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Bore taper and out of round tolerance

Engine Building Fundamentals

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I've just got my 3S-GTE block back from the machinist this morning and proceeded to check on the work done. A few bores are slightly out of round and there seems to be quite a lot of taper on the bore.

I have only measured the top and bottom of the bore and did not measure the center (around 20 to 30mm from the top and bottom) and is reading about 0.005 to 0.01mm out of round and 0.01 to 0.025mm taper.

I am not sure how much the typical tolerance would be but the 0.025mm taper definitely seemed excessive given I only specificed a clearance of 0.08mm.

Here are the measurements I've got from each cylinder, each pair of measurement are taken at 90 degrees from each other:

#1 top: 0.09, 0.09 bottom: 0.08, 0.08

#2 top: 0.095, 0.095 bottom: 0.07, 0.08

#3 top: 0.09, 0.095 bottom: 0.08, 0.075

#4 top: 0.08, 0.09 bottom: 0.75, 0.08

(measurements are in mm)

Numbers are not perfect but considering I am located in Hong Kong where there are no good machinist available, this is already the best I could get. People here send their block overseas if perfect work needs to be done but I cannot afford that. I am willing to accept less than perfect results but my measurements on number two cylinder worries me and I am inclined to get it re-honed. However, it seems like the top of the cylinder is already 0.015mm over my specified clearance and re-honing will probably increase the clearance even more.

Should I take the risk of increasing clearance even more and it get rehoned or is the taper still acceptable and I should just call it a day?

The recommended minimum clearance for the wiseco piston that I am using is actually 0.0025" (0.0635mm) and 0.003" (0.0762mm) for high boost applications. I already specified an even higher clearance of 0.08mm due to the fact that I do run my car for sessions at the track. Now going up to 0.095mm (and possibly end up with 0.1mm clearance after another hone) may be too much for my liking.

Some small amount of taper or out of round is hard to eliminate unless you're using very high end machining equipment and a knowledgable operator. Obviously the closer we can get to zero the better and the further away we get the more effect we will have on the ability for the rings to seal correctly. A guide to what is acceptable can be taken from the manufacturer's workshop manual and in this case for the 3SGTE Toyota specify that the bores should be within 0.01 mm in terms of out of round and taper. You've got two cylinders that exceed this limit but your options to improve this situation sound difficult and expensive.

Thank you Andre. Yes I tried looking all over the different shop manuals for the recommended taper and out of round tolerances but did not find any specific specification.

However, it did give a range for the standard bore diameter. I guess this range already imply the tolerance of out of round a taper? Since any taper or out of round larger than the range would not be witin the specifications. And it's exactly 0.01mm as you have mentioned (see attached).

I am not sure exactly how the honing process works (other than from watching youtube videos) but I plan to just bring the block back to the shop to run their honing bit a few passes along the lower portion of the cylinder block only which would enlarge it slightly to become inline with the top part. Or will that create other problems of it own such as straightness of the bore bowing in the middle? Given you mentioned it would be difficult and expensive.

Attached Files

The honing process can be used to help correct taper in the bores but it will do this by removing material and the net result of this is that the piston to bore clearance may become excessive. Realistically though your options are going to be limited without going to a new set of oversize pistons and having the block bored and honed again. Even then you may not end up with a better result. The hone however won't remedy an out of round situation.