You're welcome Bill
Following on from the previous post the technique used to create the conditions for a good piston/liner seal in a compression ignition model engine - usually mis-referred to as a 'diesel' is fundamentally more of the same but using a finer grit - in this case F600 - to smooth out the bore and induce a very slow taper from the bottom of the liner to just about where Top Dead Centre would be preferably slightly lower than higher. Technically it isn't neccessary to produce a taper at all but by doing so it provides a the means to lap the piston exactly to the dimension required and also once finished and in place reduces friction in the lower part of the bore. The taper is very small dimensionaly - certainly not measurable with basic kit but again the plug gauge will give an indication.
The lap is thoroughly cleaned of all traces of the 320 grit and a similar paste mixed using F600. The procedure is exactly as with the 320 but this time the lap is allowed to dwell at the bottom end and the lap is not taken all the way up the cylinder. Ideally the area taken up by the contra piston should remain as previously lapped but allowing the lap to go through this area every so often will not affect this too much - certainly not enough to cause concern. It does not take long to produce a situation where the plug gauge, with everything nice and clean, will now enter the bottom of the bore with ease but beginning to tighten at around the exhaust position. As soon as this state is reached any further lapping is really to 'polish'. Lots of lubricant but no more paste. just expanding the lap to provide a slight contact if it gets too loose.
Previously a further lapping using F1000 powder has been carried out but on this latest batch it was felt that the finish acheived with 600 was sufficient for the next stage which is to lap the piston to the bore. The resultant seals are just as good if not slightly better than previous.
These are the liners done for the Racers - the first of the scaled up engines as you can see the 1000 gives a high degree of polish
on the left is cast iron the right is high tensile steel which appears to polish to a higher finish than En1a
These are the two Tigre liners as shown previously having been lapped with 600 - very smooth but no sheen.
Before the piston can be lapped to a perfect fit with the liner the piston itself requires lapping. For this an external lap requires making. Whilst all that was available was a home made lap something that was considered to be very desirable was an external hone. After along wait for the right kind I was finally able to secure two sets at very reasonable cost off E-Bay. Much to my surprise when they arrived they were absolutely untouched - just one, easily replaced, screw missing
Desirable yes but not exactly neccessary - Used on the Tigre engines they do work well but appear to have no advantage at all over this....
Easily made, this piston lap works extremely well and removes metal just as easily as the hone - the hones were left in the box on this latest batch - the lap you see has done all seven Etas and the original gap cut to allow closing just short of nipping up on the last piston
The procedure is the same for the liner - begining with 320 the piston is coated and a smear applied to the lap
Constantly checking the dimension, the diameter is reduced until the top of the piston will just enter the bore by about 3mm or so then the lap is cleaned and the grit changed to 600.
The lapping continues until the piston will enter the lower bore becoming tight as the crown reaches just below the exhaust area. Lapping on the lathe stops at this point and the remaining lapping of piston to liner is carried out by hand
Once again the piston and liner is thoroughly cleaned of all traces of the previous compound. A tee handle driver is required and this has an end that will fit inside the piston with a cross hole that will take a reasonably tight fitting brass 'wrist pin'
The piston is attached to the end of the rod and a very small amount of F1000 compound previously mixed applied to the piston and smeared around
The piston is then inserted into the liner and with a very slow wringing and in and out action the piston/liner are gradually lapped together.
By it's very nature this is a slow process and one that cannot be rushed if a good result is to be achieved. If the piston begins to tighten and stick it shouldn't be forced - it can tend to pull itself in - tapping it gently from the other end using a wooden dowel or similar as a driver will soon release it. Wipe some of the paste off and apply plenty of lube and continue. This op does not require much compound. It can take as much as 15-20 mins for this op - but once the piston is fully inside and about halfway up the bore past the exhaust it's removed and
wiped clean - no more - and relubed.
All the lapping from this point uses the residual compound and lots of lube. When it begins to squeak it's getting dry so it's removed, more lube applied and the process continued
The piston is lapped in this fashion until the skirt is just level with the bottom of the exhaust....
... then removed and thoroughly cleaned in solvent. If an ultrasonic cleaner is available then a quick rinse in that is ideal after cleaning.
That's enough for tonight - sailing day tomorrow
and an early start
regards - Ramon