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Tight Crank During Rebuild


sadlotus
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Engine builder's just rung and said he's bolted the crankcases together and the crank is really tight.

With a firm grip you can just about turn the engine over. It should turn smoothly and easily shouldn't it?

He's thinking the crank could be out of alignment.

He said everything seemed ok when the first bearing went in, then went pear shaped when the second went in and the two cases were tightened up.

I have to add that I'm not very familiar with how the engine goes together but I think I trust him.

The next step is to have it apart and have the alignment checked this weekend(ish).

Any clues as to where something could have gone wrong?

Ta verily

1973 Bultaco 92

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Yep the crank can get "out of alignment" this has to be checked before assembling the engine again.

To check the alignment or any misalignment mount the crank in a lace or on two V prism on a surface plate, lathe is easier!

Then place measure sensor near the flywheel of the crank. Turn the crank slowly a full turn and look if there is any "unroundness" it should not be more then 0.01 mm on the left side in drive direction or 0.030 on the right side in drive direction.

If the crank is above the spec you need an overhaul, then you should too replace the conrod, the lower conrod pin an the lower conrod bearing if you havn't done this, keeps the crank assembly trouble free for the next couple of years.

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The general description you give of assembly doesn't sound quite correct. The crank seal carriers should be temporarily bolted to crankcase then mains should be assembled onto crank. Fully assembled left side of case should be laid on supports and heated to 145oC. While hot, crank should be dropped into it. Mains should slide into position with a gentle tap (not heafty blows and certainly not pressed). Centre gasket then applied and right side of case heated to same temperature then dropped onto left case over crank & mains. Again case should just slip into position.

Getting cases to correct temperature is key and assembly as stated should just slip together. As cases cool mains are gripped by different contraction rates.

Bultaco UK have a very good workshop manual which describes this operation in detail and has crank assembly and check procedures and dimensions.

Left case best heated with torch and right case best heated in an oven or I use a gas BBQ with a lid to avoid wife complaints.

Hope this helps

Tim

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Hi all, the rod, crank and bearing are all new.

We have the Bultaco UK engine manual and the Haynes manual.

When I saw the engine only one crank bearing retainer was in place and only nipped tight with two nuts/bolts.

Tim, your assembly description was how he described it to me (except the wifey bit) and things only tightened up after the second casing was nipped up.

He's having it apart this weekend and put on a lathe to check alignment - see what comes of that.

Paul

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It is surprising how easy it it to get the crank our of round if the cases are just pulled together using the case nuts/bolts. I have had two friends with bent cranks, the bultaco one only went for about an hour or so before it started to run lean[magneto seal leaking] the other was a rotax engine which used to lose power as it heated up after about an hours running- no other symptoms other than the power dropping off and the motor running hot. The way we have been putting the cases together is to pull the crank into the case using the crank as the puller.

You need to have several lengths of a suitable diameter pipe, this is put over the crank as it is protrudes out of the case, a washer is then put on the pipe - the crank nut installed and then tightened up gradually, as the nut is tightened it pulls against the pipe and pulls the crank into the bearing squarely without any chance of 'bending' the crank. Once one side is in place you repeat the procedure on the other side. It is also important to use the correct 'clearance' main bearings as this can also cause the crank to be tight.

Cheers Greg

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Hi i had a similar problem on a model 92 i rebuilt a few years ago, check your clearances on each side of the crank flywheels to the engine case with a feeler gauge the crank may be not centered in the engine case it only needs to be slightly out, if it is out use the method in Post #7 to pull it back to center,

Cheers nick

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