago that I don't recall, and haven't found that info in my notes. Most likely .0005" - .001" for both the aluminum inserts and the steel washers, chilling them both in the freezer overnight, and warming the head a bit. I might have made the steel washer fit another .0005" tighter, but I'm not sure. Both are captive when the head is bolted on the cylinder, so all that is needed is enough fit to keep them in place when handling the head.

Ken

Stephen Hill said:

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.

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I can measure the top diameter, but I'm afraid it's going to remain a guess (as far as my head is concerned) until next time I have to remove it

The bottom holes for the inserts are not bored all the way through. The aluminum inserts are pressed in until they bottom in the hole. The inserts are then fly-cut until they are flush with the bottom of the head. On the top side, the holes for the hardened washers are also not bored all the way through, just deep enough to fit the washers. If you look closely at the pictures I posted, you can see the bottoms of the holes, both top and bottom.

The top steel washers on Steve's converted heads that I have measure .875" within a thou or so. He may also have used the same size for the bottom holes for the aluminum inserts, but I haven't measured those. There is no need for the upper and lower holes to be different diameters. They have to be large enough to overlap the edge of the original bolt holes, but small enough to not encroach on the combustion chamber counterbore. .875" fits both criteria. There is a little margin to allow one to use other diameters, but not much. In my case, I bored the holes with a 13/32" end mill, both top and bottom, and made individual inserts and steel washers to fit. In the first head I did, the diameters of the aluminum inserts varied from .883" to .889", and I made inserts to suit. I don't recall the exact diameters I used for the steel washers. In the second head, I used a boring bar to clean up the bottom holes to the same diameter, .890", and used matching inserts. Again, I don't recall the precise size for the steel washers, but they would be similar to the size for the bottom inserts. I no longer have either of the heads I did for friends, so I am not able to make new measurements.

Ken

These are pics of a Maney 1007 cc kit Stage 3 head that was converted from a 750 head. The bottom aluminum insert and the top steel one both measure .875" You might notice that the combustion chamber counterbore is cut into the inserts, and that the centers of the bolt holes are offset a little more than in the pictures I posted above. That's because the bore centers in the 1007 kit are moved apart to allow for the larger 83 mm bore size, requiring the bolt holes to be moved and the counterbore opened up to suit the larger bore. But the technique is the same as used when just converting to a standard 850 pattern.





Ken

lcrken said:

These are pics of a Maney 1007 cc kit Stage 3 head that was converted from a 750 head. The bottom aluminum insert and the top steel one both measure .875" You might notice that the combustion chamber counterbore is cut into the inserts, and that the centers of the bolt holes are offset a little more than in the pictures I posted above. That's because the bore centers in the 1007 kit are moved apart to allow for the larger 83 mm bore size, requiring the bolt holes to be moved and the counterbore opened up to suit the larger bore. But the technique is the same as used when just converting to a standard 850 pattern.

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Ken

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It really is superb work

Yes, it is.....

Just amazing. Never fail to be impressed by the skill sets within this forum. Gotta think that had this been available back in the day ….

My interest is using sleeved 850 cylinders to 750 to fit in class and putting brass rings in the 850 head to match the chambers. I think Jim has done this. Do I have to machine some gaskets or are there some 750 gaskets with 850 bolt pattern?

George Hirvela said:

My interest is using sleeved 850 cylinders to 750 to fit in class and putting brass rings in the 850 head to match the chambers. I think Jim has done this. Do I have to machine some gaskets or are there some 750 gaskets with 850 bolt pattern?

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You can just machine (or file) the four holes near the spark plugs of a 750 HG to match an 850. Two of the holes will have thin areas on the outside but it will work.

jseng1 said:

You can just machine (or file) the four holes near the spark plugs of a 750 HG to match an 850. Two of the holes will have thin areas on the outside but it will work.

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I don't have any 750 parts all I have are 850. So I wanted to sleeve the 850 cylinders down to 750 and match the 850 combustion chamber to the new 750 bore by pressing in a brass ring. This can be done as I understand it.

I would never go down this path. I would rather use a 750 motor and get more horsepower by helping it to rev higher. Bigger valves mean more tendency to get valve float, so then you go backwards by changing cam profile to get slower lift rate. I would make aluminium barrels before doing head mods. The weak point in a Commando motor is the bottom end - revving higher gives more power than most other things. The way the power is delivered is also important. A high-revving motor is usually more peaky.

The 750 head is better, especially if you have a big port 850 head. But even if not, the 750s guides a smaller, thus better for flow.

TBW said:

Just amazing. Never fail to be impressed by the skill sets within this forum. Gotta think that had this been available back in the day ….

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Back in the day, they made a different two-valve cylinder head for the 850 - it could have had four valves like the Rickman which did not make Triumphs any faster. A lot of stuff is bullshit. Japanese motorcycles which have two-valve motors are usually quicker than British motorcycles which have two-valve motors of the same size. The Japanese do not use wide ratio 4 speed gearboxes, and they get the tuning right.
A normal 850 motor is a quick motor, the change to the crank balance factor and what is usually combined with the motor, holds it back. A 750 motor should have lighter pistons, and rev more easily. It should be just as quick.
My motor has had almost nothing done to it, and it amazes me that it is actually competitive - it does not need to be any faster. What it races against are four cylinder and 400cc bigger. They have had a motza spent on them.

I have never heard of a Triumph 650cc or 750cc splitting crankcases through the main bearing on the drive side. Some have pulled the back out of the crankcases. - it is not rocket science. In the old days when we built a race bike, the first thing we did was raise the crankshaft balance factor to help the motor rev higher.

In most road races these days, the lead bikes are usually line astern, up on the high line at full lean. I do not join that procession. I use my bike differently. An 1100cc 4 cylinder motor delivers heaps of power, but it needs to be in a frame which is light and handles well. With modified CB750 Hondas, that is not the combination they have. They are extremely fast down the straights but stupid in corners.
A 750 Commando motor in the right frame with a 6 speed close box should be very good, without getting into dramatic modifications.

acotrel said:

I have never heard of a Triumph 650cc or 750cc splitting crankcases through the main bearing on the drive side.

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I have. I’ve done it !