MF 40 New pistons hitting valves!

[danielapowell]

Rebuilding a Perkins AD3.152 in my MF 40. New head, pistons, rings, rod bearings. #3 piston is hitting the exhaust valve and locks the engine. It's on a stand so no damage. I'm thinking take the pistons out and have the tops ground down a tad. Any thoughts on that?? Thanks in advance. I'll probably just go ahead and do it but thought I'd throw it in here, just in case. I have tried what little can be done and it's no use. I have to back off the lash adjuster all the way in order for the piston to travel past TDC. I'll check into where I got the pistons but the 3
#'s were correct for my model and upon inspection before they were installed, they seemed an identical match to my old pistons, so WTF!!! Gotta be the piston, huh?

 

[ptfarmer]

If they all 3 have the same part number I'd say 1 was machined wrong (just because they are new doesn't mean they are good). Get a caliper, and get to measuring

 

[danielapowell]

If they all 3 have the same part number I'd say 1 was machined wrong (just because they are new doesn't mean they are good). Get a caliper, and get to measuring

Thank you. What I found is that all three pistons are 0.016 in. HIGHER than the block! I'm taking them to a machine shop and have the tops milled down. They all sit higher than the new sleeves I installed. Anyways, measure I did and thank you ptfarmer for your advice.

 

[danielapowell]

I have pictures of the gaps.

 

[damn1981]

Someone deck the block

 

[Dieseltech]

Slide the wrist pin into both old and new pistons, THEN see what the difference is. Perkins used to have some unfinished pistons available that the tops had to be machined, I wonder if you got some like that.

 

[danielapowell]

Someone deck the block

I don't see any signs that the block has been 'decked'. The sleeves have a land at the ytop that fits into a groove, so any decking would require that to be re-milled. Not likely. The head is new. Thank you for your input.

 

[danielapowell]

Slide the wrist pin into both old and new pistons, THEN see what the difference is. Perkins used to have some unfinished pistons available that the tops had to be machined, I wonder if you got some like that.

They are the same, as most AD3.152's are. The pistons came together in a set with the rebuild kit from England. I think the most likely scenario is that I ordered the wrong set due to the fact that the preceding letters in my serial number are missing so it's nearly impossible to know what I have and what fits it, other than trial and error, which so far has resulted in the wrong pistons. I took them to a machine shop to have the tops milled 0.018in to put them where they should be. Problem solved!.... almost. The machine shop called and said the first piston done was well over done at .025 instead of the 0.018 I requested. So, I ordered a new piston (their buy!) knowing that it is probably not the right one for my engine but that it can be machined. TAH DAH! It's science!

 

[miangus]

Interesting, in an automotive application I would have immediately rechecked camshaft timing. I milled the dome off a MF 65 gas piston for a guy he was just replacing one piston and could only get the high compression version. The other pistons were all flat top, I couldn't convince him having just one with a dome would be no issue. I used to blueprint Chevrolet 327s the difference in the length of factory rods is surprising, piston height is also an area where there was a difference. Even crank throws had a measurable difference.

 

[danielapowell]

Slide the wrist pin into both old and new pistons, THEN see what the difference is. Perkins used to have some unfinished pistons available that the tops had to be machined, I wonder if you got some like that.

That's exactly what I got! I am now at 0.016" machined off and they still hit. Going to go another 0.020" and see what happens. Thank you!

 

[danielapowell]

Interesting, in an automotive application I would have immediately rechecked camshaft timing. I milled the dome off a MF 65 gas piston for a guy he was just replacing one piston and could only get the high compression version. The other pistons were all flat top, I couldn't convince him having just one with a dome would be no issue. I used to blueprint Chevrolet 327s the difference in the length of factory rods is surprising, piston height is also an area where there was a difference. Even crank throws had a measurable difference.

Ya know .... I thought about the timing and how it would affect piston position vs valve position. I removed the timing cover and checked and I'm right on the money, which was disappointing because I'd rather adjust timing than to have to have the pistons machined AGAIN! But that's where I'm headed. Thank you!

 

[danielapowell]

Okay, I'm big enough to admit a mistake and I made one. Turns out that I had the cam gear in the wrong position on the camshaft. What a dope. 2 sets of pistons and rings and 2 sets of liners all because I screwed up. There is a mark on the camshaft end, a "D" and there is a corresponding "D" on the gear that are supposed to align. I didn't do that initially and didn't know. So, hopefully the problem is solved. I'll go back to it in the morning and put it all together AGAIN and check the gaps. Appreciate the help.

 

[Duner Wi]

Good to know ,thank you.

 

[danielapowell]

An update. After correcting the gear position on the camshaft and reassembling the timing gears and cover I find that the piston/valve positioning to be correct, according to my manual. Waiting for a new head gasket to arrive so I can put it together. Thinking, though, wouldn't hurt and wouldn't take long to use the old head gasket, put the head and valve train on and at least know that it works. I'll probably do that just out of boredom. At my age though, it seems that cylinder head gets heavier every trip off and on. I apologize that I don't have a picture to post of the cam/gear alignment marks. My bad.

 

[Duner Wi]

I have used modeling clay to check clearance , piston to valve. If you do need clearance for the valve you can use an old valve and make a cutter for a pocket.