McCulloch 790 build

[promac850]

It's back!! I didn't post a link to the original thread, as I can't recall if it has any important information in it. I'll be looking at it after this thread is posted, and if there is something important in there, I'll quote it and post it in this thread. Or if there is enough stuff in there, I'll link the thread here.

EDIT: Here is the link to the old thread. http://www.arboristsite.com/chainsaw/170558.htm A bit of info in there and a reminder for me slightly dumb arse... don't make eccentric spill his beer.

So... got the cylinder back from getting bored out to the next size (.030" over) since this thing has seen so much use and already wore out two previous over bores (.010" and .020")

Also, I am working on the intake ports a little bit... (after some input from the Emperor, RB1) After working one of them over a little bit, I can see how some gain can be had by tweaking the profile of the area surrounding the three individual intake port holes... there is plenty of material to work with to get good lead ins into the ports. Obviously, they aren't done yet.

Here is what the 790 workbench area of my shop looks like right now. Godforsaken mess...

We need a bigger barn...:biggrinbounce2:

Here are the parts that I've blasted so far, (except one, that's the bar oil tank) the flywheel cover, misc. aluminum tin that covers something (have yet to look at the IPL to see where it goes) and the air filter cover.

Here's the stuff I gotta do yet. Too bad it's raining right now. Otherwise they would be done.

And I polished the top of the piston. I like shiny stuff...

How am I doing?

 

[nmurph]

I'll be following this thread as I have an 890 that needs a little TLC. Compression is about 130 and I haven't had the time to delve into it.

I glanced at the other thread and I didn't see the reason for have the cyliner bored?

 

[Roanoker494]

I'll be following this thread as I have an 890 that needs a little TLC. Compression is about 130 and I haven't had the time to delve into it.

I glanced at the other thread and I didn't see the reason for have the cyliner bored?

There was too much piston to cylinder clearance which was causing very low compression.

 

[promac850]

I'll be following this thread as I have an 890 that needs a little TLC. Compression is about 130 and I haven't had the time to delve into it.

I glanced at the other thread and I didn't see the reason for have the cyliner bored?

The cylinder was tapered pretty bad... worn out .010" (.003" over max spec. of .004-.007" for the .020" over piston) near the top, and about .0079" or something like that at the bottom.

I have the .030" over piston.

Measured it many, many times. I wouldn't have had it bored out if it didn't need it. I probably could've slapped rings in, but that would only be a band aid for the bad compression it had when I received it. It's be more work to tear it all down again and rebuild it later than it's worth, IMHO. I wanted/still want it done right, so I am doing everything I can to make sure it does get done right.

If it was to be an ebay saw and I was/wanted to an #######, I would just have put rings in and given the cylinder a quick hone. But I am not planning on selling this six cuber, and I don't sell on ebay either. I can sometimes be an #######, though.

In fact, I might actually need the 790 to take care of some trees. Maybe a buddy of mine can get me into some tree work with real big stuff. That would be quite fun.

 

[promac850]

There was too much piston to cylinder clearance which was causing very low compression.

Wow. I take way too long when I make a post...

Yep, that was the problem. It should be resolved now...

 

[cpr]

I'll be following this thread as I have an 890 that needs a little TLC. Compression is about 130 and I haven't had the time to delve into it.

I glanced at the other thread and I didn't see the reason for have the cyliner bored?

It had NO compression when I sent it to him.

Kyle, what's that hammer on the floor for? It isn't a mini!

 

[promac850]

It had NO compression when I sent it to him.

Kyle, what's that hammer on the floor for? It isn't a mini!

Just for removing those stubborn wheels on cars... sometimes they kinda stick to the hubs. A lot.

The mini sledge is for smaller BFH situations. Like bending stuff.


I did not use either to tear the engine down, and will not use either to put the saw together.

You can also see that old trimmer engine that's sharing the bench with the 790 project now. I'm infamous for having more than two projects going on at one time.


Holy crap... I just realized how the explanation for the sledge hammer sounds so criminal... I'm meaning on my own/brother's/parents' and very rarely, a customer's vehicle. The wheels like to rust to the hubs here in Michigan, so I sometimes have to use a heavy object for some persuasion. I beat on the tire, not too hard (you don't want to pop the bead off of the wheel...) but enough that a few hits usually frees them right up. Be sure to thread at least one lug nut part way on, some wheels will be damaged if they fall face flat on the ground...

 

[promac850]

I am going to go tinker for a while...

 

[promac850]

All done with the intake ports. They look rough but are actually quite smooth. Still haven't decided on whether I am going to polish them up or leave them as they are.

I used the Dremel and 5 different style bits, and a riffler file set. (basically bent mini files)

Should I keep going? Is there a particular spot that can use a little more work?

 

[cpr]

Kyle, look at my 101 thread for transfer port ideas. You can get new freeze plugs to do the bottom transfers. You can also square up the exhaust ports, just don't take out the bridges or you'll have to shape it and pin the rings.

I'm going to J & S tomorrow to hunt for a carcass for my other 101. Worst case, I'll have another 790 jug you'll have to bore...

 

[promac850]

Kyle, look at my 101 thread for transfer port ideas. You can get new freeze plugs to do the bottom transfers. You can also square up the exhaust ports, just don't take out the bridges or you'll have to shape it and pin the rings.

I'm going to J & S tomorrow to hunt for a carcass for my other 101. Worst case, I'll have another 790 jug you'll have to bore...

Crap, I've been calling the transfer ports the intake ports this whole time? I do realize the reed valve plate bolts on top of this area, so maybe it technically qualifies to be called both?

Exhaust ports sure could use the clean up... I keep looking at them little recesses where the round holes in the liner don't fit the square profile in the aluminum casting... kind of an eyesore. And carbon collector.

I thought about the bottom transfers... maybe... sometimes it all just has to be done, doesn't it? Guess that might be my answer right there...

Looked at one of your many build threads, and I see how I could make things a little bit better. What were you using to work the ports over? It looks like it makes a better finish and gives you more consistent lead ins... (teardrops)


RandyMac, do I have the green light to go ahead with more port work?

 

[cpr]

At the extent of my knowledge as R M Motorsports did that port job...

Sorry.

 

[leeha]

As cpr stated, Read the port info in his thread.
Porting the 3 ports your working on is useless
unless you do the other 3 on the bottom. It
balances out the intake charge.



Lee

 

[promac850]

As cpr stated, Read the port info in his thread.
Porting the 3 ports your working on is useless
unless you do the other 3 on the bottom. It
balances out the intake charge.



Lee

Are you referring to this?

I see the ports on the other side through the ports closest to the camera... is the cylinder wall modified like this on both sides or only the bottom side?

Are those ports the bottom transfers? You all are calling them 'bottom transfers' because it's on the bottom of the engine, as if the engine was installed in a saw, am I correct?

 

[cpr]

Are you referring to this?

I see the ports on the other side through the ports closest to the camera... is the cylinder wall modified like this on both sides or only the bottom side?

Are those ports the bottom transfers? You all are calling them 'bottom transfers' because it's on the bottom of the engine, as if the engine was installed in a saw, am I correct?

Yes and yes.

 

[RandyMac]

I'm trying to find that pic with the teardropped ports, it looks easy enough, just a bit of a taper up towards the head on the inside.

 

[promac850]

When I teardrop them, do I do the sides opposite each other in a way to help swirl the air/fuel mix as it moves into the cylinder? Or is that not really too critical?

Say if that picture from cpr's thread shows the ports on the opposite wall pointing to the right, then the ones on the wall closest to us would be pointing to the left...

 

[promac850]

When I teardrop them, do I do the sides opposite each other in a way to help swirl the air/fuel mix as it moves into the cylinder? Or is that not really too critical?

Say if that picture from cpr's thread shows the ports on the opposite wall pointing to the right, then the ones on the wall closest to us would be pointing to the left...

Bump and update...

Worked the exhaust port over a bit, and yes, I left the bridges in. In fact, the holes still are round, but turn into a square as they approach the muffler inlet... I just wanted to get rid of the dreaded recessed spot where carbon loves to collect.

Haven't teardropped the transfers on the inside of the cylinder, but the transfers on the upper side are real nice now.

As for removing the transfer freeze plugs, what should I use? Can I punch them out from inside using the punch and putting it through a transfer port on the intake/upper side of the cylinder, then tapping the punch with a hammer, or is it a better idea to drill them and then pry them out?


I'm starting to think a lot about getting one of these... I realize I really am doing quite a bit of porting now... and this definitely won't be the last saw, or engine that I'll port.

1/8" Air Micro Die Grinder With Case

should I?

 

[promac850]

Went to HF this morning and bought me the 1 ton arbor press, the micro air die grinder, and a couple of other things...

Worked on the upper transfer and exhaust ports some more (still gotta pull the freeze plugs and do the bottom transfers) and now the upper transfer/intake side is, well, really shiny... exhaust isn't far behind...

Apparently, 56,000 rpm and a polishing wheel makes quick work of something that usually takes forever... this is what the upper transfer area looks like now... really smooth where the mirror shine is, and plenty smooth enough where the ports are... still some roughness, and I don't think I'm going to try polishing all that out. It'd take a long time since it's much rougher than the flatter surface of the area leading up to the ports...

Also swapped the pistons. If you notice the silver dot on the connecting rod and the black one on the piston, that was to make sure they were assembled so that the rod would be bolted up just as it was before (i.e. not running backwards) I have read that some engines have rods that are straight, and some have angled connecting rods... this one looks straight, but I try not to leave stuff to chance... if I can help it, that is.

Is that wrist pin in far enough? I did find out it's possible to go too far, and when I did that, I had to press it out and start over. I'm not exactly sure where it's gotta be set.

 

[heimannm]

If your 790 port job works out well, I may send a 795 or 840 block up so you can continue your exotic dental work.

Mark