wrist pin fit

[rdssdi]

Are TC wrist pins a light interference fit?

The crossflow engine I am currently having rebuilt requires that the pistons be heated to allow the wrist pins to be moved as they are an interference fit.

Unfortunately the initial engine builder / machine shop noticed the tight fit and honed the piston to allow the wrist pins to easily be pushed in by hand.

I am confident that they were designed that way to provide the correct fit once the piston and engine are at operating temperature. The book on crossflow engines clearly shows the pistons being heated in order to move the wrist pins. The engine builder was given that book by me to familiarize him with dimensions and this procedure.

I am now confronted with a dilemma. Do I get a new set of pistons hoping they match the bores already matched to this first set or do I leave them this way and risk piston and bore wear with no .030 over pistons available for the pre-uprated engine. The pistons fit now are .020 over.

I will call this first engine builder to demand a substantial refund to pay for the remediation of the cam bearing and now this.

Bob

[andyelan]

Hi Bob

How I hate "Experts" This is why I do every thing I possibly can myself

However to get back to your question, Pin on a twin cam should not be an interference fit although they shouldn't be loose either. Sometime the pins can get a little stuck if they're new and have been coated with oil which has dryed. In these cases a little heat might help but only something like some hot water.

In general I would say if its a warm day the pins should push through the pistons with your fingers with just a little resistance. Once in place they they should not fall out under their own weight though, if they do they're too loose.

I would guess, unless the guy who you gave the job to was totally incompetant, you'll most likley be ok, the point is however you should'nt really have been put in the position where you have to ask questions like this. Pins are ground to a tolerances of one or two tenths, why anyone should consider it necessary to "open them up a bit" I cant imagin.

Regards
Andy

[neilsjuke]

I put the pistons in the warm indoors and the pins in ice box over night
Neil

[rdssdi]

That is what happens with some "experts" who learned by doing not by thinking. "I've rebuild 500 such and such and thats the way it is done". I would ask that since the rebuild book indicates that the pistons on the crossflow be heated to allow pin removal, why would you defeat that?

Many engine builders have distain for engineers thinking them "egg heads" as their "real world" experience allows them to know better. It never seems to sink in that designs vary and if a job is done one way on a certain engine does not mean it is done the same way on others.

I would also add that there had to be a reason a tight interference fit was specified on the crossflow. It certainly was not added as a superfluous feature to confuse know it all technicians. I assume the material used to make the piston will expand enough as to need these tight tolerances.

I am hoping that another new set of pistons will be so close in diameter as to fit in my bores as presently dimensioned. I will then ask for a substantial refund to cover my expenses to rectify his error even though he will respond "they are all supposed to be like that!:.

I am tired of paying and then paying again to get it done right. I will sue the idiot if need be. I will not go gently into that dark night!

Bob

Bob

[CBUEB1771]

The Elan and Elan +2 work shop manuals both say "heat the piston in water to allow the gudgeon pin to be fitted". I have always let the pistons sit in boiling water for about ten minutes and after that the gudgeon pins slip in easily, pressing with a finger. The gudgeon pins are a light interference in the piston when both parts are at the same temperature. I have answered your question about TC pin fit however you are rebuilding a crossflow. I don't know what the pin fit should be for the crossflow. If it was a TC the mechanic who honed the pin bores would owe you a new set of pistons and pins.

[elj221c]

My original engine had Cosworth forged pistons. The gudgeon pins (wrist pins) were a tight fit so the pistons needed warming. My latest engine has QED pistons which are a light push fit cold. To quote Miles Wilkins (from his book 'Lotus Twincam Engine') "The pins are a sliding fit but if they are slightly tight, warm the pistons by using a fan or radiant heater or by placing them in hot water..."
In any case, surely the pins won't fall out as long as you fit the circlips?

Roy
'65 S2

[rdssdi]

My primary concern is that the interference fit is designed so as to allow the alloy piston to expand when at operating temperature and not have excessive "play". If it does develop a sloppy pin to piston fit I am concerned that the piston may knock or rock within the bore. I have no concern that the pin will dislocate.

Bob

[andyelan]

Hi Bob

I don't really know that much about X-Flows but do the pistons actually have any circlips. I had a feeling that the piston pin was an interference fit in the connecting rod and the piston then pivoted about the pin. If this is the case then chances are you should be ok provided that clearance isn't excessive, it isn't an issue with the Twin cam after all. Don't forget, because the parts are moving relative to each other, there has to be a little clearance to get an oil film between.

I'm with you however in that I do wish people wouldn't always seem think they know better than the original designers. I do believe Ford probably knew a little bit about engines after all.

Regards
Andy

[types26/36]

I've worked on lots of Xflows and they have a fully floating gudgeon pin with circlips and for all intents and purposes the pin fitment is the same as Twin Cams, I have had Xflow pins that both went in with a finger push and some need a bit of heat (boiling water poured over them works well) also depending on ambient temperature but as long as the pins are not a sloppy fit I doubt weather there will be a problem, although I would not have too much faith in an engine builder who felt it necessary to alter the pin to piston fit when they are supplied ready to fit with the correct tolerance.

[rdssdi]

I agree. I have no faith in the initial engine builder which is why the engine is at another shop now.

The pin to piston clearance is between .0001 to .0002 inch.

The cam bearing problem was satisfactorily resolved with the installation of a new set of cam bearings. It is possible the initial engine shop improperly fit the #1 cam bearing. Not a surprise.

I will also need new small end con rod bushings.

Bob

[andyelan]

Hi Again Bob

Others might like to confirm this, but I would guess you should be ok with 1 to 2 tenths clearance

Regards
Andy

[dlbutler]

My JE forged pistons came with an easy push fit out of the box. The racecar machine shop honed the pistons to increase the clearance slightly because "we do that for all JE pistons." I was worried for a while, but after 3000 miles of hard driving with a 7500 rpm redline still no problems.

Don
1700cc's - Dave Bean #112 cams

[rgh0]

Used JE pistons for years in my engines - I think they know more about setting up the clearances between their pistons and gudgeon pins than me or any other "race machine shop". "Honing ths clearance" may not have done much damage if they did not alter it signficantly but I doubt it did any good either and I bet they charged you for it.

I would also bet they had no specific dimension or tolerances on the new fit they created that they specified for you and I doubt they had any engineering basis for what they were doing.

If JE had it wrong I think they would have changed what they supply by now. I have always used the as supplied fit and never had any problems and JE keep there tolerance very tight so the fit is always the same on every piston I have got from them

cheers
Rohan

PS In the business I manage I spend a lot of time with the maintenance and engineering departments to ensure that when they make a change like this it is properly documented and has a sound engineering basis and that the outcomes are rigorously tracked in the future. Uncontrolled changes whether it be by DPO or race engine shops is a recipe for future problems.

[rdssdi]

Rohan

I do agree with you.

Unfortunately at this point I have little choice but to use them as "modified". The first engine builder did this and I am most upset because he was only to fit the pistons to the engine bore. Not the wrist pin to piston.

To fit new pistons would be difficult as they are .020 over and that is all that is available. At best I would have to ask the vendor to "select" pistons of certain diameters and make up a set.

Another DPO find. One con rod is 15 grams lighter than the others. The adjustment "pad" was completely ground off and the sides of the rod thinned. "New" used one on the way.

Bob

[dlbutler]

Well, the machine shop I use is a real racecar machine shop not one in quotes. You can go to their website for a partial list of pro customers, but the short story is they have been winning for almost 40 years, so I trust them. http://www.accellodyne.com/

They dimension all of the JE wrist pins the same and I can find out the number they use if it is important.

Don