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Pistons in - how much drag is acceptable?

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garmo
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PostPosted: Sun Aug 25, 2013 12:18 pm    Post subject: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

Hi guys,

Just put new rings on my pistons, honed the cylinders to remove the glaze and had the crank polished and all new bearings. Now that I have the crank in, and the rods and pistons I'm wondering how much effort it should take to turn the crank. Seems the oil rings are fairly tight. All end gaps are within spec. The rings I pulled off of the tractor had huge gaps, some as much as close to 1/8" .

It has been about 25 years since I rebuilt a motor of any kind and would appreciate some feedback here.

There seems to be one spot in the rotation that is causing a bit of resistance more than in other areas.
This is a 1949 8N, with the thicker cylinders.
 
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David G
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PostPosted: Sun Aug 25, 2013 1:16 pm    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

There should be resistance, but none excessive. Make sure you cleaned the ring grooves good.
 
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Texas!
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PostPosted: Sun Aug 25, 2013 3:17 pm    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

Yep, did you clean the carbon out of the ring grooves? A buddy of mine put new rings in his 289 Mustang back in about 1970 and when he got all of the pistons in he could barely turn the crank. Turned out that he had not cleaned the grooves.
 
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garmo
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PostPosted: Sun Aug 25, 2013 5:52 pm    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

I did clean the grooves well. I pulled things apart and used plastigage. The mains and rod checked out to be in spec. It looks like the #1 piston is ever so slightly cocked toward the front of the motor. I did not check the exact measurements, but there is just a bit more room to the rear side of the piston than the front side.

I can turn the crank with not a lot of problem using a 24" piece of square tubing. I just notice that at certain times I hear more of a scraping than at other times. I am not gouging the cylinder and everything else looks ok.

Maybe I am being overly cautious.
 
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HCooke
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PostPosted: Sun Aug 25, 2013 6:56 pm    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

On two recent rebuilds I was able to turn the engine while on a stand with a socket on the pulley nut and a 20" breaker bar.
 
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36 Coupe
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PostPosted: Mon Aug 26, 2013 3:51 am    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

The cylinders must have some taper so you have to be careful measuring ring end gap.
 
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TheOldHokie
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PostPosted: Mon Aug 26, 2013 4:27 am    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

36 coupe wrote:
(quoted from post at 07:51:27 08/26/13) The cylinders must have some taper so you have to be careful measuring ring end gap.
.

Where did you get that idea? A freshly bored and honed cylinder should have as close to zero taper as the engine builder can get. High dollar engine boring and honing machinery goes to great lengths to ensure that. A top quality CNC engine shop shoots for something less than .0005 maximum taper - a low budget shop with a manual boring bar and hone is working closer to .001. The Ford 120 specification for maximum allowable cylinder taper when fitting new parts to a worn cylinder is .004, max out off round is .003.

TOH
 
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JMOR
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PostPosted: Mon Aug 26, 2013 5:10 am    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

Rope seals may be causing more 'drag' than old pistons in old worn sleeves, assuming rings/grooves are all good.
 
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garmo
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PostPosted: Mon Aug 26, 2013 5:50 am    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

Thanks all. I pulled all the caps and checked clearance with plastigage. They all came within spec. This is my first rebuild with rope seals. I'm gonna put it back together and hope it is just that. It's not like I have to jump on the breaker bar, it is just a bit more resistant in certain places in the rotation.
 
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TheOldHokie
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PostPosted: Mon Aug 26, 2013 6:01 am    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

Garmo wrote:
(quoted from post at 09:50:11 08/26/13) Thanks all. I pulled all the caps and checked clearance with plastigage. They all came within spec. This is my first rebuild with rope seals. I'm gonna put it back together and hope it is just that. It's not like I have to jump on the breaker bar, it is just a bit more resistant in certain places in the rotation.


Every engine I have ever built taook more effort when the pistons went over the top and bottom. Sounds normal to me. New rope seals are a lot more drag than modern rubber ones and can make it very hard to turn at any position....

TOH
 
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garmo
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PostPosted: Mon Aug 26, 2013 10:34 am    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

Great. I'll put it back together. thanks for the input folks!
 
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Ultradog MN
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PostPosted: Mon Aug 26, 2013 1:20 pm    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

TOH,
Coupe doesn't need my defense but one could take his reply to mean that since Garmo did not replace the sleeves he "should" have some taper.
 
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TheOldHokie
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PostPosted: Mon Aug 26, 2013 1:29 pm    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

Ultradog MN wrote:
(quoted from post at 17:20:38 08/26/13) TOH,
Coupe doesn't need my defense but one could take his reply to mean that since Garmo did not replace the sleeves he "should" have some taper.


I considered that before I replied which is why I added teh bit about new rings in a worn cylinder. But it's not what he said so just consider my reply a more detailed clarification on the taper issue.

TOH
 


Last edited by TheOldHokie on Mon Aug 26, 2013 1:30 pm; edited 1 time in total
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36 Coupe
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PostPosted: Mon Aug 26, 2013 1:51 pm    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

He did not bore the cylinder EXPERT.
 
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Charner MS
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PostPosted: Mon Aug 26, 2013 2:15 pm    Post subject: Re: Pistons in - how much drag is acceptable? Reply to specific post Reply with quote

Most all rebuilt engines are tight and some more than others. What I do is before I put the head on, I put a generous helping of lite old, 10w, and crank the engine about 30 second. Clean the oil and do it several times. That will seat the rings somewhat. Sometimes with a tight engine it will run hot until broke in. I remove the lower rad. hose and stick a water hose(sealing with a rag) in the lower rad. and fill the water chamber. Leave water on as it runs out the lower water outlet in the engine. Crank engine a run for and hour or so. After that it should be smooth.
 
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