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High Performance Bicycle Engine Tips and Motorized Race Bicycles We all want our bicycle engines to push our bikes faster. Get exclusive engine modification tips and suggestions from us. Also check out some of our racing members builds

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  #1  
Old 10-18-2016, 03:58 PM
vhatley vhatley is offline
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Default How Many Cyl Bottom Gasket(s)

Hi, I recently bought an engine that managed to break a motor mounting bolt off smooth with the hole upon first start and the seller has agreed to send me a replacement "engine block" so while I had the cylinder off the old block I decided to follow the guide on Port matching the intake and exhaust...

After doing my due diligence I got out the dremel and started to work on the port work in question, (did the head too,) and realized that the port timing on the new engine block may be different than the old one...the old configuration had 3 gaskets on the bottom of the jug and a very thick head gasket.

I don't have a dial caliper, (yet, gotta talk the wife out of the cash,) but with the old configuration the piston was at least .040 below the deck AND the head gasket is really thick, .060, (or maybe more, again, no calipers.)

Here's the question: When the new "engine block" arrives, how do I tell how thick I need to make the lower gasket? I know this affects (intake,exhaust,and transfer,) timing, I just need to know where, (or what,) to look at to determine OPTIMAL base gasket thickness.

From there I can use the extra thick head gasket to adjust compression and squish ring for extra power. After setting up the cylinder for optimal transfer port and port timing at the base gaskets I don't really want to have to remove that stuff again.

I think I remember reading in my research somewhere that you need to set base gasket thickness so that the top piston ring almost, (or just barely,) shows at the top of the intake runner when the piston is at BDC...but I can't find the exact post now to get the exact procedure and measurements/observations.

Thanks guys for the excellent information I've been getting here. HOPE to be inputting something valuable in the near future too...
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  #2  
Old 10-19-2016, 12:04 AM
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sbest sbest is offline
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Default Re: How Many Cyl Bottom Gasket(s)

First off, don't port match the intake. The mismatch works in your favour.

Raising and lowering the cylinder with basegaskets will alter your powerband and there is no perfect spot. You have to find it. All that would be if you kept your squish gap constant.

What should your gap be? 0.5mm to 1.25mm would be fine, 0.75mm to 1mm best.
Just saw your inch measure, 0.020" to 0.050" is fine, 0.030" to 0.040" perfect.

1) So set your gap any way you can, basegaskets, headgaskets, or filing metal off the head of cylinder. 2) Now run the motor and jet it. 3) Now experiment to see if more cylinder height helps or hinders. Yup, a fair amount of work.

You want quick and easy? Stick with step one only.

Don't want to have to remove that stuff again?
Its only 6 nuts dude! Stick with step one.

Steve
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Old 10-19-2016, 10:34 AM
vhatley vhatley is offline
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Smile Re: How Many Cyl Bottom Gasket(s)

Quote:
Originally Posted by sbest View Post
First off, don't port match the intake. The mismatch works in your favour.

Raising and lowering the cylinder with basegaskets will alter your powerband and there is no perfect spot. You have to find it. All that would be if you kept your squish gap constant.

What should your gap be? 0.5mm to 1.25mm would be fine, 0.75mm to 1mm best.
Just saw your inch measure, 0.020" to 0.050" is fine, 0.030" to 0.040" perfect.

1) So set your gap any way you can, basegaskets, headgaskets, or filing metal off the head of cylinder. 2) Now run the motor and jet it. 3) Now experiment to see if more cylinder height helps or hinders. Yup, a fair amount of work.

You want quick and easy? Stick with step one only.

Don't want to have to remove that stuff again?
Its only 6 nuts dude! Stick with step one.

Steve
Thanks Steve...I appreciate the advice. The real reason I don't want to have to keep removing and reinstalling everything is that it's my main mode of x-port. But guess what,

The seller sent me an ENTIRE engine to replace the one with the broke bolt, so now I have a complete extra motor for experimentation and a daily driver. I will now resign myself to getting the broke bolt out of the case and modding THAT engine.

I do have one question though Steve, how would the mismatch help??? In my experience any voids in the fuel circuit will cause eddies and pooling. At least in cars it does...

The new (replacement) engine has a similar (timing) setup, but has 4 base gaskets the head gasket is the same thickness. I'm not gonna crack the cylinder of this one right away, (it'll be my daily driver,) I'll only use the intake and exhaust I opened up for the old motor and go with that, keep things mainly stock for the run-in.

In the end, for my peace of mind and to know I have understood my research correctly: Adding base gaskets will ADVANCE intake transfer timing BUT decrease compression unless you use thinner head gaskets/surface cyl or head. AND removing gaskets retards the timing...provided I take care of the piston/head relationship by adding space at the top...right?

IF I have this right, then this is VERY similar to advance/retard of a perf. cam in a 4 stroke. (I have LOTS of drag-strip experience, but these 2 smokers have somewhat of a learning curve...

So IF I'm right about the timing, does it work the same as with a car, advance gives bottom end and retard gives top?
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  #4  
Old 10-19-2016, 12:33 PM
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sbest sbest is offline
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Default Re: How Many Cyl Bottom Gasket(s)

Quote:
Originally Posted by vhatley View Post
Thanks Steve...I appreciate the advice. The real reason I don't want to have to keep removing and reinstalling everything is that it's my main mode of x-port. <SNIPPED>
A head gasket or to replace the head is 4 nuts and a gasket. 10min tops.
Slide some 1.5mm (0.060") electronics solder into the spark plug hole sideways until you feel the cylinder wall (to the side, not front or back) and roll the bike ahead. Measure the compressed section. No compressed section? Over 0.060" so you might think of reducing squish.

Quote:
Originally Posted by vhatley View Post
I do have one question though Steve, how would the mismatch help??? In my experience any voids in the fuel circuit will cause eddies and pooling. At least in cars it does...
Not always and not directly comparable with cars and 4 strokes. My guess would be the the intake port mismatch acts like a one-way valve, reducing the back pulse to the carb. Theory doesn't matter, real life effects are what I follow. Even the best theories are just made up stories to cover what happened. The worst theories are the made up stories for what I WANT to happen!

Quote:
Originally Posted by vhatley View Post
The new (replacement) engine has a similar (timing) setup, but has 4 base gaskets the head gasket is the same thickness. I'm not gonna crack the cylinder of this one right away, (it'll be my daily driver,) I'll only use the intake and exhaust I opened up for the old motor and go with that, keep things mainly stock for the run-in.
Good plan, start it up stock, as the manufacturer intended, break it in and baseline (record) its performance. When you make mods, do them one at a time and measure (you need to be able to measure changes) whether it gained or lost performance. Don't trust the old Butt Dyno. it lies. Speedo, stopwatch, or GPS and a good long steep hill are your tools.

Quote:
Originally Posted by vhatley View Post
In the end, for my peace of mind and to know I have understood my research correctly: Adding base gaskets will ADVANCE intake transfer timing BUT decrease compression unless you use thinner head gaskets/surface cyl or head. AND removing gaskets retards the timing... provided I take care of the piston/head relationship by adding space at the top...right?

IF I have this right, then this is VERY similar to advance/retard of a perf. cam in a 4 stroke. (I have LOTS of drag-strip experience, but these 2 smokers have somewhat of a learning curve...provided I take care of the piston/head relationship by adding space at the top...right?

IF I have this right, then this is VERY similar to advance/retard of a perf. cam in a 4 stroke. (I have LOTS of drag-strip experience, but these 2 smokers have somewhat of a learning curve...
No, not really. Adding base gaskets increases the port open DURATION, not timing. Same as duration on a 4 stroke cam. That duration is always centered on BDC.

Quote:
Originally Posted by vhatley View Post
So IF I'm right about the timing, does it work the same as with a car, advance gives bottom end and retard gives top?
Duration does work similar to duration on a 4 stroke.
We are faced with huge overlap and cannot move the durations relative to centerline. Real handicaps! More duration will typically gain peak rpm hp at the loss of mid range torque. It doesn't have to work that way...

Ports that have an appropriate cross sectional area to keep velocity up and yet fill the cylinder within the port open time will do miracles for power. If you want to do calculations about it read Gordon Blair's books on the theory and numbers. Most of the knowledge came from blue collared experimenters like you and I, he just made up the theory to fit the facts.

So, you have 14.5mm carb, your intake ports should be similar. The average intake port window area should be similar, keep in mind it is an opening valve. So have it open drastically at first, a flat edge where it first opens to get port momentum going and a strong pull. Widen it only where it first opens. leave the rest alone.

The unported cylinder wall is your power generating area. The more port you put up there, the more torque you lose. Think about it. Dremel that area with reluctance.

The exhaust needs to open fast and hard. It wants no restrictions but it too likes a constant velocity. The pipe is 19mm inside so keep that diameter. From the window it can funnel a bit, but volume is your enemy. Flatten the top where it opens, don't touch the bottom. Port match the pipe and gasket to the port. I find a 19mm (3/4") drill works great for that. Polish all you want, you will never feel it on these engines.

Cylinder head: Burn central!
A bit of squish (0.2mm to 1mm gap) around the edges of the cylinder will help create combustion turbulence and resist knock. It also speeds up the burn and reduces the need for excessive spark timing advance. With common tools you can modify the stock head to get this.

But then again, these are all theories...
One thing at a time, see what works for you.

Steve
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  #5  
Old 10-19-2016, 02:10 PM
vhatley vhatley is offline
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Join Date: Sep 2010
Location: Oklahoma City, Ok. 73118
Posts: 27
Cool Re: How Many Cyl Bottom Gasket(s)

Quote:
Originally Posted by sbest View Post
A head gasket or to replace the head is 4 nuts and a gasket. 10min tops.
Slide some 1.5mm (0.060") electronics solder into the spark plug hole sideways until you feel the cylinder wall (to the side, not front or back) and roll the bike ahead. Measure the compressed section. No compressed section? Over 0.060" so you might think of reducing squish.



Not always and not directly comparable with cars and 4 strokes. My guess would be the the intake port mismatch acts like a one-way valve, reducing the back pulse to the carb. Theory doesn't matter, real life effects are what I follow. Even the best theories are just made up stories to cover what happened. The worst theories are the made up stories for what I WANT to happen!



Good plan, start it up stock, as the manufacturer intended, break it in and baseline (record) its performance. When you make mods, do them one at a time and measure (you need to be able to measure changes) whether it gained or lost performance. Don't trust the old Butt Dyno. it lies. Speedo, stopwatch, or GPS and a good long steep hill are your tools.



No, not really. Adding base gaskets increases the port open DURATION, not timing. Same as duration on a 4 stroke cam. That duration is always centered on BDC.



Duration does work similar to duration on a 4 stroke.
We are faced with huge overlap and cannot move the durations relative to centerline. Real handicaps! More duration will typically gain peak rpm hp at the loss of mid range torque. It doesn't have to work that way...

Ports that have an appropriate cross sectional area to keep velocity up and yet fill the cylinder within the port open time will do miracles for power. If you want to do calculations about it read Gordon Blair's books on the theory and numbers. Most of the knowledge came from blue collared experimenters like you and I, he just made up the theory to fit the facts.

So, you have 14.5mm carb, your intake ports should be similar. The average intake port window area should be similar, keep in mind it is an opening valve. So have it open drastically at first, a flat edge where it first opens to get port momentum going and a strong pull. Widen it only where it first opens. leave the rest alone.

The unported cylinder wall is your power generating area. The more port you put up there, the more torque you lose. Think about it. Dremel that area with reluctance.

The exhaust needs to open fast and hard. It wants no restrictions but it too likes a constant velocity. The pipe is 19mm inside so keep that diameter. From the window it can funnel a bit, but volume is your enemy. Flatten the top where it opens, don't touch the bottom. Port match the pipe and gasket to the port. I find a 19mm (3/4") drill works great for that. Polish all you want, you will never feel it on these engines.

Cylinder head: Burn central!
A bit of squish (0.2mm to 1mm gap) around the edges of the cylinder will help create combustion turbulence and resist knock. It also speeds up the burn and reduces the need for excessive spark timing advance. With common tools you can modify the stock head to get this.

But then again, these are all theories...
One thing at a time, see what works for you.

Steve
DUDE...Steve, Thank You. Is there any way to get this post marked as a Sticky?!? That is EXACTLY the kind of thing I have been trolling this Forum for days to put together in my own head...again, Thanks. This is more relevant info in one place than anywhere I have found. (Not to be dissing any of the other posts or posters.) And and excellent reference. But just a few more questions...

1.) I'll index the plug, as a habit, but I read in a post that, if using the stock CDI, cutting the plug ground tine even with the middle of the center electode aids in putting the spark more accessible, I have an extra plug, and will probably try it anyways.

2.) I have not gutted the stock "catalytic" yet, but was considering it...what's your thoughts? I've already cut the outlet tube on the bottom a bit, 3 in. I think, and like the sound, but staying stock, is it worth the expense to get a tuned/expansion exhaust?
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