This might be a stupid question but I'm new to this. What is the best way to check the compression on an engine that is out of the vehicle? I have a compression tester but it says to run the vehicle for ten minutes then test it. I obviously cant do that since its half apart minus the heads and bottom end. I tried turning it over by hand but that doesnt get much of a reading at all. Do I have to hook a starter to it and get it going faster? Also, once I get to actually testing the compression, what kind of readings are ok? I have heard that the lowest reading should be within 75 percent of the highest one. Is this true or are there other rules or actual pressures to look for? Thanks in advance.

Ryan

 

You can try doing it with a starter, but with the engine cold, you won't get a true operating temp reading. Compression loss should be no more than 25% of stated value for the particular engine. You didn't mention the size of the engine, so I can't tell you what it's supposed to be. The average should be somewhere between 110 to 130 give or take 5psi. Anything below needs attention prior to operating.

 

You have the engine out. Thats half the battle. Pop the heads and check the ridge at the top of the cylinder. Since the engine is out, new rings and bearings are cheap and easy to install. Head work is more expensive but a machinist can check the valve seats.

 

I have never done anything to the lower end of an engine. I dont really want to screw anything up. Is there anything I can do to make sure the lower end will with stand higher than normal stress? I plan on putting a carb on this motor and pushing about 350 horse on the stock bottom end. I could afford a few extra hundred bucks for maybe connecting rods or different pistons. I just wonder how hard it is to change these parts. How much skill are we talking here. I have pulled trannies and changed axles before on my truck but never anything that requires real close tolerances like pistons. Thanks for the info.

Ryan

 

BTW the engine is an 88 351w soon to be carbed. Thanks again.

Ryan

 

Do you know what the mileage is on the engine? Where's the FI system?(might take that off your hands if you have it). Depending on the mileage, you might get away with nothing other than putting new rings and bearings in seeing how you have it this far down. I would suggest getting a Haynes or Chilton Manual that covers that particular year, they have all the dimensions and torques to check and reassemble it and gives you a guideline on general engine overhaul.

 

Do you know how many miles are on the engine? It's just such a pain to remove an engine I thought it might be a good time to replace the bearings. You can do this without messing with the pistons or heads. If it has more than 100k miles on it I would at least replace the rod bearings. You would have to pull the pan and pop each rod cap one by one and replace the bearings. Then re-torque the rod cap. Use a little assembly grease on each rod cap bearing and thats it. Main bearings can be replaced also by lifting the crank. Maybe a little more technical. Just my opinion.

 

Hmmm. The truck has 135000 on it but the engine is not original according to the owner. He said it had 32000 when I got it which would make it have about 35000 by now. I dont know if I would take his word for it or not. I just wondered if the stock rods would be strong enough to hold that horsepower. According to others in this forum, people have gotten over 400 out of the stock bottom end on this motor. I am trying to stay cheap as possible. So far I have gotten some gt40p heads off an explorer for 200 that I am in the process of porting. Am I going for the Xtreme Energy Comp Cam 262H kit with roller rockers which will be the most expensive thing at around 500 I think . Along with a 650cfm carb and a performer rpm intake. According to my desktop dyno, it will be putting out around 350-370 peak hp around 4500-5000 rpms. I just want to make sure that this bottom end wont fail. How hard is it to change rings? Do I just disconnect the rod from the crank and pull it out? I do have the chiltons and haynes manual. So I feel like if I spend the extra hundred bucks or so for rings and bearing I will be ok as long as the pistons are ok. Another thing I was wondering, does anyone know the piston down bore is for this engine? I havent been able to figure this out. Could I call Ford Motorsports? Thanks for all the info guys. Im out.

Ryan

 

Oh someone already yoinked the efi setup LoosMaster.

 

Sorry to hear about the FI.

Another thing I was wondering, does anyone know the piston down bore is for this engine?I've never heard of this. Are wanting to know maximum cylinder bore or the stroke?
As far as the bottom end holding up, what are your plans for the truck?

 

One test you can do, with the engine out, is a cylinder leak down test. I believe that this is a better test for the condition of the valves, cylinder wall, and rings than a compression test!!

 

The rule of Thumb..

Is 10% on compression..So if you have 180 on the highest cylinder the lowest should be no less than 162 psi..ECT.

Hope this helps ya-Russ

 

Piston down bore or deck clearance i think its also called is the distance between the top of the cylinder and the TDC of the piston. It would be how much it lacks before it actually gets to the top of the cylinder. As far as plans for the truck it will be a daily driver and some weekend muddin/racing and occasional towing. Also, what is a leak down test? Im thinking you compress the cylinder and see if it holds the pressure... As always, thanks for the info.

Ryan

 

Without block cast #, I'll give you the most prominent #'s.

Deck Height - 9.503" This is measured from center of crank to top of block-head mating surface.
Deck Clearance - .015" Assuming it's never been decked. .010 max removal, measured from top of piston to top of block.

This stock configuration.

 

Thanks for all the info LoosMaster. You seem to be answering all my questions in more than one forum too. Just curious, is there a site somewhere you can look up technical data like that? I assume that the stock pistons on this motor are flat topped. Is that a bad assumption? Thanks.

Ryan