The Hodaka Club Forum

Well I ran into another little glitch today when I pulled this cylinder head out of the box to paint it. I hadn't noticed this before, but I don't know how I missed it. The question is: is it repairable or do I look for a new head? Any ideas on what may have caused this? Thanks
JackM

Does it have a heli-coil or similar thread repair? If so, it looks like whoever did it didn't do a very good job. You can probably get a different thread repair kit (one with a one piece insert - not the spring style like a Heli-coil) and re-do it properly. But, that is just a guess, as I don't have the original in front of me to examine, just a picture.

Wow. Your photos don’t show it clear enough and the image breaks down quickly when I zoom in. Is it actually cracked there or is it just carbon build up? Can’t see it clearly enough.

And, can you tighten a spark plug in it snugly?
Victor

Vic, did you click on the pics? They blow up real good for me. You can definitely see a triangle shaped hole and faint crack lines leading away from it. I believe DR is right. A heli-coil gone bad. I ran a spark plug in and it felt a bit loose going in, so I'll take it down to the shop and see what they say. Thanks

A competent motor shop should be able to remove the damaged part, reweld the hole, retap the threads and do any machine work to make the head good as new. My opinion.

I am going to agree with throw chain 100 . I would never recommend a new heli-coil. -----------Clarence

Probably cheaper to just pick up another head. When you are talking about welding and machining that's going to cost. If you can find a one piece tread repair that might be good. But if you have to get a machine shop to install it its going to be expensive.

New is, what, $150+ and used is under $100. Not bad, but not cheap.

Depends on your machinist capability. Repair is do-able but parts are scarce. If you get new/used, fine but do save the old one for possible future repair.

Clicking on the photos doesn't improve the photo clarity. Both images are slightly out of focus because the depth of field is so shallow that only limited portions are actually in focus. But it looks to me like it was previously welded, and then either tapped or heli-coiled. What may appear to be cracks look to me like weld margins running around the plug hole.

Is it repairable? Yes. Whether it is economical to do it may depend on who you take it to.

Right on Jack. It has been welded before but could be redone by the professional. Kinda need a welder with talent. ---Clarence

Looks like someone tried to MIG weld the head. I wouldn't do that. TIG would be far better, and yes it does take talent to do this one right. While you are at it, it could be built up some, and remachined into a toroidal chamber. As long as you are going to spend the money.....

Thank you Jack! Glad I’m not the only one who thought the pics were not real clear.

Here’s a link to what is described to be a Super Rat Head. But I can’t read the part number in the sentence ons picture to determine for sure if it is or not. Might ask for a clearer picture of photo #2! Victor

https://m.ebay.com/itm/Hodaka-Super-Rat ... iid%253A17

Went back and looked into the expanded description. Here’s the number!

For sale is a Hodaka head with the numbers stamp 92501 made in Japan stamp on the head. I am nost sure if is from 100 0r a 125. If you know please inform me. He head is in very good condition
.If need more pictures please email me and i will ASP.
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Im guessing its a typo. The Super Rat heads were all 932501. So I’m guessing it’s just a typo.

Victor

Probably a road toad head .. as for jack m pics they were a clear as day for mine ☺

Also the 912501R head is the same as the 932501 I do believe.-------Clarence

viclioce wrote:For sale is a Hodaka head with the numbers stamp 92501 made in Japan........

Im guessing its a typo.

Victor

Lots of Hodaka heads were cast with the part number including one blank spot in the second position, as in 9_2501. Probably what this guy is seeing, so not necessarily a typo, but it also doesn't really tell you which model engine it came from.

The main difference I see right off in the photo Victor shared is the size of the cylinder bolt holes. Those on the 93 are only about 1/4". Thanks to H Jack I learned a new word today = Torodial = shaped like a doughnut. Even spell correct doesn't recognize it as a word.

So there is really no difference between the Ace 100 heads & the Super Rat Head? Is that what you’re saying Clarence? Did they not make any differences between the heads for the Aces & Super Rats? I’m asking because I truthfully do not know. Victor

Victor there are big differences between Ace 100 heads and 93 super rat head. Hodaka did make 91 series racing parts for the Ace 90, you could use them on the Ace 100 also. The 912501R was the part number for the Ace 90 racing head, I am using one on the old Ace 90, vintage race bike.---------Clarence

So what are the differences between the regular Ace heads & the “R” heads for the Super Rat. Learn me please! Victor

Compression due to chamber volume, squish band profile.

The differences between the ACE 100 heads and the Super Rat heads are found not only in the shape of the fins but most importantly in the compression ratio. If you compare visually you will see that the combustion chamber of the heads is very different. The Rat head is not very deep where as the ACE head is much more deep and has more volume. Having more volume makes for a lower compression ratio. This is the main difference. The super rat head has a different fin shape also. You can tell a Rat head versus an Ace head right away if you are familiar with both.
If the head pictured in the previous photo does not leak, I would not even bother with repairing it. If it was welded in the past what you think are cracks does not mean they are or that they go through the head. Many times when compression releases are installed on cylinder heads they crack between the spark plug thread and compression release threads. But the crack does not go all the way through causing release of compression.
I have seen this back in the day when I raced flat track. Thank you Gentlemen

Since this post has been posted I have been thinking about some of the heads I have with bad spark plug holes. I have found that there is sold an adapter that converts 18mm spark plug threads to 14mm threads. Its made of steel and would be a good fix for bad spark plug threads. You would just need to drill out the old threads and tap for 18mm threads and screw in the adapter. If there were cracks around the old threads they would be hopefully removed when the hole is drilled prior to tapping the 18mm threads.
These adapters are on Ebay under 18mm to 14mm spark plug reducer. They are about $15.00 I think
This is what I will use for my repair. I will Loctite it in place after I make sure the length is correct. I would not want it to extend down into the combustion chamber at all. Possibly might could use a longer reach spark plug if needed. Thank you Gentlemen

You could even make one of these adapters on a lathe if you have one and know how to. This way you could make the external threads whatever size you want. Say for example if you have a 3/4 inch tap you could make the external threads 3/4 inch and thread your head with the 3/4 inch tap. This way you would not need to buy an 18mm tap.
You could even make this adapter using a small 3/4 inch bolt. You would just need to bore it thru and tap it for 14mm spark plug threads and trim it down to the correct length.
Maybe I will do this with my lathe. Any machine shop could do this for you also.
Those heads are not being made anymore, better save them and fix when necessary.
Thanks Gentlemen

Kruger, I'm going to try and save it. Stopping by the machine shop tomorrow and have them check it out. Will let you know the results. Thanks,
JackM