McCulloch 790 project...

promac850 · Apr 21, 2011

Here is the thread some of you must have been waiting for... this is dedicated to the rebuild of the McCulloch 790 that I received from a member on here. Thanks again sir!!! :cheers:

Once upon a time, a box showed up in the garage. It was fairly hefty and had a few rattles... hmm, what could it possibly be?

It was the 790. I opened the box, and found an engine, and some of the other parts needed to make a complete saw. I will have to acquire a few parts as I go, it will need a few things, most notably, a complete fuel tank with the upper and lower housings. Some bolts and nuts likely will be needed in the future.

Here is a pic of what I've got so far... the engine is torn down, awaiting a thorough cleaning.

Here are some pics of the cylinder... mind you, it is as is from disassembling, so there are a few specks of crud here and there.

I'd say the bore looks very good. There are no signs of piston slap on the piston, so I assume there is no reason a re-ringing of the piston and a very light hone wouldn't take care of the compression. If anyone notices something important that needs to be checked on the cylinder, piston, etc. please let me know.

promac850 · Apr 21, 2011

Here are two more pics, one of the piston, and one of the cylinder bore...

Roanoker494 · Apr 22, 2011

Two McCulloch rebuilds in a row, looks like you are starting a trend there. Macs are yellow like the sun because they make life worth living.....

Repped ya for bringing another oldie back to life.

Deprime · Apr 22, 2011

Looking forward to seeing your build progress! :rock:

promac850 · Apr 22, 2011

Roanoker494 said:
Two McCulloch rebuilds in a row, looks like you are starting a trend there. Macs are yellow like the sun because they make life worth living.....

Repped ya for bringing another oldie back to life.

Thanks!!

It won't let me hit ya, outta ammo.

I love bringing a good old saw back to life.

Still gotta figure out the little running problem I have with the 850... should've rebuilt the carb at the beginning, lol. Pretty sure that's the problem. Otherwise, I'll have to look deeper.

Deprime said:
Looking forward to seeing your build progress!

Thanks!! I'm excited too... I have been trolling around feebay and it is very hard to find parts for this thing. What I can find, is quite expensive, lol. Guess the rest of my graduation money has a place to go...

(yes, I saved some from last June... it's lasted that long!! Bless the family and friends that dropped money into the box, I bought a welder, more tools, a saw for my pops, and possibly a bunch of parts for this 790 soon)

Also can't wait for your tread on your beastly Mac too!

Brian13 · Apr 22, 2011

Why are there so many hammers right there

.

Might not be a bad idea to check your clearance and taper.

heimannm · Apr 22, 2011

I expect it is worth replacing the rings as long as you have it down that far, rings are not so expensive and can make a big difference in compression...if you want the saw to really pull hard in the cut.

Standard rings will be 58353 is the bore is 2.119.

Mark

Ambull · Apr 22, 2011

You have a bit of work ahead of you. Keep us posted.

promac850 · Apr 22, 2011

Brian13 said:
Why are there so many hammers right there.

Might not be a bad idea to check your clearance and taper.

Hammers... the rubber mallet worked nicely to coax the crank out of the cylinder and cover plate.

Anyone have a way to measure the bore of the cylinder accurately without one of these fancy cylinder bore gauge sets? I don't have one yet.

heimannm said:
I expect it is worth replacing the rings as long as you have it down that far, rings are not so expensive and can make a big difference in compression...if you want the saw to really pull hard in the cut.

Standard rings will be 58353 is the bore is 2.119.

Mark

That's what I'm hoping... that it only needs a set of rings and a very light hone. I found .030" oversize rings on feebay... is it possible to use these rings in a regular size bore if I file the end gap to the proper specs?

Ambull said:
You have a bit of work ahead of you. Keep us posted.

Yep... gonna take longer than the 850 took, as I will be looking for specific parts and trying to get them for a price that doesn't chafe my cheap ass. :laugh:

I suspect this will be one of the most 'expensive' saws I have when I'm done with it. I will likely end up with some money in it.

promac850 · Apr 22, 2011

heimannm said:
I expect it is worth replacing the rings as long as you have it down that far, rings are not so expensive and can make a big difference in compression...if you want the saw to really pull hard in the cut.

Standard rings will be 58353 is the bore is 2.119.

Mark

I measured the piston with my micrometer, and it came out to 2.2345" I kinda forgot how to read a micrometer... :bang: I may be wrong, so I will post a pic of the measurement after I eat some lunch. If I'm and this measurement is right, I assume that means the piston may be an overbore size.

dieselsmoke · Apr 22, 2011

610, Looks very good from the pics. Can you take the cyl. to your school and get one of the instructors to help with measuring the bore? YOU really need to check it at top, middle, and bottom of the stroke, to determine how worn it is. IIRC the man did say it had almost zero compression. Clean up the piston and look for over-size markings. Not sure if Mcc did that. Good luck, oh and I see now we all can see the surface of you work bench!! Good job.

promac850 · Apr 22, 2011

dieselsmoke said:
610, Looks very good from the pics. Can you take the cyl. to your school and get one of the instructors to help with measuring the bore? YOU really need to check it at top, middle, and bottom of the stroke, to determine how worn it is. IIRC the man did say it had almost zero compression. Clean up the piston and look for over-size markings. Not sure if Mcc did that. Good luck, oh and I see now we all can see the surface of you work bench!! Good job.

Well, if I had the snap gauges, I could check it myself... I still remember that at least a four-point check should be done. I will likely check it at six, or maybe eight points for the hell of it.

I will have to see if Napa or Autozone has the gauges in their store. Will check stores tomorrow. Right now, I think I need a nap, lol. Stayed up until 2:40 last night... looking for stuff for the 790 on feebay.

dieselsmoke · Apr 22, 2011

Make your self a rod out of 1/4" C.R. round stock that is EXACTLY as long as your piston dia. Then check your clearances with a feeler gauge. A quick project for a novice welder, student mechanic, sawaholic.

promac850 · Apr 22, 2011

dieselsmoke said:
Make your self a rod out of 1/4" C.R. round stock that is EXACTLY as long as your piston dia. Then check your clearances with a feeler gauge. A quick project for a novice welder, student mechanic, sawaholic.

hmm... you gave me an idea!!

I'll have to see if there is something like that I can use... don't have any 1/4" C.R.<-- what? round stock. Either I'm stupid or it's just the aftereffects of taking a nap for two hours, lol.

cbfarmall · Apr 22, 2011

promac610 said:
1/4" C.R.<-- what? round stock. Either I'm stupid or it's just the aftereffects of taking a nap for two hours, lol.

CR is cold rolled

Chris B.

promac850 · Apr 22, 2011

cbfarmall said:
CR is cold rolled

Chris B.

Doh, heard of that before. :bang: Don't know how I didn't think of it. Must've been the brains not waking up yet.

Cold rolled round stock... I don't have any in stock. I'll look around and see if I can find something that can be substituted for the cold rolled 1/4" round stock.

Roanoker494 · Apr 22, 2011

Harbor Freight has a lot of engine building tools available, in store or on the website. I buy most all my tools there and have never had any problems. The prices are quite a bit cheaper than most other places and the tools seem to hold up very well.

promac850 · Apr 22, 2011

Roanoker494 said:
Harbor Freight has a lot of engine building tools available, in store or on the website. I buy most all my tools there and have never had any problems. The prices are quite a bit cheaper than most other places and the tools seem to hold up very well.

Yeah, some of the stuff they sell is pretty good. I went there to pick up a set of hose pliers the other day and their completely out of stock, warehouse and all. I needed those damn pliers to pull the injectors on the VW. Since they stated they won't get them for months, I just spent yesterday fabricating my own injector puller with some scrap angle iron and the handy Hitachi grinder, and welding the pieces together with the Hobart. Results in a very nice home-grown tool made out of necessity. Will be trying to pull some injectors tomorrow... I heard it's like pulling teeth when the seals are dried up. :laugh:

It is quite difficult to get them things to move up... they spin in the bushings, but they don't come out of the bushings... so much for those seals that should've been replaced when that shade tree ******* worked on it. Won't bring another car to that guy, ever.

Back onto 790 topic... it still is waiting to be cleaned up good so the measurements can be as accurate as possible. I might boil the cylinder later tonight, this time with water mixed with some dish soap in the pan, instead of plain water... not saying plain water won't work, it just leaves the p/c more of a dark gray color, rather than a shinier lighter gray.

slipknot · Apr 22, 2011

promac610 said:
Well, if I had the snap gauges, I could check it myself... I still remember that at least a four-point check should be done. I will likely check it at six, or maybe eight points for the hell of it. I will have to see if Napa or Autozone has the gauges in their store. Will check stores tomorrow. Right now, I think I need a nap, lol. Stayed up until 2:40 last night... looking for stuff for the 790 on feebay.

I got a whole set of cheap t-gauges off of ebay for less than $20 shipped. They arent the best but get the job done. Good job on this thread...a very interesting subject. I may be inspired to tear mine down. I need a frikin clutch so get mine goin...I robbed it to fix my 1-70. You are really getting a worth while collection going.

promac850 · Apr 22, 2011

slipknot said:
I got a whole set of cheap t-gauges off of ebay for less than $20 shipped. They arent the best but get the job done. Good job on this thread...a very interesting subject. I may be inspired to tear mine down. I need a frikin clutch so get mine goin...I robbed it to fix my 1-70. You are really getting a worth while collection going.

I will have to look at a set of those gauges. Who was the seller? How fast did they ship it?

A clutch... will keep my eyes open for one and let you know if I see one somewhere. :cheers:

I know exactly how you feel when it comes to the final stretch and not have what you need to get it done. Can be frustrating, but totally worth it to keep whittling away at it until you get everything you need.

McCulloch 790 project...

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