Engine knock

[Trouty]

Hi Guys. Mike here from England.

I'm trying to help a friend who has a newly built engine fitted with a Thomas head. It has been re-bored, new pistons etc. It develops a knock under load as the revs go up and you have the timing advanced the normal amount. The timing has been checked and is right. Cam timing has been checked and is right. We discovered that the knocking can be stopped when shorting out #2 cylinder, so the oil pan and head have been off, #2 piston taken out, and nothing found to suggest a problem.

When he first built the engine he mistakenly ran it for a while with no coolant in the system (only for a mile or so) and it obviously did get hot. I'm wondering if this is a centre main problem? I wasn't there when the engine was built, so did not have a chance to check clearances. I just want to gather opinions before going to the trouble of stripping the engine.

Any input most appreciated.

MIKE

[40 Deluxe]

Since the pan is off, take a few minutes to drop the center main cap and have a look. Don't guess or assume!

[100IH]

I saw what was done with the pistons and cylinders but no mention of the bottom end: rod bearings and main bearings. Also, did the shop understand the ongoing clearance problems with issues as stated by some of the piston manufacturers. The clearances are critical because of the rust buildup at #3 & #4. Coolant circulation is less efficient at the rear and rust buildup makes it more likely to not tolerate running without coolant - collapsed skirt on those pistons. How was the block cleaned if the bearings were not redone?

[Trouty]

The bearings were all redone. know what you're saying about the piston clearances, but there is no sign of any damage at all to the piston. If I had built the engine I'd know much more, and I may end up doing it anyway.

Having read how the centre main can cause problems, and seeing as the knock stops when shorting out #2, I will check that bearing first. It's a deep sounding knock, not at all like piston slap (pinging) and the big end is OK.

All input greatly aprreciated.

M

[Marshall V. Daut]

Short out #2 AND #3 at the same time. If the knock goes away or is greatly reduced, the center main is most likely the problem. Shorting out one cylinder at a time will only tell you whether the problem is in that cylinder if the knocking stops or lessens (rod bearing, wrist pin, piston slap, broken piston).
Marshall

[100IH]

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With only 1 mile and some warmup time, things should have been ok at the lower bearings as long as there was oil in the dipper tray at startup -in theory. There would have been oil where it's needed the whole time. Now the pistons. measure the skirts of the pistons for signs of collapse, there might not have been enough time to score the skirts and/or cylinders. There is some technique in the measuring process, might have the machinist do it. If they are not numbered do it first. I'm not saying don't do a full inspection, be thorough now that it is apart. Good luck.

[Tom Wesenberg]

If the pistons didn't lock up after driving a mile with no coolant, I wonder if they have too much clearance?

[Kohnke Rebabbitting]

If the rod bearing are Ok, along with the center main.

I would look close at the top of the Piston, and head to see if there is a print from the piston, in the soot in the head chamber.

I don't know what size your rebore job is, but you have to have a piston that is cut at a 45 degree, on the top, to miss the head gasket . Depending on who made the Pistons, .060 to .125 can hit, and you have to look close.

The only other thing I can think of is a Rod that is out of Alignment, for twist, Bend, and Off Set.

I bet less then 1% of Rods, are ever checked for Alignment, and they all need it.

All three are very important, and will create a knock that get worse with R.P.M's.

Herm.

[James Rogers]

With the engine being overly hot, I would suspect a galled piston. This could be any of the 4 but quite possibly #4. The offending piston could also have a damaged wrist pin/bushing that could cause this.

[pooch]

What idiot would drive a mile with no coolant?

[George Miller]

Originally Posted by James Rogers

With the engine being overly hot, I would suspect a galled piston. This could be any of the 4 but quite possibly #4. The offending piston could also have a damaged wrist pin/bushing that could cause this.

Plus 1

[Benson]

I had an original Thomas head that I got in early 60s ... 8.25 to one.

Bore was either 0.080 or 0.100 ( I had two engines not sure which one had problem).

It had no flycut for piston clearance like original heads have ... had to flycut head to clear pistons when engine was warm.

Also one engine I put it on had oversized valves and some clay showed that they would have hit the head (1.70 inch valves maybe). Had to cut head to clear oversized valves.

[Trouty]

Originally Posted by Kohnke Rebabbitting

If the rod bearing are Ok, along with the center main.

I would look close at the top of the Piston, and head to see if there is a print from the piston, in the soot in the head chamber.

I don't know what size your rebore job is, but you have to have a piston that is cut at a 45 degree, on the top, to miss the head gasket . Depending on who made the Pistons, .060 to .125 can hit, and you have to look close.

The only other thing I can think of is a Rod that is out of Alignment, for twist, Bend, and Off Set.

I bet less then 1% of Rods, are ever checked for Alignment, and they all need it.

All three are very important, and will create a knock that get worse with R.P.M's.

Herm.

I think the bores are + 30 so shouldn't be a problem. I'll check all this when we get it apart.

Thanks for the input.