Relieving the exhaust port bridge - a 41 year old mystery SOLVED?

[spudman]

I've been reading as many posts here as I can since acquiring my '75 model 99 Road Toad about a month ago. It's exactly like the one my dad bought for me as a kid, so I'm in the process of doing all the "restoration" work I can right now. I've seen several posts relating to "relieving the exhaust port bridge" on some bikes and last night I finally had time to delve into the topic a bit more. Then it dawned on me...my original Road Toad had seized up on me the first year I had it. I never knew why at the time, as I knew the oil injection was working fine and I never road it harder than what I knew it was capable of. Tearing that engine apart to remove broken pieces of piston was a real experience for myself and my childhood best friend. I remember having a few of what I now know to be counter shaft ball bearings left when the job was done, but you know...the bike started and ran fine until the day I traded it in. That says a lot for two kids with very few tools but a lot of ambition. Getting back to my "new" Toad though, with only a little over 1000 original miles on it, I'm thinking I better address the exhaust port bridge issue now, because it sounds as though this isn't a question of "if" the engine will seize if left as it is, but rather a question of "when". I'm wondering if someone could point me to a good tutorial here as to what I need to do. I must be doing something wrong when I use the "search" feature here, as I'm never able to find much on a given topic as opposed to just using Google which will often take me directly to a thread/post here anyway. But what I'm wondering is...what is the best "modern day method" (i.e. not using the "stone" shown in the Hodaka service bulletin) of remedying this problem? Is this a job for a Dremel tool? Is relieving the bridge the best way to go, or is drilling the small holes in the piston the better option? What about both? If anyone can direct me to some posts or links that would help me in this endeavor, I'd be eternally (well...as long as the engine never seized, anyway) grateful!

[hodakamax]

Here's the info. Use a stone as shown and push a ring down to the bridge where you can use a feeler gauge to check the clearance. I personally wouldn't use anything but the stone.

Max

[Bullfrog]

I too recommend ONLY using the stone method. It assures a smooth gentle slope for the contour of the exhaust bridge . . . and it won't take very long at all to do.

Ed

[spudman]

Well...it looks so far like the original stone method is the choice of the experts. I do have a few questions regarding this method though. First, where exactly can I find such a tool (stone)? I was somewhat hoping I'd see them on the SH website under the "Tools" section. I get the impression that the stone has to be sort of "shaped" for a specific job before use...am I wrong in assuming this? In Max's attachment, there is reference made to two different stones (oil stone and flex stone); can I assume that one is simply less coarse than the other? Thanks guys for the above information!

[hodakamax]

As for the tool or stone, it's whatever works. You are only removing a few thousandths of an inch. I rummaged around in my tackle box and found a hook sharpener that worked well. Check your local hardware store or tackle shop. If it's too rough, smooth the bridge with sandpaper.

Max

[dcooke007]

I use these for a lot of different tasks. The medium grit should work well. You can shape stones on your grinder and give it a little radius at the end.
http://www.brownells.com/gunsmith-tools ... d9622.aspx
Danny Cooke

[viclioce]

I did mine out a dremel. It was easy and I was able to steady the dremel against the top of the cylinder opening. I used a fine grade sandpaper wheel on the dremel and polished it down with a slight now in the middle. Took about 5 minutes total. The only stone I had was a debuting stone I use on my ski edges. It didn't have a edge which I could fit against the port and end up with a smooth grind because it was about an inch wide and 4 inches long. ; D Victor

[spudman]

Thank you all very much for the help! All the tips are much appreciated.

[hodakamax]

Hey Victor, After studying and blowing up the pictures of the exhaust port bridge done with a Dremel tool, I'm starting to have serious concerns about the clearances and abruptness of the ramps. The bridge needs to be relieved because it gets hot and expands into the cylinder space causing too close of tolerances promoting a seizure in that area. The bridge is put there to keep the ring from expanding into the exhaust port. Too much relief on the bridge will allow this to happen and the results can be catastrophic. The danger is that the ring will expand into the port and snag on the upper and/or lower edges of the exhaust port. Also a flutter could be set up in the ring that could cause failure of both ring and piston.

The relief should not be over .004'' and centered to keep the ring from entering the exhaust port and the entry ramps must have a very smooth transition. If the transition is too steep or deep as it appears in your picture the ring could actually snag in the port.

Maybe it's just the angle that I'm looking from but it's something I would double check. I'm certainly not being critical but concerned. I would hate for you to have a major failure without me at least commenting.

Max

[spudman]

I think I can see now why it was recommended to use the stone versus any type of power tool and to do this very slowly, lightly and methodically. I was going to do mine today but upon removing the cylinder I found scoring on both the piston and cylinder. Looks like at least one of the rings was sticking. So...I ordered a .20 over piston kit and I now won't be doing the exhaust bridge relief until I get the cylinder bored. Plenty of time for me to find and shape just the right stone for the job.
I actually found a local shop owner who is very experienced with all of this, so I'll likely have him take care of the bridge after he does the cylinder bore.

[Zeek]

. I must be doing something wrong when I use the "search" feature here, as I'm never able to find much on a given topic as opposed to just using Google which will often take me directly to a thread/post here anyway.

A lot of forums aren't indexed well, so search is very hap hazard.
Try this instead....Google is much more powerful than the site.
In your Google search box type:
Where for instance you want to search for "relieve exhaust port"

Code: Select all

site:http://hodakaforum.com  relieve exhaust port

This works for all forums and sites

[spudman]

Thanks Zeek!

[dirtsquirt80]

wow the picture of that exhaust bridge does not look good all. hopefully it is just the camera angle!

[taber hodaka]

It is hard to be negative when you try to live in a positive world. The cylinder Does not look usable to me I would be afraid to use it considering every thing else that goes with building a dependable engine. The bridge looks bad and the taper on the port edges do not appear uniform. I think working with some one that has a feel for this stuff would be helpful. Are you considering boring this cylinder out? It is good that you are trying to do this by yourself but in looking for the acceptable standard we all need to read, study, research or have one on one guidance. This is a area that requires some precision. ----------------Clarence

[spudman]

Just to be clear...the photo of the cylinder above is NOT mine (I'm the originator of this thread). And yes...since I now need a new piston and cylinder bore anyway, I plan on having the exhaust port work done by someone with far more knowledge about it than myself as well. There's no way I want to risk getting it wrong and throwing away $170.00+ for the piston kit by having a ring or piston skirt hanging up on the bridge.

[hodakamax]

I must say that this is not a difficult procedure. The tools required are a small stone and a feeler gauge. The instructions are straight forward and require minimum skills. If you do it yourself and follow the instructions it will meet the requirements. I like the satisfaction of doing things yourself (within my skills) and knowing that its done correctly. This is a somewhat unusual procedure limited to engines with an exhaust bridge and even expert boring people might not be aware of this required procedure. Whether you or someone else does this this only requires following important instructions and using the right tools.

Max

PS--Again, the micrometer is not required when using the ring and feeler gauge method. Use the piston to push the ring to the center of the bridge and keep checking it with the feeler gauge.

[spudman]

Thanks Max. Yeah...the only reason I'm letting the person doing the boring do the exhaust port bridge is because he's well aware of the issue from working on 2 strokes "back in the day". I also gave him the printout you provided above so he knows exactly how it has to be done. I'm with you...I also prefer to do things myself. In this case though, if the "pro" has all the tools and expertise (and he'll already have my cylinder as well for the bore job), I figure why not let someone with more experience than myself handle the task. I do appreciate all the info you've provided though! Looking forward to getting everything back together and running in the coming weeks.

[jakecb420]

I figure I will throw some input into this discussion since my bridge relieving may have contributed to my piston getting eaten up. I have about 500 miles on since I did the work to the cylinder. I used the same piston and rings, and had it honed. I feel that I should have done a smoother job, I used a file and a dremel with a stone on it. It didn't do as gradual as a job as I anticipated. I will post a couple pics of my work for general discussion so it may help people in the future as to what a good job should or should not look like. Maybe I should have taken it into the cylinder wall a little more based on the service bulletin.

[bchappy]

Notice how wide the exhaust port is. The ring has to still use the bridge for support so if it is relieved too much it allows the ring to drop out too far and get caught. As the ring has to touch the bridge it should be smooth. Properly done and under working conditions the bridge will heat up and become straight. The Dremel is just not a good tool for this project.

[Bullfrog]

A few detail points:
1. The continuation of the curve shown in the exhaust bridge relief drawing only illustrates what ought to be pretty much a match with the chamfering of the edge of the exhaust port. The drawing is certainly not to scale - you can't illustrate a distance of .003" with lines which are ten times wider than that. So the curve shown is exaggerated to make it visible . . . and dimensioned to give you accurate information regarding the "depth" of the ground in curve. So the visualization is that the ring starts to "pooch out" into the exhaust port and the bridge helps control how far it "pooches out" AND the bridge gently coaxes the ring back "in" so it won't catch on the edge of the port.
2. Having visualized the description above . . . now throw that thought away . . . because . . . the exhaust bridge is undoubtedly the HOTTEST part of the whole cylinder what with the exhaust gasses surrounding it completely. The bridge can conduct heat away to the top and bottom of the port where it connects with the rest of cast-in iron liner . . . so the center of the bridge is likely the hottest part of the bridge. Since iron expands when you heat it, and the center of the bridge heats the most - it makes sense that the center of the bridge gets ground to have the most clearance, right? Al-in-all, at operating temperatures, the relieved bridge may offer a relatively straight surface to guide the ring - still serving the purpose of not allowing the ring to "pooch out" into the exhaust port too far. Nifty.
3, Without relief, the exhaust bridge heats up, expands and moves "in" toward the center of the piston. Not good.

Hope these comments help the understanding of how the relief works and why a smooth profile to the curve is important.

Ed

[spudman]

Just ordered one of these this morning: http://www.ebay.com/itm/311663463696?_t ... EBIDX%3AIT
I'll put a little radius on the edge like Danny suggested and then hand it off to the cylinder bore pro when I drop it off. I feel more comfortable in using the method recommended in that original Hodaka service bulletin anyway...and it looks like it would be pretty darn hard to mess things up with the stone.

[MTrat]

Is anyone recommending the small bridge cooling holes as per Bultaco?

[taber hodaka]

Spudman That sounds really good. It just scares me when someone grabs a dremel and goes grinding, besides in a confined area they can be very dangerous. --------Clarence

[dcooke007]

Is anyone recommending the small bridge cooling holes as per Bultaco?

I drill the small holes in all pistons that fit engines with exhaust port bridges. This is also a Wiseco recommendation.

Danny Cooke

[hodakamax]

Jake, your pictures on page one graphically shows what has happened. The bridge center shows no wear from the ring meaning too much material has been removed allowing the ring to enter the exhaust port. The top ramp seems to have been working but the bottom one is too steep and you can even see where the ring was striking the lower port edge which probably broke the piston area above the ring. It's hard to tell what happened first, ring or piston breakage but it was all surely caused by the ring snagging the lower port edge. Thanks for your illustrative pictures!

Max