No cylinder gasket questions

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quentrm250

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Location
Endicott, NE
I am rebuilding a Cy-f270. I have two questions:
Anybody ever tried running a super thin layer of silicon and no cylinder gasket for better compression?
I believe there is plenty of quench room to do this is that correct?
 
I am currently doing it on my cy290 . You should have room, the gasket on the f270 is about .45mm give or take a bit. The squish on a stock motor is typically .7-1mm. It would be best if you had a way of checking it just to be safe. There may be a bit of shrinkage in the squish as the piston heats up and expands. It should go back very close to normal once the cylinder is up to temp and expands but that brief period in between would be enough to destroy it if the squish is too small. Always better safe than sorry. You may want to get a thinner gasket and start there. It would be safer if you have no means of measuring.
 
rovanmadlunatic said:
I am currently doing it on my cy290 . You should have room, the gasket on the f270 is about .45mm give or take a bit. The squish on a stock motor is typically .7-1mm. It would be best if you had a way of checking it just to be safe. There may be a bit of shrinkage in the squish as the piston heats up and expands. It should go back very close to normal once the cylinder is up to temp and expands but that brief period in between would be enough to destroy it if the squish is too small. Always better safe than sorry. You may want to get a thinner gasket and start there. It would be safer if you have no means of measuring.
Click to expand...

Ok, so I found some things out last night. I had a brand new complete top end for a F270 and was going to try the no gasket test but I decided against and chose to disassemble a brand new Rovan 27.5cc. Basically there doesn't seem to be a lot of info. on what parts you can mix and match without just replacing everything (crank and top end).

Found some interesting things. First of all I did not have an accurate way to measure quench or exact wrist pin position in comparison to each other (CY vs Rovan). It looks like to me that the Rovan piston is really close to 1mm shorter than the CY (top of wrist pin to top of piston). I dry assembled the entire top end on top of the Rovan short block with no gasket. Turning the crankshaft slowly the piston would actually hit the top of the cylinder. With minimal force I could rotate it 360 deg.
I had two choices: Pull it back apart and insert the CY cylinder gasket (.5mm quench) OR use the Rovan piston and no cylinder gasket and have approximately 1mm of quench.
I chose the .5mm for obvious reasons. 1. I would be putting a gasket back in where it belongs and not have to worry about the two halves of the crankcase meeting together prefectly. 2. I fealt the CY piston quality looked better. 3. More compression.

I got it running today and have started the break-in on the motor. At this point it has been through 2 heat/cool cycles and it appears as though it's going to work.

Essentially moving the piston up .5mm from it's intended position in the cylinder will alter the port timing. Is this anything to worry about? So far still at 50% throttle and it seems to run good throughout the powerband.
Thoughts?
 
Last edited:
quentrm250 said:
Ok, so I found some things out last night. I had a brand new complete top end for a F270 and was going to try the no gasket test but I decided against and chose to disassemble a brand new Rovan 27.5cc. Basically there doesn't seem to be a lot of info. on what parts you can mix and match without just replacing everything (crank and top end).

Found some interesting things. First of all I did not have an accurate way to measure quench or exact wrist pin position in comparison to each other (CY vs Rovan). It looks like to me that the Rovan piston is really close to 1mm shorter than the CY (top of wrist pin to top of piston). I dry assembled the entire top end on top of the Rovan short block with no gasket. Turning the crankshaft slowly the piston would actually hit the top of the cylinder. With minimal force I could rotate it 360 deg.
I had two choices: Pull it back apart and insert the CY cylinder gasket (.5mm quench) OR use the Rovan piston and no cylinder gasket and have approximately 1mm of quench.
I chose the .5mm for obvious reasons. 1. I would be putting a gasket back in where it belongs and not have to worry about the two halves of the crankcase meeting together prefectly. 2. I fealt the CY piston quality looked better. 3. More compression.

I got it running today and have started the break-in on the motor. At this point it has been through 2 heat/cool cycles and it appears as though it's going to work.

Essentially moving the piston up .5mm from it's intended position in the cylinder will alter the port timing. Is this anything to worry about? So far still at 50% throttle and it seems to run good throughout the powerband.
Thoughts?
Click to expand...

It sounds like a good choice. If it hasnt gone boom yet its likely ok.You will have altered the timings but by a very small amount. Likely moving it a shade more to the lower rpm range.It might actually work quite well with the longer stroke. You may gain a bit more grunt, and being a smaller bore, still rev hard as well. Please keep us posted. Im always interested in hearing how these types of changes work out.
 
rovanmadlunatic said:
It sounds like a good choice. If it hasnt gone boom yet its likely ok.You will have altered the timings but by a very small amount. Likely moving it a shade more to the lower rpm range.It might actually work quite well with the longer stroke. You may gain a bit more grunt, and being a smaller bore, still rev hard as well. Please keep us posted. Im always interested in hearing how these types of changes work out.
Click to expand...

Went on vacation so delayed my finish break-in process. 5 heat cycles in and about 60% throttle so far. My feeling is that it has good power completely through the range and I believe the lower rpm power is real good. Only have the 6k clutch in this one and so it is engaging 2k lower than my 30.5cc car and has no problem revving up from there. I was afraid I might have to switch it to 8k but I think it's going to be good.
 
quentrm250 said:
The stock square muffler right now with the restrictive extension pipe removed (removing that extension makes a huge difference from mid and up)
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Hmm well you.ll deff want to change to a pipe at some point. Two strokes are all about the exhaust;) Keep us posted on the progress of your engine. If it ever stops freakin raining here I might get to run mine.:mad:
 
Did some more driving of the higher compression Rovan 27.5cc with the CY top end this weekend and just the stock muffler (Pipe is in the future). Comparing it to my 30.5cc Rovan all testing done on asphalt with knobby tires. 6k clutch on 27.5 and 8k clutch on 30.5.
My 30.5cc from a dead stop gets off off the line faster but the low to mid climb is slow. This 27.5 catches up with it until the 30.5 gets toward the high end and then outruns it on the top end. Checking the spark plugs separately on lower rpm casual driving and high speed runs it appears as though I have both cars tuned very similar.
The results basically were what I expected. I was pretty sure the 30.5 would get off the line quicker and perform better on the upper end. What I did not expect was the 27.5 to be a pretty even match on everything in between. Can't wait to try a tuned pipe and see the results!
 
quentrm250 said:
Did some more driving of the higher compression Rovan 27.5cc with the CY top end this weekend and just the stock muffler (Pipe is in the future). Comparing it to my 30.5cc Rovan all testing done on asphalt with knobby tires. 6k clutch on 27.5 and 8k clutch on 30.5.
My 30.5cc from a dead stop gets off off the line faster but the low to mid climb is slow. This 27.5 catches up with it until the 30.5 gets toward the high end and then outruns it on the top end. Checking the spark plugs separately on lower rpm casual driving and high speed runs it appears as though I have both cars tuned very similar.
The results basically were what I expected. I was pretty sure the 30.5 would get off the line quicker and perform better on the upper end. What I did not expect was the 27.5 to be a pretty even match on everything in between. Can't wait to try a tuned pipe and see the results!
Click to expand...

If the rovan is anything like the km 30.5 it is def built for low end power.It will be interesting to see what a pipe will do. I,m getting very frustrated. I finally got my new modified gizmo carb and of course its raining. Hopefully it wont be long before i get it out.
 
Finally got a bit of runt time on my modified cy290. With a gizzmo mod carb and thor pipe it seems to be working very well. Not fully broken in but already looking strong. It pulls well right of the line. I was a bit surprised at how well it worked in the mid/top end. Kinda thought the thor would fall off before that but it doesn't seem to. More testing to be done yet. The thor is very quiet compared to my dragon pipe.
 
Rovan 30.5cc engine question

Hello Guys, I have a question. I have a Baja 5b with a 30.5cc Rovan engine in it, the guy I got it from never started it because when he started it, it was too loud. I went through and did all of the checks and put locktite everything needed, seems to run great as I have only had 1 1/2 tanks through it so far. My question is who actually produces the engine and what is the track record. This engine feels strong and smooth running. Any input as to what is should expect I am running HPI 2 stroke at 28:1 with 91 octane, Thanks
 
Todd5b said:
Hello Guys, I have a question. I have a Baja 5b with a 30.5cc Rovan engine in it, the guy I got it from never started it because when he started it, it was too loud. I went through and did all of the checks and put locktite everything needed, seems to run great as I have only had 1 1/2 tanks through it so far. My question is who actually produces the engine and what is the track record. This engine feels strong and smooth running. Any input as to what is should expect I am running HPI 2 stroke at 28:1 with 91 octane, Thanks
Click to expand...

I believe RovanSport makes the engine themselves. I have one with about 6 tanks through it that I race dirt oval. Holding up fine. One thing I know for sure is be sure to clean and oil the air filter every tank. I just rebuilt a top end for a guy that didn't have enough oil on his air filter and it wore the ring and cylinder. I have had 2 of the engines apart so far (Rovan) and they have a stuffed crank and the machining is pretty good on the bottom end. The cylinder's are ok but definitely not as pretty as the cy engines. There are a few faulty engines with all brands so you will see a few guys complain of how they are junk but I would disagree. They look ok from what I have seen so far. Are they super perfectly machined billet motors? No they are not but the cost is much cheaper. Keep the air filter and the rest of the car clean and run good synthetic oil for your mix and you will have fun.
 
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