13 replies to this topic

[markosparko1994]

i have just built a bottom end of a 998 engine, new bearings etc. the crank, and pistons are fitted and I have lubricated the bearings.

 

It just feels a little tight when turning over by hand. When it was just the crank it moved very freely and the end float was good. I can turn it over by hand but it is a bit though to do so.

 

I am especially concerned about this as the big end con rod caps got mixed up and I have matched them all up by measuring and feel but there is a chance I have mis matched one or two?

 

so.... how hard should it be to turn over a freshly built and bottom end?

 

thank you in advance for answers (and I know it was silly to mix up the con rod caps)

 

 

[Artful Dodger]

Should be faairly easy to turn over, but ther is a lot of drag when the pistons are in. If you have to give it some or use a tool (of any description) to turn it over it's wrong

If you are worried that you have mismatched the rods, take them out, and bolt them onto the crank individually outside of the bore, just so you can rotate it on the crank and get a good feel if it is tight or not. This action should be super smooth. Any tightness and it's wrong.

[Tamworthbay]

Are the piston rings fitted? If they are there will be very noticeable drag which is normal.

[minimadles]

it should still turn over nice and smooth just slacken off the big end bolts slightly to see if it frees up then retorque each one in turn to find the tight one or remove the pistons from the block and recheck that the caps are on the right rod . its fairly easy to match them remove the shells take one rod and try each cap in turn the matching cap will line up perfectly on the parting line in the tunnel bore as they are machined as a unit hope this helps

[Cooperman]

When building an engine it is necessary to carry out a 'dummy build'. This involves assembling the block, crank & pistons with the piston rings removed. Torque the caps to about 80% of the final torque and check that it all rotates smoothly and easily.

That way the correct fit of all parts is confirmed and the amount to be skimmed from the deck of the block to ensure that the pistons come right to the top of the bores. It also enables the compression ratio to be measured and calculations made to get it right.

The rings need to be removed in order to measure/check the piston ring gaps (0.003" to 0.005" for each inch of bore diameter).

If this was not done, you should strip it and re-do it correctly.

Sorry about the cap mix-up. You must get the caps on the correct rods.

[MIGLIACARS]

put the bottom pully on and it should turn by hand!!!!!!!!!!!!!

 

if not there is an issue.

 

i take it the head isnt on

 

 

they are not super easy to turn when all new bearings and pistons are in.

[tommy13]

Your problem is undoubtedly due to the mixing up of the big end caps. One piece of advice I may offer is to remove the caps and carefully clean the cap and rod mating surfaces and look for any irregularities or staining on the cap surfaces which may have been mirrored on the rod to which it was originally fitted. If you are lucky enough to identify two then its simply a matter of trying the other two both ways to see which works.

Failing this its a tedious process of trying each cap on every rod an check there are no steps where the cap joins the rod, then fit the shells and bolt to a crank journal (with the rod and piston out) and check for smoothness in turning on the journal. no resistance should be felt. Eventually you should be able to systematically determine which caps do or don't suit each rod until you can sort them into their correct positions.

It's not a mistake you will make twice.

[markosparko1994]

I did the dummy build and I thought all was good but now i'm unsure. I also think it may be the caps, it all seems fine but when i torque up one of the caps (on the 2nd con rod) it becomes much harder to turn but i can still turn it by hand.

 

i'll have another look at all the caps to see if I can match them more accurately. 

 

thanks very much

[KernowCooper]

Bit late now but thats why the first job on a strip is to emgrave the caps failing that centre pop the rods/caps lightly, hope you can match them up

[tiger99]

It it is the caps, which seems quite certain, the offending bearing shells will have quite distinct pressure marks adjacent to the joint between cap and rod. One end of the shell in the cap and the opposite end of the shell in the rod will be marked,

 

Please do check carefully that they are only marks and not indentations, otherwise you will need a new pair of shells, and I don't know if you can buy anything less than a full set. Torquing up the bolts puts a wedging action on the soft bearing metal, which can be deformed. Guess how I know! (It wasn't a Mini, but it easily could have been.) You only do that once. I was very young at the time.

 

You should check all the shells for pressure marking at the ends, as more than one cap may be wrong, although one may be more mismatched than the others, and therefore tighter.

 

The pistons do add a lot of friction. I have never tried a dummy build with no rings, although I see that it is a good idea. I generally just turn the well lubricated crankshaft over by hand as I fit each item, whether main bearing cap or rod, and then as I nip up the bolts, and finally again as I torque it fully. You do feel the piston friction that way, but it is fairly obvious if one is stiffer than another. However I do see that assembling without rings is definitely better and will do it that way in future.

[tiger99]

Kernow, please be VERY careful where you pop or engrave the rod and cap. It adds stress raisers, which WILL cause fatigue failure, even on a standard engine, not tuned, unless it is done only in the area which is held in compression by the bolt. Remember that there is a stress cycle on every rev or the engine, so at 6000 rpm there are 100 cycles every second, 360000 an hour, and maybe 500000000 in the life of a normal engine. It is safer to use paint or typist's correction fluid.

[KernowCooper]

I dont pop the rods i lightly engrave never been a problem on bike engines revving to 14000rpm+ and even the factory tuning shops do it, they dont use paint as it can be chipped off or if left on can chip and block oilways, or get behind the relief valve, were not talking road drills lol its faint like ghosting. The Honda Motor Corporation engrave there conrods with the weight group on tuned engines.

 

But you had images of me with a lump hammer and a 3mm centre pop and engraving 25thou deep but yes something worth pointing out to the less experienced who have never done it

[1984mini25]

I had a similar problem were the crank, rods and pistons would turn freely by hand until left and then seize up solid. It turned out that after stripping it all down that the id stamping on the back of the bearing shells on one of the rods had some burs that prevented the bearing from fully seating and just needed filing off.

[markosparko1994]

i'll look for the stress marks on the shells before removing everything to see if i can sort it out the quicker way but don't worry, if i'm not sure then i'll take it apart again, it won't take too long. thanks for the advice, i'll keep you posted