New to porting

[Shane¹]

So I am going to attempt to port a couple dolmar cylinders. Nothing too crazy a 7900 and 6421 cylinder. I am trying to figure out timing but not figuring out what to do just yet from reading. So I understand how to check exhaust, intake and transfer height but do you only worry about the height of the port from the top side? Does the bottom of the port get checked? Anyone know a good thread or site with more info?

 

[merc_man]

Look up part one the exhaust port and start ther.


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[merc_man]

https://r.tapatalk.com/shareLink/to...hare_fid=1392778&share_type=t&link_source=app

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[Lnk]

Look up part one the exhaust port and start ther.


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https://opeforum.com/threads/part-one-the-exhaust-port.7279/

 

[Shane¹]

I have read through a lot of that thread there is a lot of just talking I just more want to know how to check all of the numbers

 

[Ketchup]

You read the exhaust port and transfers on the top (closer to the combustion chamber). Read the Intake on the bottom.

I recommend you read everything in the porting links sticky. Plenty of useless chatter, but there are some real nuggets that you may miss at first.

https://opeforum.com/threads/chainsaw-porting-thread-links.16583/

The Tree Monkey videos are really helpful too.

Also, find threads specific to the model you want to port. That will help you look at the same thing that’s being talked about and reduce confusion.

And get yourself some low cost cylinders to start on. 7900 could be an expensive jug to replace.

Welcome to the rabbit hole.

 

[Shane¹]

The one I plan to mess with is an aftermarket 7900 cylinder so if I mess it up real bad I will not be out much and will probably just replace it with another aftermarket cylinder. Anyone have a suggestion on what numbers to start with. I am think about going off numbers Dyno Joe used in his 6421-7900 video intake 78 exhaust 98 and transfers 126 but depends on where squish lays because I don't really want to have to have the base cut if I don't need to

 

[Ford3000]

You would be better to try on the cylinder on the saw, measure squish,
and measure port times.
That is the only way people can give advise, as they need to know the numbers
you are dealing with.

I know nothing about porting, but if it were me, I would get the numbers
so those in the know could advise me accordingly.

Do you know if the case you have is good, as in the seals are holding pressure and vac,
are the main bearings good, connecting rod good, best to know before putting money
and time into a project that may work out more expensive that you plan on.

 

[Shane¹]

You would be better to try on the cylinder on the saw, measure squish,
and measure port times.
That is the only way people can give advise, as they need to know the numbers
you are dealing with.

I know nothing about porting, but if it were me, I would get the numbers
so those in the know could advise me accordingly.

Do you know if the case you have is good, as in the seals are holding pressure and vac,
are the main bearings good, connecting rod good, best to know before putting money
and time into a project that may work out more expensive that you plan on.

I have plans to get numbers just need to find time to get them

 

[Ford3000]

So I am going to attempt to port a couple dolmar cylinders. Nothing too crazy a 7900 and 6421 cylinder. I am trying to figure out timing but not figuring out what to do just yet from reading. So I understand how to check exhaust, intake and transfer height but do you only worry about the height of the port from the top side? Does the bottom of the port get checked? Anyone know a good thread or site with more info?

I would imagine the bottom of the port also counts.
Hopefully someone with knowledge will explain why, @huskihl anyone.
Its got me thinking, what keeps the fuel charge sucked in on the upstroke,
from being pushed back out the same port on the down stroke, does it mean
the case has to hold more volume than what would hold the charge so that charge
does not get blown back up the intake port before the piston closes it preventing such

 

[huskihl]

You’re on the right track. At this stage in the game, just use port openings and don’t pay attention to the opposite side. 98 on the exhaust, at least on how I do mine, makes them pretty spicy and maybe not the greatest worksaw if you like to dig the dogs in

 

[Shane¹]

You’re on the right track. At this stage in the game, just use port openings and don’t pay attention to the opposite side. 98 on the exhaust, at least on how I do mine, makes them pretty spicy and maybe not the greatest worksaw if you like to dig the dogs in

So what you are saying is only worry about the bottom of the intake and top of exhaust?

 

[Shane¹]

My stock numbers came out to intake 78 exhaust 105 and transfers were 127-130. The squish measured a little weird because of some flash at the top of the cylinder I got .019-.026. Plan is to take the exhaust up to 98 or 99 clean up transfers and widen intake and exhaust. Going to try to find a way to sand the top of the band even and remeasure squish first

 

[huskihl]

So what you are saying is only worry about the bottom of the intake and top of exhaust?

Correct

 

[huskihl]

My stock numbers came out to intake 78 exhaust 105 and transfers were 127-130. The squish measured a little weird because of some flash at the top of the cylinder I got .019-.026. Plan is to take the exhaust up to 98 or 99 clean up transfers and widen intake and exhaust. Going to try to find a way to sand the top of the band even and remeasure squish first

The oem 7900/10 is too tight at the outer edge to delete the gasket. Squish at the outer edge is .013”

 

[Shane¹]

The oem 7900/10 is too tight at the outer edge to delete the gasket. Squish at the outer edge is .013”

This is an aftermarket cylinder and it is close to .020. Saw has already been ran with no gasket and hasn't blew up yet I will find my good solder and recheck squish tomorrow

 

[huskihl]

This is an aftermarket cylinder and it is close to .020. Saw has already been ran with no gasket and hasn't blew up yet I will find my good solder and recheck squish tomorrow

Yep. Just reminding you that even though the AM cylinder fits, it’s a bit tight for the oem

 

[Shane¹]

Made up a cutter for the squish band tonight and got it pretty even around .025- .026. Now time to mark it up and grind it!

 

[Paul Fithian]

I’m starting this endeavor as well. These tools have worked well so far for me, and add additional capability in my shop for other work.



Most of my learnings on this subject has come from this forum, big thanks to OPE contributors!

 

[Shane¹]

Got the 7900 cylinder done for now. Exhaust raised to 98 transfers are at 128 intake is at 78. Widened the exhaust to about 28mm and intake to about 27mm think I can go more? Going to try it first then maybe take it back off if it is not very strong.