More ado about Through Bolts etc

[Guest DJH]

Well actually OME, 4 thou. is a very generous piston clearance and would be considered " a rattling good fit " by any engine builder worth his salt. i.e. worn out.

 

 

[Guest Andys@coffs]

I was taught that liquid cooled with greater certainty and narrower operating ranges allowed for tighter clearance tolerances and air cooled with the wide operating ranges was the opposite of that....As to whether 4thou is or isnt a tight fit.....others with more experience can comment....

 

Other than to say if it ws a tight fit, perhaps too tight then wouldnt you expect to hear of the engines nipping up regularly and of all the issues we hear about with the J engines, siezure due to tolerance, other than as a result of catestrophic failur,e isnt something I recall hearing of regularly?

 

Andy

 

 

[JabSP6]

The clearance OME has listed is what the clearance of the earlier Jab engine used to be (4 thou.)

 

Jabiru have revised the clearance some time ago to 6 thou after numerous issues came up regarding what Andy has mentioned.

 

Air cooled engine must have bigger piston to bore clearances then water cooled engines and there will always be differences of opinion on this matter.

 

Some suggest that even 6 thou is not enough.

 

The design of the piston skirt and the piston material type will determine what is acceptable for clearance.

 

Safe Flying

 

JabSP6

 

 

[facthunter]

We are talking about running CLEARANCES. Tolerence is a permitted variation to a specified size. Be careful what terminology you use, and where you measure the clearances. Some do it just below thw gudgeon pin at right angles to the pin and MOST measure it at the bottom of the skirt...Air cooled aircraft engines use piston clearances that seem excessive to auto engine builders. As for cost, the last set of lycoming cylinder assemblies I got cost me $13k. for 4. There is no comparison with Jab stuff nor is the price close. Jaburu stuff is not expensive..Nev

 

 

[Guest DJH]

Did you check what the Lycoming piston to bore clearances are ?

 

 

[old man emu]

Yeah. Off the top of my head they are at least 10 thou' and, I think, more.

 

Those Jab pistons are pretty tight.

 

OME

 

 

[facthunter]

The clearance relates to the bore size (amongst other things). Other factors being equal the bigger bore will run a proportionately larger clearence. I have the manual for the Continental C-90 which is nearly identical to the 0-200. The lycoming cylinders were checked (from new) at a place in Brisbane prior to assembly on the engine, down here. I don't know the exact clearance but would expect them to be around .010" as OME says, and I think the Cont. ones are a little less.

 

The thing about these clearances , once the motor is at running temp, they reduce to very little and if they get too hot or the original fit is too tight the engine will seize or at least drag the side out of the piston, so you are never very safe running the MINIMUM.. There is an upper limit specified too which is still allowable but engines assembled like that are noticeably noisier mechanically and the rings will wear barrell faced slightly and not seal as well. genuine Rotax two stroke pistons are supplied in "class" sizes so you get the chance to get a better fit than if they were only available in one size for each nominal bore size. The variation will often be in increments of 1/2 thou they ar colour coded or stamped ABCD etc. Nev

 

 

[Phillip King]

Hi guys, would like to add my two bobs worth to your discussion. I fly a club jab which started to use a lot of oil (140ml per hour). It caused a lot of tension in the club, trying to work out what the problem was, when a club member (engineer by trade) explained his idea on how to fix the problem. Once he got the go ahead from the club, he made up torque plates for the barrels and heads the barrels are then bolted to the torque plates to the right tension then honed so when you bolt the barrels back on the block to the right tension you you have round bores. The heads are bolted to the there torque plates valves reseated add KY valve guides.Now our club jab burns very little oil (with original pistons and rings) and runs sweet, and I mean sweet, no more typical jab clatter fixes alot of jab motor probs. Cheers Phil

 

 

[JabSP6]

Phillip

 

Just curious how many hours were on the motor when you carried out this work?

 

Any chance of posting a picture of the torque plates for the Barrels and Heads?

 

Also can you clarify what was done to the valve guides?

 

Good to hear you have sorted out your oil consumption problems.

 

Safe Flying

 

JabSP6

 

 

[Keenaviator]

Yes pictures of these torque plates please. This could be of interest to lots of Jab operators. Laurie.

 

 

[billwoodmason]

I have seen these plates, and it makes really good sense, to have the barrels and heads as straight as possible, I believe also the engine in question starts really well as the starter is able to crank the motor with ease. (finally some good communication without all the BS that the haters and sensationalisers on here come up with).

 

 

[Guernsey]

Yes pictures of these torque plates please. This could be of interest to lots of Jab operators. Laurie.

They would probably be of interest to the Jabiru Factory, or am I just wishfull thinking. ?

 

Alan.

 

 

[Yenn]

Did you use the torque plates with the barels at operating emperature?

 

 

[Phillip King]

Hi guys photos are on the way . For the valves Google k liners ,they are a spiral bronse insert desined for extreme conditions. Our motor hours were in the hi 300s .cheers Phil

 

 

[facthunter]

Fitting plates and torquing them and then finish honing or even doing the original boring process that way is not a new idea. The Jab cyls have very small bolts and I have never seen evidence if distortion due to torquing with the Jab cylinders. It may occurr, If this is happening it will be evident by the hone pattern near the bolt bosses being different from other areas due to distortion after a fairly short running time. The high spots will be polished more. Distortion HAS happened at the bottom retaining nuts which had to be modified, as they were contacting on the radius.. This tended to cause cracking Jabiru cylinders distort in use due to uneven heating due to the positioning of the very hot exhaust port to one side. ( Which is common to most designs).. This area will often show a blue colour due to the excess heat Nev

 

 

[Phillip King]

Hi all the honeing is done cold.Went there Alan yes you are wishfull thinking. Cheers

 

 

[Thirsty]

But if the aircraft is used for training you can't be making non approved changes can you?

 

 

[facthunter]

That is not a change as such. You could argue it is just a refining of a repair technique. It is done all the time building performance engines. Nev

 

 

[damkia]

That is not a change as such. You could argue it is just a refining of a repair technique. It is done all the time building performance engines. Nev

It is a change in standard assembly practice as set out by the engine manufacturer, therefore is a "change".

 

As an example, there is a set way of drilling and dressing rivet holes that is followed by all LAME's. This is to give consistent results rather than better results. If everyone went off and did their own thing there could be an argument made that anything goes, if YOU see it as an improvement.

 

 

[facthunter]

I'm not suggesting anything goes or an attitude that alludes to it. An engine must be assembled to wear limits and tolerances which are stipulated. If an organisation did reseat valves or hone with torqued bolts how would this be normally detectable? I question the need for this but can't see a problem as a principle. I would question the ability of most shops or individuals to hone cyls correctly and in any case there is always some metal removed so you are increasing clearances. If you didn't hold the cylinder correctly you could distort it. Nev

 

 

[Thirsty]

I was referring to the valve guides mostly as they're not standard to the engine. These engines are supposedly certified and in a training environment I don't think we're allowed to change anything like that. Just playing devils advocate, I'm all in favour of improving the engines.

 

 

[facthunter]

I'm a little concerned about using K-liners there myself. I have had them shrink in high temp situations and grab the valve stem..

 

4 APlat ( A contributor from france) was using guides sourced from a VW Golf. The material would be a concern from many sources. ( Bronze of which there are many different specifications). Nev

 

 

[billwoodmason]

Changing valve guides to a different type may effect warranties or certification if not approved. My interest is in the use of patterning plates which is just an installation technique. The distortion as I understand is at the top end where the head bolts to the barrel causing the barrel to distort inwards and as a result the rings have contract more at TDC as tolerances are tightened causing more friction between piston and liner - possibly causing premature wear. This idea is not new and has been used for decades particularly for air cooled engines with horizontally opposed cylinders.

 

 

[ianboag]

Changing valve guides to a different type may effect warranties

Can we have a response from anyone who has ever been paid out under a Jab warranty ?

 

 

[jetjr]

Plenty have had problems made good, they have flown LAME to remote sites for full rebuilds and covered the lot. Have sent me parts for free, replaced a prop, repaired 2 heads and generally given good backup. Mostly they charge postage only.