Super Rat Model 98 Modification

[JackM]

I'm just wondering if anyone on here has modified a model 98 by putting on a larger cylinder and head (123cc) and a larger carb (32mm Mikuni). Not positive, but I believe those parts are for a Super Combat model 97. Checking the cylinder bore, I found that a bit of the plating (chrome plating??) has worn off near the bottom of the cylinder. I don't know whether to have the cylinder replated (ouch) or try to find a used 98cc cylinder and go with a smaller carb. Not knowing much about Hodakas when I bought it, I figured it would be a good trail bike, but this one looks like a motocrosser. Any suggestions welcome. Meanwhile I'm going back to stripping the paint off the tank. Thanks. Jack

[dcooke007]

Jack,
None of the Hodaka cylinders came from the factory plated. All were either solid cast iron or aluminum with cast iron liner. Plating the cylinder is a modification that is done though. What size is you bore? If indeed plated might be able to get under the plating and bore to next over size.

The super combat top end can be installed if the cases are bored for the larger cylinder spigot on the super combat cylinder.

Danny Cooke

[JackM]

Danny,
The bore looks to be about 55.6 or 55.7 mm. if I measured correctly. The super combat shows a 56 mm bore, so it looks like maybe thats what I have. I believe they called it an alumiferric cylinder. The cases don't look like they have been bored. The engine # L-18xxx is correct for a model 98. A mechanic friend had told me that the cylinders were chrome plated, so now I'm confused. Imagine that, a confused old man. lol. I'll try and get a photo of it and post it later. Thanks, Jack

[Hydraulic Jack]

Jack,

Danny is correct that no Hodaka was ever plated by the factory. All have iron bores. Alimiferric is just a trade name applied to the later used process of casting an iron liner into an aluminum cylinder.I would not expect a 100cc, 50mm cylinder to be susceptible of being simply bored out to nearly 56mm. I also don't know how one may look at a cylinder and tell it has never been bored unless the bore is, in fact, stock in dimension. If your bore is worn, it is hard to measure accurately without proper machinists tools, but you can get reasonably close.

You mention that it appears plating has worn off at the bottom of the stroke. If you can see clear plating with a line of demarcation showing extreme wear, it may well have been plated at some point, in which case who knows what you have, but it would have to have been bored at least once. Someone may have already converted it to a model 97 top end, or anything could have been done. What does the piston skirt measure, and how much clearance is there between piston and cylinder at top of stroke, mid stroke, and bottom of stroke? And, how thin is the wall of the cylinder spigot?

[JackM]

Jack,
I believe you may have misunderstood me. It's not that I want to modify, but that someone already has converted it to a model 97 top end. I took some measurements but I'm using a cheap digital caliper. The outside diameter of the bottom of the cylinder (spigot) is 62.1mm. Inside diameter at bottom is 55.6mm, which makes the wall of the spigot about 3.2mm. My question now is whether to have this cylinder replated, or just go back to what it was supposed to be in the first place, by trying to find a good, used 99cc cylinder/piston and smaller carb? What should the standard thickness of the cylinder spigot be? I've included a couple pics, don't know if they will be any help. Thanks, Jack

[dcooke007]

JackM,

That is definitely a super combat cylinder and you cannot go back to a super rat mdl 98 cylinder. At least not with the engine cases you have. The cylinder spigot on a model 98 super rat is much smaller than the model 97 super combat. Either someone has bored your cases for the super combat cylinder or re-marked the engine case serial #.

It is difficult to tell if your cylinder is plated or just scored. If it is plated it looks very thin. I think I would just try to get an accurate measurement, bore and fit new piston.

I would suggest checking the other top end parts. Correct carb and cylinder head comes to mind.

Danny Cooke

[JackM]

Danny,
I double checked the engine serial # and is an L-18xxx and the frame is G-12xxx, indicating that it is a model 98, so I suppose the cases were bored. The carb is a 32mm, and the cylinder 123cc. I'm posting a pic of the head which is marked 9U2501. I don't know what the U means. Should be a 972501 for a super combat, I believe.

[Hydraulic Jack]

Personally, I can't tell if the cylinder in the photo is a Super Combat cylinder or from another compatible model. I suppose you could map out the ports and compare dimensions on the port timing to confirm. The cylinder head frequently does not have a complete part number cast into it because the same casting blank was used for several different heads, with only the combustion chamber profile and deck clearance being specific to one engine versus another.

Your last post tells me it is definitely confirmed as a 125 top end. And, no you can't just drop in a 100cc cylinder and go back to model 98 spec because the cases have to have been bored out to accept the much larger diameter spigot on the 125 cylinder. There would be a substantial air gap between cylinder and cases that would allow the 100cc cylinder to now wander around, and I doubt it would last more than a few minutes if you tried it. The cases can be welded and recut for 100cc use, but it would be cheaper and easier just to source some 100cc cases if that's what you want to do.

Assuming you stay with what you have, your cylinder is toast and needs to be either rebored to the next size up that will remove the scoring damage in the cylinder walls, or you can have it nicasiled and use a stock dimension piston. Both routes cost money. Rebored will give you a bit more displacement which you won't notice in the way it runs, the other will give you a 21st century bore surface that defies wear unless you explode the engine. Your choice. Plating costs more.

If you haven't yet considered it, at this point I would continue to tear down the engine. Model 97/98 engines were usually ridden hard, and your cylinder shows signs of such use. I would expect perhaps gears and clutch pack to show some signs of abuse that may need some attention. Now is a good time to find out, and while the engine is apart you can redo the seals, crank bearings, and verify the bottom end rod bearing and the shift shaft and springs. No reason to put it back together without checking these things only to have to rebuild it again in a few months. Rebuilding a Hodaka is not hard compared to sliding dog gear bikes like Suzuki and Yamaha, and parts are generally available for your engine. Choice is yours. And there are folks on the forum who can do the work if you are not comfortable doing it yourself.

[taber hodaka]

I have a Super Rat that had the engine cases bored out for the Webco 125cc cylinder. In my opinion they didn't work out that well. I am going to find another Super Rat engine and go back to the100 cylinder. It is my thinking that using a piston built for the victory port and having the holes drilled through to the ports, would work fine especially for low end. I would not be afraid to try the reed as it is, after the cylinder is bored. It was fun playing with this stuff especially in the day, porting included. -------Clarence

[viclioce]

Didn't they make a 125 cylinder which drops into the Super Rat cases? I remember Greg telling me his bike is set up that way as well, with a 125cc cylinder/head kit. I can't remember if it was Hodaka made or aftermarket tho. Victor

[taber hodaka]

Webco Victor and it it fit the Ace 90, Ace 100 the Ace 100B and the Super Rat. You had to machine out the cases for the cylinder to fit.------Clarence

[Hydraulic Jack]

Whether a 125 cylinder will "drop in" to 100cc cases all depends on the cylinder bolt spacing and the spigot outer diameter. Whether a 125 piston will mount to a 100 rod depends on the wrist pin diameter at the very least. The compatibility of 97/98 parts differs a bit from a Webco 125 kit built for a 100 Rat of early vintage. As far as I know, the bolt pattern on the 97 and 98 cases is the same, so potentially one could swap for the other - if the spigot hole is big enough. The bolt patterns for the early 100's did not match the 125's, and at least early on, there weren't any 125 Hodaka to steal parts from, hence the Webco aftermarket kit.

The Webco kit required boring out the 100cc donor cases. There is a 6mm difference in bore between 100's and 125's, and the spigot was not thick enough to simply bore out the difference, so a new cylinder was created, and the cases were bore to match. Webco made a piston that used the same wrist pin as the Rat and Ace, so one did not need to swap out the crank to use the kit. But, once the cases were machined to take the larger bore cylinder, the original cylinder could no longer be used because there was nothing to keep the cylinder centered over the rod, which function the spigot does perform.

Hodaka apparently decided to make the 97 and 98 cases the same as far as bolt pattern, likely to save money. Without having the two cylinders side by side, I can't say with any authority that the case holes are the same and only the 100cc spigot was thicker to take up the difference. Actually, I think that there is a machining difference in the cases, and that the two spigots are not the same outer diameter. If they are the same diameter, the cylinders would swap easily. Danny has suggested that cases need machining to swap to the 125. He would know better than I.

Clarence, if your 100cc cylinder fits loosely in the cases because the cases were machined to accept a larger spigot, I would not trust the cylinder bolts alone to center and hold the cylinder over the rod. I would instead expect heat and stress to shift the cylinder around, which would not be good. You will probably eventually bend a rod. Even if Hodaka cylinders are not "tight" in the cases, still the clearances are in the thousandths of an inch, not plus one to three millimeters. Once the engine is fully warmed, I would expect the spigot to be tightly held by the cases, with zero clearance. If you want to try retrofitting a small cylinder on fly cut cases, be my guest. It would make me very nervous though. However, if you could machine a spacer to fill the difference, with a lip at the top to keep the spacer from dropping into the cases, that might very well work fine. I have never seen it done, but that doesn't mean it could not be done.

[JackM]

Thanks for all of the comments/advice guys. The next thing I'm going to have to mull over is how much money do I want to spend and how much the bike may be worth afterwords. Having the cylinder replated will cost me around $200.00 alone, not to mention the cost of a new piston/ring. I would not open up the bottom end without a service manual, which I do not have yet. One thing that attracted me to this bike in the first place was the very good condition of the seat, tank and exhaust pipe. The fork tubes and shocks look good too, now that I've cleaned everything up. My only intent on any of the old bikes I've done is to bring them back and save them from the junk pile. Here's a pic of an XR100 I did over the past couple years.

[Hydraulic Jack]

Jack,

Hodaka never published a shop manual for the model 98 far as I know. However, the methodology is the same as any of the newer five speeds that Hodaka made, and not all that different from every other bike Hodaka produced, with the possible exception of the 175's and 250's. So if you can find a shop manual from, for example, Clymers, Haynes, or one of the other publishers of the day that covers the era of your bike, basically 1974 Hodaka 100/125, buy that one. Anything that might not be as well covered in such a book as you might need can be addressed here by Hodaka folks. They truly are easy to work on with very few special tools needed. If you were able to rebuild an overhead cam Honda, you will find a Hodaka to be silly simple.

Also, ask around the listed preferred mechanics shown on the Strictly Hodaka website, and see if you can find someone to rebuild your top end within your budget. There are several with gold plated reps. As for what the bike is worth after it is rideable, hard to say. My experience is that as an owner, you can't make money buying and rebuilding Hodaka bikes unless you are really, really good at doing it yourself. Cost and value are personally subjective topics.

[dcooke007]

JackM,

I have attached a picture of a super combat cylinder head. Yours looks like it was damaged and a repair attempted. Hard to tell what is going on the outer edge of the combustion chamber.

And just to demonstrate....the late super rat cylinder spigot will fit inside the super combat bore. So there is a lot of difference between these items. The spigot part of the cylinder should be a close fit to the engine cases to help support and center the cylinder.

Your cylinder is absolutely a Super Combat. It is the only 125 model with that intake passage layout. The most cost effective way to get your engine going again is to bore your cylinder and fit a new piston. Plating is much more expensive and you will still need a new piston assembly. From the pictures I cannot tell if your head is still usable as is. More detail on the outer combustion chamber area would be helpful.

The best repair info can be found in the Hodaka Workshop Manual. Manual covers Wombat 94 / 94a and the Combat Wombat. There are some differences between the Combat and the Super but a lot of the info over laps. Add the Hodaka Super Combat Owners manual and you will be good to go. Paul may have these on his web sight.

Danny Cooke

[JackM]

Danny, thanks for the workshop info. You guys on here have been a big help and i do appreciate it. I'm posting another pic of the cylinder head with it cleaned up a little. Without seeing the two heads side by side its hard to tell, but I believe there is a difference, especially in the middle part.

[dcooke007]

JackM,

Looks like it has been machined and the outer / squish area machined. It would run but might be more subject to detonation.

Danny Cooke

[viclioce]

Danny. What causes the kind of pitting observed in JackM's squish area? That looks almost like it got hit with buckshot! Could it be it got rusty from sitting and then was Cleaned up leaving the pitting behind? I'm just curious.

And yes Clarence! Webco made it! Greg emailed me and said that's what he has and that indeed the cases need to be machined for the upgrade. Victor

[MTrat]

Looks like it ate a broken ring. Also, overall, I kind of question the longterm value of plating the bore.. How do others weigh the pros and cons of a plated cylinder.

[viclioce]

I guess the plating issue depends on how much life you have left in a cylinder. Danny told me you can buy Pistons all the way up to .080 in oversized, but, he doesn't like to go past .050 because it makes the spigot too thin.

I have a Wombat cylinder which had a cracked .040 ring set in it. Piston is also shot. So my question in my mind is, should I bore to .050, or should I plate it and bring it back to maybe .030 or possibly .020 depending. I know it's less costly to just bore, but if I exceed .050 with the next bore, I would probably have to toss the cylinder or nickaseal it! Victor

[matt glascock]

On one of my 78.5 RM 250s, I ran out of overbore and had a real problem sourcing a replacement jug. I sent the top end off to Superior Sleeve and had a sleeve installed with matching piston. My original plan was to run it until it died and hopefully, by that time, could source a replacement jug. I found the jug, but the sleeved cylinder is still performing perfectly. In fact it is a magical top end I've beaten the crap out of and the bike still rips. I'm going to weep when it dies, but I'm on the third set of rings and the sleeve looks and measures perfect. I know the pros and cons of sleeving a cylinder, but it is sure an option when having a difficult time finding a replacement jug.

[taber hodaka]

Hydraulic Jack I changed my post to reflect on the discussion. I am not going to put the smaller 100cc cylinder on the engine, set up for the 125. I will rebuild a different Super Rat engine. With your input and Danny's and with some thought I totally agree. ----------------------Clarence

[dcooke007]

Vic,

Hit by buck shot is not to far from what occurred. I agree with Mtrat, broken ring, bearing cage.

Danny

[viclioce]

I thought it might be, but was confused because there are so many nicks!

When my Kawasaki F6 cracked its top ring, it folded apart and it got blown out the exhaust port into the pipe! I was able to buy new rings, put them on & keep riding! I've never seen that much damage from rings! Wow! Victor

[dirty_rat]

JackM,
I don't know what it cost near you, but around here it only cost $60 or so for a GOOD boring job on a cylinder. That is quite a bit less than plating a cylinder, and less than the cost of all the parts and work you would have to do to convert the engine back to 100cc's. With the cross thatch showing at the bottom of the bore, I would doubt that is was ever plated. If it were me, I would get the next oversize piston and have it bored and run the engine as is, a 125.