Engine knock

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Model T Ford Forum: Forum 2008: Engine knock
Top of pagePrevious messageNext messageBottom of page Link to this message  By Richard Gould on Tuesday, July 01, 2008 - 12:08 pm:

Just rebuilt and installed my nonstarter engine and now have about 50 miles or so on it. Been driving it easy. I am experiencing what I think is an engine knock and want to hear what the experts say is the cause. It doesn't sound as low and hollow as a rod knock but not as high and clicky as valve chatter. I hear the noise only when the engine is under load in high gear. I don't hear the knock on idle or when speeding up the engine when the car is in neutral. When driving down the road in high, I also notice it disappears when I advance or retart the spark a lot. I have an idea what it might be but want to hear what others say.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jim Weir on Tuesday, July 01, 2008 - 02:58 pm:

Richard; I have found that knocks under load at lower RPM generally fall in two headings. Main bearings and piston slap.

Sincerely

Jim Weir


Top of pagePrevious messageNext messageBottom of page Link to this message  By warren rollins on Tuesday, July 01, 2008 - 06:21 pm:

Adjust the spark advance where it runs best. No rocket science here just takes a little practice.


Top of pagePrevious messageNext messageBottom of page Link to this message  By cecil paoletti on Tuesday, July 01, 2008 - 07:27 pm:

Richard, if you used aluminium pistons in the rebuild and the knock goes away or diminishes when the pistons are hot you have piston slap. Piston slap is related to piston clearance and this changes as the piston is heated. Aluminium pistons expand much more than cast iron pistons so the temperature effect on piston slap is greater for AL pistons. Either advancing the spark "A LOT" or retarding the apark "A LOT" will increase piston temperature. Main bearing knocks have a sort of dull hollow sound and are usually always present if the bearings are too loose, not especially load sensitive. Con rod knocks are sharp metallic raps and usually occur when the engine speed is changed significantly, as when blipping the throttle. At least that has been my experience.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Luke Chennell on Tuesday, July 01, 2008 - 07:56 pm:

One of the best ways to test for main bearing knocks are to short the cylinders adjacent to the suspected bearing - in Model T's, that's usually the center main, though the front often knocks too. Try holding down the points on the coils (or have an assistant do it while you're driving - BE CAREFUL!) one at a time and see if it doesn't go away. If it goes away on no. 2 and/or 3, it's the center main. If it's 1 or 4, well, I think you can figure it out from there. Can also be used to isolate rod knocks - but that's where your "gut feeling" helps a lot. I usually try to get some second opinions from people on noises before I start tearing things apart. Sometimes a second set of ears really helps.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Speedster Inc. on Tuesday, July 01, 2008 - 07:58 pm:

It sounds more like a wrist pin to me.
JS


Top of pagePrevious messageNext messageBottom of page Link to this message  By DaGunny on Tuesday, July 01, 2008 - 08:16 pm:

Is your bottom pulley loose? If it's a split one stick a box wrench on the allen wrench and really tighten the allen screw. Mine loosens occasionally and embarrasses me.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Norman T. Kling on Tuesday, July 01, 2008 - 10:24 pm:

That type of knock is quite common on both Model T's and Model A's It can be brought on by overheating the engine. If it didn't knock when you first installed the engine, but at some time shortly thereafter it overheated then cooled off you might have "collapsed" the pistons. Aluminum pistons are made slightly out of round so they are tighter in the direction perpendicular to the wrist pin. When you get the engine much too hot, they expand and become more round. Then when they cool off, they will be too loose and knock. If it started out that way when you rebuilt it, the pistons were probably fitted too loose. If that's the case, there are 3 things that can be done. 1. replace the pistons with the correct fit. 2. Follow an earlier post and "expand" them to fit. 3. Live with the problem.
One other cause of piston slap which was not mentioned above is a crooked connecting rod. This will cause the piston to move along the wrist pin in one direction when the rod goes up and the other when the rod goes down. The only correction for this is to either straighten the rod or replace it with a straight one. You can check for both of the above conditions by removing the inspection plate. for a collapsed piston or a loose fit, insert a feeler gauge between the skirt of the piston perpendicular to the wrist pin. It should be less than 5 thousandths. If it is more, you have a loose piston. For the bent rod. look upward you should find the rod approximately centered between the piston bosses and when you rotate the crankshaft the rod should not move front to back at the wrist pin. Sometimes excessive end clearance at the crankshaft end will allow you to move the rod back and forth, but it should not move back and forth on its own when you rotate the crankshaft.

Norm


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce on Tuesday, July 01, 2008 - 10:30 pm:

What kind of pistons did you use? Some of the "Hecho in China" pistons are offset ground the wrong direction and cause piston slap which sounds kinda like a rod knock, but is more noticeable when the engine is under partial or no load.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Richard Gould on Wednesday, July 02, 2008 - 09:37 am:

Thanks all for the good advice. The engine was first rebuilt last year, but the mag coil was grounding out and I was leaking much oil through the tailshaft (didn't insert a plug like I should have). So I took the engine out about 2 months ago.
In addition to correcting the above I discovered a little grit was present and was imbedded in the babbitt. So I lapped everything again with Timesaver. The pistons were scored a bit as well so I sanded them about .001". After the sanding, they fit the cylinders with a .005" feeler guage. They are Egge pistons.
I replaced the rings with new ones and reassembled. The rod bearings were rechecked and were all .0015" using Plastigage. I didn't check the mains but Timesaver should leave them with about .0015" clearance.
I lapped the wrist pins and there might be my problem. They came out not a loose fit but not near as tight as what you find with new pistons. The pistons rock easily on the pins but have no detectable play, sort of like an engine with a few thousand miles. I didn't think it was a problem but now I wonder.
All the rods were checked on an alignment jig on the first go round. They were straightened if the piston was tilted more than .003". Just after the rebuild, the car did run hot on three occasions due to a compression leak caused by failure to retighten the head bolts soon enough.
I installed a new aluminum pulley with ridges since my belt was falling off with the water pump. It is a tight fit on the crank.
Trying to eliminate potential chassis knocks and rattles, I unhooked the water pump, placed a shim where the steering post exits the column and tied the gas and spark rods closer together. The fan housing to bolt clearance is good. Today I will shim the crank handle in the pan.
From the advice given, I suspect the culprit is the pistons. Either sanding them or opening up the wrist pins bores is the likely culprit.
What do you think?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Kenneth H. Todd on Wednesday, July 02, 2008 - 10:11 am:

Just drive it, if it's serious it'll get worse.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Walter on Wednesday, July 02, 2008 - 11:16 am:

Cam bearings???


Top of pagePrevious messageNext messageBottom of page Link to this message  By Richard Gould on Wednesday, July 02, 2008 - 11:50 am:

I just drove the car this AM and the noise is getting louder. Sounds more like a chassis noise but since I have checked everything I can think of in that dept., I think I need to pull the engine. Its a shame since its a 1910 with no inspection cover on the pan. I'll make a phone call first.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Speedster Inc. on Wednesday, July 02, 2008 - 11:52 am:

Recheck your wrist pins. Thats what I am betting on.
JS


Top of pagePrevious messageNext messageBottom of page Link to this message  By steamboat on Wednesday, July 02, 2008 - 12:36 pm:

You can check for loose pistons by putting them at top center and trying to shake them from side to side with your hand on the piston head. Try to rotate them in the bore to check for loose rod bearings. You only have to remove the cylinder head to do this.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry VanOoteghem on Wednesday, July 02, 2008 - 02:49 pm:

I'm thinking you shouldn't have lapped the wrist pins. They're supposed to be on the snug side.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Art Wilson on Thursday, July 03, 2008 - 01:09 am:

Is the front wishbone loose where it connects to the engine pan? Is the hood former or hood vibrating or rattling? Is the steering shaft vibrating against the column? There are lots of places where strange noises can occur which sound like engine noise. The fact that it only makes the noise under load and it is not a thumping main bearing noise makes me think it may not be in the engine.

Art


Top of pagePrevious messageNext messageBottom of page Link to this message  By Richard Gould on Monday, July 07, 2008 - 12:44 pm:

Update, tore the engine down and discovered I put the main caps on backwards. The caps had dual marks and I followed the wrong set. Yea, that would cause some knocks! I initially tore the engine down about six months ago to have the crank, rods, flywheel and drums dynamically balanced and it was a while before I got the parts back and reassembled everything. I must have forgotten about the dual marks. I was fortunate I did little driving with the caps on backwards. When I turned them around, I had a uniform .003" clearance and will file and lap them to get them down to about .002". The rods were fine.
I discovered a gummy oil residue on all intake valve stems that caused the valves to stick until the engine was warmed up. The oil appears to be coming from the cylinders and is probably due to that fact the engine hadn't yet broke in. I also opened up the lifter guides finding the clearances were between .0005 and .001" This is an open valve motor and I replaced the bronze lifter guides and reamed them too close the first go around. After reaming and lapping, the final clearances were between .0015 and .002". I'd rather have oil leaks than a sticky lifter problem.
I still am going to replace the pistons as I don't want the wrist pins as loose as they are. Lapping the wrist pins was a mistake. I also noticed a bit of scoring on the pistons and cylinder walls. They were bored to a tight .005", and I'll open them up to .0055" Thats what I did with my 12 and there are is no scoring. I'll post back if I find anything else. A good lesson you can't be too careful. Thanks to all who responded.


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