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"Getting it on" with a Honda 175

Another article sent in by Axel, this time about how to modify a Honda 175 into a proper racer, using the proper tools. It dates from 1969 or 1971 and I have not been able to track the author of this article and where it has been published. Enjoy!


"Getting it on" with a Honda 175

In 1954, Soichiro Honda, president of Honda "Motor Co., announced that his firm would compete in the Isle of Man TT races sometime in the future. Then, as a spectator, the Honda president attended the Isle of Man races that year. Five years later, in 1959, the “future” had come, and Honda entered three 125cc machines in the classic event, To everyone’s astonishment, the three Hondas finished sixth, seventh and eighth in their first outing. Aside from the personal satisfaction, this initial effort held a bigger reward: the Manufacturer’s Team Prize.

In the years that followed, Honda posted a most enviable race record. However, in the last few years the Honda factory has not participated in racing, at least not officially. Even though the Honda factory has not been involved for a while, there are still many Hondas racing in all types of competition. As long as motorcycles are built someone is going to tear ‘em apart and tinker around hoping to extract more power. A number of manufacturers offer a “race-ready” (or close to it) motorcycle, which anyone can purchase at his local dealer. The largest number of these are two-strokes in the 250cc class and under. In the larger displacement classes, many of the “private” entry Hondas have more than held their own against the other factory supported efforts. Yet, in many of the smaller classes, people are now finding that it is hard to make a Honda competitive. In recent years the two-strokes have come a long, long way, and most forms of racing are rapidly gaining in sophistication. Many of the Honda models are available in two slightly different forms: the CL and CB series (and in some cases a third, the SL). The CL series of bikes have a few changes which make them more of a dual-purpose machine. The CB models are usually strictly street machines. The days of “run whacha brung” are quickly ending for those people thinking of winning. Many people enter a race just for the fun and enjoyment they experience. For them the thought of winning is exciting but it is not prime motivation.

If your “thing” in racing is winning, and you own a Honda 175, read on while we tell you what Rick Toberoff has done to put his 175 at the front of the pack in many a So. Calif. TT race. Then you can take it from there.

Rick has blended a few Honda 305 components into his 175 to come up with a “combination” that has been competitive in the 250cc class. With the exception of the Norris cam, Rick has used readily available Honda parts for all of the engine modifications. If you plan on making some of these modifications yourself, we strongly recommend that you have a Honda 175 Shop Manual, and the proper tools. If you haven’t already found out, these manuals are quite easily read, and they’re a tremendous help when tearing an engine down. And, as for the tools, NO, a crescent wrench and screwdriver aren't enough to be considered proper. Also, some of these modifications will require the services of a machine shop, so you might start checking for a good one in your area. The total cost for all of the engine parts and necessary machine work shouldn’t be over $200.

The displacement of the 175 has been increased to 231.8 cc’s through the use of std. 305 Honda pistons. The stock 175’s bore is 52mm, and the std. 305 measures 60mm. Obviously, it ap- pears that the 175 cylinder will have to be bored 8mm if the 305 piston is to be used in any form other than pieces. Well, you’re wrong!! You can’t bore the 175 cylinders this big. However, you can use the 250 piston, and the 175 sleeve can be bored large enough. But you’d best be careful. For short periods of use, this may work fine, but in the long run it’s much better to in- stall the larger diameter 250 sleeves in the 175 cylinder. Stock sleeves, when bored out this large became thin, and will have a tendency to distort. If this happens, you can plan on seizing a piston before too long, and that’s a No, No! Besides, if it does happen, and chances are it will, you’d end up spending the few extra bucks doing it the Tight way the second time around. When using the 305 pistons, you have to install the larger 305 sleeves.

So, while you’re over at the Honda shop buying the other ‘goodies’, pick up a couple of standard size 305 cylinder sleeves. Although the dimensions will be different for the 250 sleeve, the procedure is the same as described for the 305. Now we'll tell you exactly what to do with them. Stuff them up your 175 cylinders, after they have been machined to fit. There are a number of ways to do this, but we will tell you what Rick does as it seems to work pretty good. The 175’s steel cylinder sleeves will have to be pressed out. Then there are three items which will need to be machined to adapt the 305 sleeves to the 175 cylinder. They are, the aluminium cylinder, cylinder sleeves and upper crankcase, The inner diameter of the aluminum cylinders will have to be enlarged to 2.522-inch. Then the outer diameter of the 305 sleeves will have to be turned down to 2.525-inch. The difference between the diameters of the sleeve and cylinder are now plus .003- inch. This is the same amount of “press” fit that Honda recommends. You’re probably wondering why both pieces needed to be machined.

Why not just machine the inner diameter of the aluminum cylinder larger (plus .003), and leave the outer diameter of the sleeve alone? Well, going about it in the way we described is like “robbing from Peter to pay Paul.” But, if you take too much out of the aluminum cylinder it would have adverse effects on the heat dissipation. Going the other way, by taking more off the sleeve and leaving the aluminum cylinder as is, brings you right back to the same problem boring the 175 sleeves had: a very thin cylinder wall. The procedure described is a good compromise that leaves you plenty of cylinder wall thickness and a minimal loss in the heat dissipating aluminium. If you still don’t understand, just take our word for it and tell the machinist the dimensions we mentioned. Also, since the 305 sleeve is longer than the 175, it will have to be machined on the bottom, to a length of 3.411-inch. This is the same measure- ment as the 175 sleeve. Now let’s get to the upper case. Since the sleeve pro- trudes out of the bottom of the aluminum cylinder, and registers in the bore on top of the upper case, and the outer diameter of this sleeve is now larger than the 175 was, the inside diameter of the upper case bores will have to be increased also. Believe it or not, if you have gone by all of the other dimensions given, the diameter of this bore should be enlarged to 2.51-inch. This will give a total clearance of .012-inch for the sleeve. While it sounds like a lot of work, it really isn’t, and you're probably not doing it anyway. All you have to do is pay for it.

Now, if you haven’t already noticed, the 305 wrist pin won’t fit the 175 tod. Before you sit down to cry, let me pass along something else so you can get all the bad news at once. The piston will require some machine work on the dome. Don’t sweat it; we’ll never get you into anything you can’t at least buy your way out of. The 305 wrist pin is .038-inch larger in diameter than the 175, but this is quickly remedied with either a Honda 250, or 305, wrist pin reamer. If you can borrow one to do the two rods, you'll save a buck or two, Yet they’re not expensive if you have to buy one. If you only plan on using it once, it might be cheaper to have the machinist ream the small end of the rods for you; he should have the right size reamer. As for the other machine work we mentioned, the dome of the 305 piston will need to be cut down and contoured. If the height is cut so that it measures the same distance as the 175 does from the center of the wrist pin hole fo the top of the piston, you'll be looking good. As far as contour, it should look similar to the dome of the 175. The photographs will better illustrate what we’re saying. Also, the skirt of the 305 should be machined even with, the lightening recess on although all it involves is cutting it off, the inside of the piston.

Look at the inside of the 305 piston, and you'll see what we mean. The skirt of the 305 piston should be machined off at the highest point of this undercut. Then, if you want to really get carried away, you can lighten the 305 pistons. Between the amount of material that is removed from the dome, and what you can take out of the inside, the piston can be lightened considerably. Yet, be sure that you are extremely careful when do- ing this. The skirt thickness should remain constant. The stock 305 with the piston rings, weighs approximately 165 grams. We measured one of the pistons Rick uses, also with the rings, and it came in at 120-grams. If you choose to do this, aside from being careful, be sure that when you’re through both pistons weigh the same. Piston to cylinder clearance should be .0015—-.002-inch. The stock 305 piston rings are used, along with stock end gap clearances. That takes care of all the big work, but don’t stop here.
Rick uses stock valves, springs and retainers in the 175 head. The installed spring height, is 29.5mm measured at the outer springs surfaces. This should net close to 30 lbs. seat pressure and 100-110 lbs. open pressure. The passageways in the head on Rick’s 175, have been cleaned up a little. Nothing exotic, just a mild porting job.

Rick has found that, the best cam for his 175 “combination”, is the Norris’ #185SRA (grind number). This Norris “powerstick”, has .286-inch lift, and 304-degrees duration. Rick has been fortunate, in that, he does a lot of experimental cam testing for Norris, and they have been helpful with a lot of the machine work we have described.
About the only other major modification to this “175 powerhouse”, is, the marriage of “late” 350 Honda carbure- tors to the inlet side of the 175 head. As you can see in the photographs, this necessitated making new intake manifold stubs to accomodate these bigger brutes.

The exhaust pipe length on Rick’s bike, if you’re interested, is 38-inches. This is measured from the backside of the exhaust valve (bottom of exhaust port with head on engine) to the end of the megaphones.

Using the earlier (1968) 175 frame, the total weight of the machine can be lessened by about 25 lbs. This frame, is almost identical to the Honda 160 unit. It has no downtubes, as the engine provides this strength. Rick has added a Cerriani front fork assembly, to aid in the handling. ‘To further help in getting this 175 around the course a bit faster than most, Rick has added an addition- al 3/4-inch to the length of the swing- ing arm assembly.

That’s all there is to it. Approximately $200. (for the engine parts, etc.) plus a good amount of time, and you can make a real charger out of your 175 Honda. Don’t you just dig all this sophistication. Yet, this is part of what it takes to be competitive; the rest is up to you. Rick will be happy to do the work for you, if for some reason you don’t want to do it yourself. We said Rick would be “happy” to, but the price of his happiness is money. For further information contact, Richard Toberoff, c/o Norris Racing Cams, 14754 Cal- vert St., Van Nuys, Calif. 91401.

(A) Honda 305 cylinder sleeve. Outer diameter of sleeve needs to be machined to a smaller size, before it will fit the 175 cylinder
(B) Stock 175 cylinder on left. Inner diameter of the cylinder is increased to 2.522 inch (on right) for the proper “press fit" in the modified sleeve.
(C) With the increased “sleeve” diameter, upper crankcase bores have to be enlarged so that the sleeve will register and fit in the hole.
(D) (Upper) The larger 60mm 305 pistons require a certain amount of machine work before they can be used in 175 engine. Wrist pin diameter of the 305, is .038-inch larger than the 175 pin. A Honda 250, or 305, wrist pin reamer quickly solves this problem. (Lower) The 305 pistons can be lightened considerably as seen here, but be extra careful.
(E) Stock Honda 175 valves, springs and refainers are used. Nothing exotic, a mild porting job on the 175 head helps performance considerably. Note also, special adaptors for the “late’’ Honda 350 carburetors.
(F) Rocker arm weight can be reduced, but caution should be taken, Norris’ #185SRA (grind number) cam is used in this 175 “combination.”
(G) Honda’s “late 350 carburetors are used in this modification (on left). The 175 comes stock with 22mm Keihan (on right).
(H) The 175 intake ports have been “matched,” to mate with the custom intake manifolds adapting the late 350 carburetors to the 175,
(I) Custom head gasket for the modi- fied 175, Steel rings (around cylinder bore) are carefully removed from the 305 gasket. Then, the 175 head gasket is enlarged around the cylinder and the steel rings from the 305 are CAREFULLY installed. Stock 175 head gasket (on right). The custom made head gasket (on left) is a time consuming task.
(J) The finished 175 now boasts a larger, 231.8cc displacement. Horsepower increase makes the 175 Hon- da a “sophisticated” competitor.
(K) In the 1968 model, the 175 frame was similar to the 160 Honda. It has no downtubes, and is about 25 Ibs lighter than the current 175 frame. Ceriani front end, and a ¾ inch addition to the swinging arm, improved the handling.

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