effect of 3 flat top pistons in a domed top piston motor? (1 Viewer)

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Hi all, I put off taking the winter cover off of my 74 '40 for as long as I could because I knew once the cover came off I'd be in full obsession mode. I was right...

May 1974 FJ40 with the original F155 engine. When I pulled the head last year to have the valves done, I found the engine had 3 flat top pistons and 3 domed top pistons. From what I understand, my particular F155 should have domed top pistons.

That being said, how much of an effect will having 3 flat top pistons in the mix make on engine performance? I've never driven another Landcruiser so I have nothing to compare it to, but it seems low on power/torque even for what it is.

Thank you!

 

thebigredrocker

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Couple more from FJ40Jim

I have seen plenty of 2F engines with flattops and open chamber. The have 6:1CR and no power. Toyota didn't build em that way. It's the result of cracking one of the relatively crack- prone close chamber heads and replacing with an indestructible open head from the JY.

At least, that's what I've seen.

Yes, you just call me to order. There is one set in stock. Commodity prices are up, so the set of pistons, pins, rings is now $989 delivered in USA.
These are domed pistons, .060" oversize, for use with 75-80 2F cylinder head.
Flattop pistons are still available from OE or equivalent sources, AFAIK.
 

brian

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that engine should have pulled and replaced with a another...obviously that mill saw some grief and then was worked on by a clown.
anybody in there right mind would have known mixing different types of pistons was not a permanent repair....or a good idea.
 
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The F.5 pistons were still available last year from ITM. Heres rockauto. 6 in stock .030. $33.99 each

Screenshot_20180610-111649_Samsung Internet.jpg
 
Joined
Mar 20, 2016
Messages
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Lakeport, CA
thanks for all of the feedback. 6:1 compression ratio definitely explains the lack of power. As it turns out, buying a '40 with a rebuilt motor isn't always a positive thing if you don't know how it was rebuilt.

Now I'll have to decide what to do with it moving forward. I'd like to keep the original F155 but not sure if rebuilding an F155 makes much sense these days when there are 2Fs available to rebuild.

What a bummer because it runs great. I can't imagine I am doing any harm by driving it this way, so that's probably what I'll continue to do for the foreseeable future. You can't miss power you've never had!

Every time I think I've got things dialed in and can focus on things that don't have to do with reliability (like the Mastercraft seats I've been wanting). Something like this pops up that promises to soak up all of my resources. I love it anyway.
 

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