Relieving the exhaust bridge on a freshly bored 85 250R

[wonderboy]

I have my brother's 250R jug back from being bored and ported. I need to relieve the exhaust bridge. I understand it needs to be relieved 3 or 4 thousandths. To do this, is it fine to use a rotary tool or will this be too agressive? Also, I'm assuming that the relief is 3 or 4 thou at the midpoint of the bridge, and that it tapers back to nothing (no material removed) at the top and bottom of the bridge itself. Also, what surface finish am I looking for when done (same cross hatch as the rest of the bore, or smooth, etc).

I'm probably making more out of this than necessary, but any advice would be appreciated!

[barnett468]

I have my brother's 250R jug back from being bored and ported. I need to relieve the exhaust bridge. I understand it needs to be relieved 3 or 4 thousandths. To do this, is it fine to use a rotary tool or will this be too agressive?

f you are determined to do it one approach is to use a dremmel with the white sanding/polishing tips.

Also, I'm assuming that the relief is 3 or 4 thou at the midpoint of the bridge, and that it tapers back to nothing (no material removed) at the top and bottom of the bridge itself.

yes, but just .0015 is way more than enough. leave the bridge untouched 3/16" - 1/4" from both top and bottom so the rings will seal beyond those points.

Also, what surface finish am I looking for when done (same cross hatch as the rest of the bore, or smooth, etc).

i would make it all as smooth as glass wherever you remove material. Hone it after, not before.

I'm probably making more out of this than necessary, but any advice would be appreciated!

No, you are not, many people screw up their rings or cyl by doing it improperly.



You can color the bridge with black felt pen or dykem so you can see what you are doing better.

Instead of doing this or in addition to this you can also drill 2 1/32 holes in the center of the piston, one above the other 1/2" apart, starting at around 1/2" below the bottom ring. This forces gas/air/oil onto the bridge which cools and lubricates it which reduces the potential for a seizure on the bridge.

[82 250r]

Usually the machinst/builder who does the bore and hone will relieve the bridge for you.
Are you sure it's not already done?

[nstyle73]

An honest question, I know this is common practice on the Honda FL350, but is this pretty standard on the liquid cooled bikes?

[Super350]

Usually the machinst/builder who does the bore and hone will relieve the bridge for you.
Are you sure it's not already done?

I was going to say I have never done any relieving myself...I hope my machinist did it

[82 250r]

An honest question, I know this is common practice on the Honda FL350, but is this pretty standard on the liquid cooled bikes?

Yes...if you're referring to relieving the exhaust bridge.

[wonderboy]

Thanks guys. Barnett468, thank you for the details. That makes sense to leave it untouched before (rather than exactly at) the top and bottom, that way the ring is pushed back cleanly.

Unfortunately, the original machine shop that bored the cylinder didn't relieve the bridge. They also didn't even chamfer the ports (they are off my list now). The cylinder was ported by Jason Hall (after having it bored by another shop). Jason recognized that the ports weren't chamfered and did that, but didn't know if the bridge was touched or not, so he left that alone (especially figuring that I could easily take care of it)

Regarding weather or not to do it on a liquid machine: I think the problem was much worse on the air cooled machines (FL350 Odyssey) but most of the sources I read say to do it on all machines with an exhaust bridge. The way I figure, it can't do any harm as long as it is done properly.

I'll have to read up on the drilling of the holes in the piston. This sounds like a good idea too.

Thanks for all the good info guys, I appreciate it!

[atc007]

I've forgotten how many R motors I've built,and NEVER relieved it. I have always drilled 3 holes in every 250 and bigger 2 cycle piston there. Make sure your ports are chamfered smoothly,which yours are by Jason . And ride it.

[barnett468]

Thanks guys. Barnett468, thank you for the details. That makes sense to leave it untouched before (rather than exactly at) the top and bottom, that way the ring is pushed back cleanly.

no prob, yes a shallow transition is better than a steep one.

Regarding weather or not to do it on a liquid machine: I think the problem was much worse on the air cooled machines (FL350 Odyssey) but most of the sources I read say to do it on all machines with an exhaust bridge. The way I figure, it can't do any harm as long as it is done properly.

it's best not to do it on machines that are not known for seizing there. i did that and drilled holes on my old air cooled race bikes back in the 70's but they got the rings replaced often.

We never seized a stock kawi and the factory team rarely seized one. yes one might say that it's a bad comparison because the motorcycles are much lighter than a 3 wheeler and/or they are tuned by the worlds best tuners etc., but just the same, Jeff Ward, Larry Roessler and others subjected them to plenty of load I can assure you.

I'll have to read up on the drilling of the holes in the piston. This sounds like a good idea too.

it is as ATC007 suggests. You can also put 3, 1 1/2" long narrow horizontal score marks around .0015" deep spaced 1/4" apart with the edge of a small fine triangle file on the ex side too if your worried. These help retain the fuel/oil mix adding additional lubrication to this area.

[barnett468]

post correction

never seized water cooled kawi.

[just ben]

I know wiseco pistons come with directions for drilling the holes In the piston. It's pretty simple,just Install the piston and trace the bridge on both sides with a pencil and drill the holes within the marks.

[Jason Hall]

Frank, I wish I would have known the cylinder was already honed to fit the piston, I would have done the bridge. I look at it this way, the bridge Will swell into the cylinder if ran lean. It only takes a couple seconds for the bridge to swell enough to trash your rings, and possibly stick the piston. Even if, and especially if your using a wiseco
Piston.. The Wiseco usually has the holes drilled to help cool/lube the bridge, but relieving it is extra Insurance that I like to have!! There are a lot of different opinions on pistons.. I like a cast piston. A wiseco is forged aluminum. That means when it was made it was poured into its mold and cooled under pressure, then machined. A cast piston is poured into its mold then cools naturally (not under pressure), then is machined. So IMO once the wiseco gets hot (Until it has some heat cycles to allow the aluminum to find its happy place), it wants to naturally expand like a Mo fo.. Lol. Even when I use a cast piston, I relieve the bridge and drill holes for the bridge. To much is never enough as I Hate to stick or melt my pistons.. Hope you understand what I'm saying, and I didn't confuse you.. Lol.

I use a 60 grit flapper wheel in my die grinder and check the clearance by laying a straight edge from the top of the cylinder to the bottom directly over the bridge. I have done a lot of them, and found this to be the easiest way. As stated earlier by someone, don't mess with the top or bottom to much, you just want to relieve the middle looking at it from top to bottom.. Good luck..

[wonderboy]

Thanks Jason! I was gonna call you and chat about it, but didn't want to bug you. No worries about leaving it alone, as you were just doing the porting job... By they way, check out my other thread showing off the 350x head you did. I put it side by side with a spare stock head I had. It is very cool... (I'm so excited to get these builds going)

I understand what you're saying about the forged versus cast pistons.

Also, back to early comments: I went onto Wiseco's website and downloaded their general piston instructions and they specifically say to do BOTH: relieve the exhaust bridge and drill the holes over the bridge. I found quite a few discussion about this on the net, including one that had comments by an official Wiseco rep saying that the only reason they don't drill the holes is that becuase many of their pistons cover multiple years and multiple cylinder configs (some without exhaust bridges). So wiseco just has the consumer drill the holes in situations like the 250R. Sounds like it is a very easy thing to do.

So, I'm planning on doing both steps.

Thanks again for everyone's input. It is greatly appreciated.

[barnett468]

Great jason posted, here's some more fun info.

1. A stock unmodified bridge is unlikely to swell especially if properly jetted as jsason suggests. this is also evidenced by the many cyls that atc007, factory kawi racing and i have run.

2. When a cyl with a bridge is modified/ported, it is common practice by many porters to narrow the bridge. when the bridge is narrowed it has lost some structural rigidity and therefore has an increased potential to swell/warp etc.

3. When the cyls are designed the engineers have this potential swelling in mind and make the bridge thick enough so that it should not swell providing it is left alone.

4. All the mfg's run the bikes through the wringer for a few hundred hours before production. all problems are fixed before production. if a bike ever seized for any reason the prob would be corrected before production. the mfg's HATE warranties so they really do try to build a reliable bike to start with.

5. The flapper paper he recommends is also good however it is easy to round the sides of the bridge quite a bit and/or accidentally catch the nicely honed cyl if one is not careful. You might consider putting masking tape around the ex port to protect those areas.

[Jason Hall]

All you said sounds great, but when they engineered the factory setup, I'm sure they used a Cast OEM piston?? Almost every person who just orders a piston from their local cycle shop will most of the time end up with a Wiseco piston. So I would Deffinatly not trust my piston and bore on an engineer that set up the cylinder 20 years ago with a cast piston that most likely used 2 times as much oil as most people would use today. Also, most people don't have there machines jetted correctly. A factory race team, or Very experianced person would probably not ever have to worry. I (knocks on Wood) have never had someone call me and tell me they just melted their piston, and I have done LOTS of cylinders for people. I Always tell people to call or message me and I will help them jet.. I don't narrow the bridge on the piston side, but do sharpen the outside edge. So to be safe I would always suggest to relieve the bridge.. Another thing to remember is as you bore the cylinder over, the cast iron sleeve gets thinner.