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Old 09-20-2020, 10:52 AM   #1
pcpat
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Default Hard turning with new pistons

first time posting in a long while, so please bear with me...
I have a question regarding rebuilding my engine. I purchased my RPU as a basket case, and never had the engine run since it it was partially disassembled when i picked it up, but the engine turned free. I finished tearing down the engine and brought the block to a local machine shop get bored, he bored it to 30 over. I picked up the pistons and rings from Snyders and following Les Andrews book, measured the rings for proper clearance before installing.
Checked the babits, and they looked really good, Installed the crank and shimmed it so the plasti-gauge was at .002 but when turning it was way to tight, added some additional shims which brought the plasti-gauge measurement to .0025-.003 but the crank was able to turn with about 25 lbs pressure per the book.
Now the issue...Installed the pistons one at a time, again checking each piston to keep the clearance around .002 and added more shims so it wasn't binding and each piston added additional force to turn but all moved without binding; however, when all the pistons are connected, it takes quite a bit of effort to hand crank the engine (no heads are installed yet), the effort is way more than 35 lbs like the book says.
Is this normal for new pistons and rings or is there something I have missed? The rings measured correctly, but will the block need to be bored out more/? Will this amount of effort ease up after the engine has turned over and ran for a bit? Any guidance or thoughts are welcome.

Last edited by pcpat; 09-20-2020 at 11:54 AM.
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Old 09-20-2020, 12:14 PM   #2
Bob C
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Default Re: Hard turning with new pistons

Did the machine shop have the pistons before they did the final hone
after they bored it??
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Old 09-20-2020, 12:33 PM   #3
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Default Re: Hard turning with new pistons

I just rebuilt my engine a month or so ago and had the same experience. It also ran hot for the first couple hundred miles. I initially started the engine, ran it to running temp then changed the oil. Then put on 100 miles and changed the oil, then went to a 500 mile oil change interval. When i first put he pistons in it was all I could do to turn it over with the crank
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Old 09-20-2020, 12:53 PM   #4
1931 flamingo
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Default Re: Hard turning with new pistons

Did you lube everything when you assembled it??
Paul in CT
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Old 09-20-2020, 01:06 PM   #5
Patrick L.
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Default Re: Hard turning with new pistons

You added more shims to the crank bearings when the pistons were installed and the engine turned hard ?

The crank bearings shouldn't have more than .002" as far as I'm concerned. I've seen to many knock at .003". If just the crank is installed it should spin quite easily.

What clearance does the piston maker suggest ? .002" seems way too tight to me, but, its up to the manufacturer to set the spec. I'm an old .001" per inch of bore [ minimum] guy. I usually use more than that.

Last edited by Patrick L.; 09-20-2020 at 02:02 PM.
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Old 09-20-2020, 01:30 PM   #6
Jim Brierley
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Default Re: Hard turning with new pistons

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.002" was the max piston clearance recommended by Ford when new, with todays pistons and the roads and speeds we drive on, that is not enough, they will stick in the bores, and scar the block and pistons. I recommend a minimum of .0035" piston to wall clearance. I do more than that on my performance pistons. However, this should not cause tightness when cold, just ruin things when they get hot.
When you fit the bearings, were the caps on correctly? .002" clearance should be plenty, and about right IMO. Also the babbitt can be too tight in the radius, and/or the side clearance.
With .0025 - .003" clearance on the mains you should be able to spin the crank by hand, fairly easily. Also everything should be generously oiled when assembling.
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Old 09-20-2020, 01:47 PM   #7
Jack Shaft
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Default Re: Hard turning with new pistons

On assembly I turn the crankshaft after each process, set a main? Turn,set one piston? Turn.

When I get boring done I give the machinist the pistons to fit. Vendor pistons are inexpensive,they save money somewhere,don't assume anything,measure twice,fit once
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Old 09-20-2020, 02:05 PM   #8
Patrick L.
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Default Re: Hard turning with new pistons

Measure twice, fit once.

Remember the similar Measure twice, cut once. Heck I can cut something 3 times and its still too short.
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Old 09-20-2020, 03:03 PM   #9
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Default Re: Hard turning with new pistons

Yep and mine is 4 inches...folded in half
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Old 09-20-2020, 03:31 PM   #10
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Default Re: Hard turning with new pistons

- Bob, as for the machinist having the pistons, yes he measured them and said it was right on.
- Paul, as for Assembly Lube, yes I used it through out the installation on everything.
- For the crank bearing, when i set them to .002, i couldn't even get the crank to turn, that's when I added more shims till it turned free and measured to find it was between .0025 and .003. Once I had the crank set i didn't go back to the crank after setting the pistons (Is this something I should have done?).
- I initially set the piston caps to .0015 per Les Andrews book, I didn't think to look at the manufactures specs, but found it was binding (couldn't turn), .002 was even a bit too tight and could barley move it a little so I went with a few more shims. I'm not sure the final measurement on each, since I was going in the smallest increments that I could peel the shims off in (I believe they were .0003) if I measured them correctly. I did each cylinder this way pushing each out of the way so I could feel the drag of each piston. Once connect back up and torqued all to 35lbs, It is extremely hard to crank by hand.
- is there anything else I should consider? I want this to run up properly and not cause any issues when I start it.
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Old 09-20-2020, 03:48 PM   #11
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Default Re: Hard turning with new pistons

Was the crank turned prior to assembly, or was the crank already done prior to your build? If done before you, did they check the crank for straight?
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Old 09-20-2020, 03:48 PM   #12
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Default Re: Hard turning with new pistons

pat,

If it is truly that hard to turn, something is too tight.
It sounds like the crank is set correctly, this leaves just a few things to consider.
Check the rods to make sure the radius on crank is not causing the tight condition, rings are gapped properly. Assembly lube is pretty thick, I prefer to use 90wt gear oil when test fitting parts, I find it easier to feel the fit and when final assembly, I use the thicker assembly lube.

Even though these might be different, the basics still apply, slightly modified "B" motor.

John

PS, I have witnessed another condition that can cause issue, check to make sure the rods are not bent. With the width of the bearing, the straightness of the rod can cause binding.
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Old 09-20-2020, 04:11 PM   #13
pcpat
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Default Re: Hard turning with new pistons

No, the crank wasn't turned prior to starting. I guess that will be the next thing I will need to have done just to make sure that is not the issue.
Thank you for the advice
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Old 09-20-2020, 04:40 PM   #14
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Default Re: Hard turning with new pistons

I'd recommend measuring the crank [ each surface in 4 places] to make sure its round.

The rod clearance should be checked before installing the pistons or at least before sticking them in the hole.

What has the manufacturer of the pistons said about clearance. Unless he is insistent about the .002" I'd be inclined to install with .005" clearance.

Again, I wouldn't set crank clearance over .002"

Sometimes its hard to separate the shims, so them I sand them on a piece of glass and measure along 3 spots to make sure i sanded them evenly. I keep the shims equal on either side.
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Old 09-20-2020, 04:58 PM   #15
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Default Re: Hard turning with new pistons

I hope your mains are aligned straight.
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Old 09-20-2020, 05:28 PM   #16
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Default Re: Hard turning with new pistons

I'd be inclined to strip it back down to just the crank and start over again.
If you can't get the crank to rotate freely with .0015 on the mains your trouble most likely is there. Add the cam shaft into the mix next. If all is good there then do one rod at a time minus having a piston mounted to it... repeat until all 4 are good. Then add one piston at a time. You will find the problem.
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Old 09-20-2020, 05:43 PM   #17
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Default Re: Hard turning with new pistons

The center main tends to wear faster than the others. If it had a good bit of wear then de-shimming can set up a pressure on the center main that is trying to pull the crankshaft off center a bit. This is a possibility as to why it turns tighter than it should.

Different pistons sometimes required more clearance as was mentioned previously. Manufacturers of the pistons usually have recommended clearance for their product. Forged pistons require more clearance that plain cast pistons. Split skirt pistons may get away with less but I don't know how many or if any pistons are available with split skirt design in this day and age. They are somewhat weak due to the split.

If the crank is ground, the babbitt will have to be replaced.
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Old 09-20-2020, 06:01 PM   #18
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Default Re: Hard turning with new pistons

All good stuff but it still falls back to the basics,each step on assembly you turn and feel..that way you find the bind...
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Old 09-20-2020, 06:29 PM   #19
Chuck Sea/Tac
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Default Re: Hard turning with new pistons

Set your rod bearings without the pistons in the block.
I would get some Prussian blue (Napa) and check your bearing surface to crank fit. 80% I think is acceptable. First mike your crank. I’ve seen egg shaped journals.
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Old 09-20-2020, 06:32 PM   #20
pcpat
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Default Re: Hard turning with new pistons

- Thank you about the note on if the crank is reground, the babits will need replacing. I was trying to keep away from rebabiting the engine.
- I'm looking to see the clearance from the manufacturer, once I find that I will check to see if I am too tight. Snyders Pistons so it should be easy to find.
- Yes, I will pull everything back to the crank and start over, thanks for the tip on one rod at a time minus the piston, I will try going that route and see if that helps me locate my issue
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