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3cycle
New member
Username: 3cycle
Post Number: 1
Registered: 07-2011
| | Posted on Friday, July 01, 2011 - 10:17 pm: | 
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My St Lawrence engine (ser. no. 848) has developed a piston knock. The rings are new,the connecting rod is tight (good) on both ends,there is no rubbing or scuffing on the piston,the cylinder mics at right angles to no discernable free motion.(on a 95 year old cylinder this is mil. spec. With the head off and the piston set at TDC, I can grasp the deflection fin and move the piston back and forth in line with the connecting rod bearings.It will not move in line with the crank....OK stay with me now...The amount of movement is visible and is the distance from the back of the compressed rings to the bottom of the ring grooves(.187). The all -around clearance between the piston and the cylinder is.0125(in many places)The compression is good-(one pull to start)so the new rings are doing their part. As well as I can figure, the free motion is occurring inside the compressed rings.The sound is the same as the engine makes,albeit not as loud. This sound was not apparent while propeling a 16 ft. Whitehall launch. It turns a 12-13 3blade prop at 600rpm with throttle plate barely cracked. If my reasoning is correct or if this is not uncommon,would a flat Oring behind the iron ring helpI? (I can't beleive I wrote that)I need help! |
bcm
Member
Username: bcm
Post Number: 22
Registered: 11-2009
| | Posted on Saturday, July 02, 2011 - 03:06 pm: |
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Richard,
Yours is a very old St Lawrence, circa 1912 ?? so probably has been repaired by others, many times.
My suggestion is to first define the problem before deciding on a fix.
My thoughts meander in this direction………..
Clunk is usually related to excessive clearance in:
1 Piston/cylinder … these engine cylinders tend to wear oval towards the bottom of stroke.
2 Wrist pin wear, St Lawrence engines use a bronze bearing and non hardened wrist pin, not a long life arrangement.
3 Connecting rod to crankshaft wear .. Later St Lawrence used shim packs to adjust con rod clearance, not sure if used in old engines.
4 Main bearing wear
It’s interesting that new rings tend to provide sufficient seal such that the engine will run, and clunk, probably at less than full power. These engines run even when excessively worn.
I suggest the following to attempt to identify the problem or problems as there may be several.
1 Ideally with head removed and piston at TDC, rock the flywheel back and forth just a small amount and observe if the piston moves sideways.
Repeat at about ½” piston movement until bottom of stroke.
This will determine if the cylinder is worn and where the wear is located.
2 Remove the side inspection plates and observe any excessive clearance between crankshaft and con rod when the flywheel is rocked back and forth.
3 Grip the flywheel and pull up and push down then sideways and observe any main bearing play. Do the same at the back end by gripping the coupling.
4 With piston at TDC try to push and pull up/down to judge if there is con rod clearance, also at the same time observing con rod to crankshaft clearance if any. Do this at several piston positions in order to decide if con rod and/or con bearing is worn.
If as result of this, you have identified the problem you then with confidence you can undertake a fix.
Barry |
jb_castagnos
Senior Member
Username: jb_castagnos
Post Number: 602
Registered: 07-2002
| | Posted on Saturday, July 02, 2011 - 05:28 pm: |
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Check close for a loose flywheel, this is the most common knock to start suddenly. Also hold both ends of the crank and be sure it isn't broken. |
3cycle
New member
Username: 3cycle
Post Number: 2
Registered: 07-2011
| | Posted on Wednesday, July 06, 2011 - 12:32 pm: |
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Thanks for the info. After more measuring, it has become woefully apparent that the piston is a very loose fit in the cylinder. Oddly, everything else on the engine is in very good shape:mains, connecting rod bearings, wrist pin/piston,water pump,and seals.
This engine has compression enough to start with one pull and will push a 16ft. boat(a beautiful Whitehall launch)at hull speed around 400 RPM.
I would like to save it. If anyone has been through this or knows who could help me please pass the word. Thanks |
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