[Fot] piston/head clearance

Bill ponobill at gmail.com
Wed Aug 23 22:19:13 MDT 2017 

The theory about squish area in two strokes is that besides generating turbulence (thus mixing) and moving the majority of the mix to the middle of the head, that the relatively cool surfaces in close proximity to the mix slow the flame front headed for the cylinder wall and prevent detonation. If the squish band is too tight the gasses cool too much and don’t burn to the edges. The piston is still moving upwards at that point so partially burned gases push back into the non-squish area as well and screw up complete burning. One of the reasons that head and piston tops needed to be decarbonized was closing up the squish gap and losing horsepower. 

At least that’s what Gordon Jennings said. I always believed him. 

> On Aug 23, 2017, at 6:46 PM, Kas Kastner via Fot <fot at autox.team.net> wrote:
> 
> This is a very interesting subject.  In  the thousands of hours of dyno testing I did in the 60's, the squish area clearance was never addressed with exception of making certain the piston did not hit the head. Off hand, when the clearance is too close it rather sounds like there is insufficient volume in the squish at TDC to do its job of properly exciting the balance of fuel mixture when it squirts back into the combustion chamber thus making a better mixed and  thus more powerful combustion moment. Or maybe something else :-)
> 
> Never be beaten by equipment.
> 
> 
> On Wed, Aug 23, 2017 at 3:47 PM, John Hasty via Fot <fot at autox.team.net <mailto:fot at autox.team.net>> wrote:
> Interesting... We found 1.5 mm in the 250 two strokes was the best....
> 
> Sent from my iPhone
> 
> On Aug 23, 2017, at 6:31 PM, "Chris Marx via Fot" <fot at autox.team.net <mailto:fot at autox.team.net>> wrote:
> 
>> On dyno test we found a disturbing issue that lowers the power if the piston to head clearance is less than 0.039“.
>> 
>> We didn’t find a cause for this but tested thoroughly clearances from 0.0236” upwards.
>> 
>> Best power reading was with 0.0472”.
>> 
>>  
>> 
>> Cheers
>> 
>> Chris
>> 
>>  
>> 
>> Von: Fot [mailto:fot-bounces at autox.team.net <mailto:fot-bounces at autox.team.net>] Im Auftrag von fubog1 via Fot
>> Gesendet: Mittwoch, 23. August 2017 23:52
>> An: timmmurphh at gmail.com <mailto:timmmurphh at gmail.com>; fot at autox.team.net <mailto:fot at autox.team.net>
>> Betreff: Re: [Fot] FW: Grattan VSCDA next weekend? piston/head clearance
>> 
>>  
>> 
>> With the good rods and reasonable rpm you have plenty of clearance.
>> You don't want too much clearance either cuz then it lessens the "squish" effect.
>> The idea is to have enough clearance for all running conditions, plus a few thou safety margin, and no more.
>> Glen
>> 
>>  
>> 
>>  
>> 
>>  
>> 
>> -----Original Message-----
>> From: timmmurphh <timmmurphh at gmail.com <mailto:timmmurphh at gmail.com>>
>> To: 'fubog1' <fubog1 at aol.com <mailto:fubog1 at aol.com>>; fot <fot at autox.team.net <mailto:fot at autox.team.net>>
>> Sent: Wed, Aug 23, 2017 3:38 pm
>> Subject: RE: [Fot] FW: Grattan VSCDA next weekend? piston/head clearance
>> 
>> The Payen head gasket last used was compressed to 0.055 at the fire ring.  So, with the 0.010 pop-up that I have from the all efforts to deck the block, I will have 0.045 clearance.  I have Carrillo rods and we try to stay to under 6000 RPM for engine “longevity”.  (Not sure “longevity” is a term that can be used with a race engine!)  At the most we’ll hit 6200 or 6300 RPM briefly before upshifting.  Therefore, I should be within the 0.030 clearance that you recommended.  It will be interesting to see what the top of the pistons looks like on the next rebuild with regard to carbon build up in the squish area.
>> 
>>  
>> 
>> Thanks for the advice and all the other comments on this subject.  All much appreciated.  Still a lot to learn😊
>> 
>>  
>> 
>> Tim
>> 
>>  
>> 
>> P.S. Finally got everything together on the engine and about ready to mate up to the transmission and put back in the car.  Looking forward to ELVF next month.  That will be the test.
>> 
>>   <>
>>  <>From: fubog1 [mailto:fubog1 at aol.com <mailto:fubog1 at aol.com>] 
>> Sent: Wednesday, August 16, 2017 5:27 PM
>> To: timmmurphh at gmail.com <mailto:timmmurphh at gmail.com>; fot at autox.team.net <mailto:fot at autox.team.net>
>> Subject: Re: [Fot] FW: Grattan VSCDA next weekend?
>> 
>>  
>> 
>> Check the gasket thickness, it depends on what piston/wall clearance, rods, and RPM you use.
>> You generally need around .030 piston/head with stock rods, so add that to your pop-up of .010 for minimum gasket thickness.
>> If you run good rods you can run a little less before the pistons smack the head.
>> It's perfect when there isn't any carbon build up on the piston tops under the squish area.
>> When it's not right, it goes BRRRRRRRRRRRRR...
>> HTH
>> Glen
>> 
>>  
>> 
>> After all the machining to get the stick out
>> right I now have the pistons sticking up about 0.010 above the tops of the
>> liners. No problem with the composite Payen head gasket.
>> 
>>  
>> 
>>  
>> 
>>  
>> 
>> -----Original Message-----
>> From: timmmurphh--- via Fot <fot at autox.team.net <mailto:fot at autox.team.net>>
>> To: fot <fot at autox.team.net <mailto:fot at autox.team.net>>
>> Sent: Wed, Aug 16, 2017 5:48 pm
>> Subject: [Fot] FW: Grattan VSCDA next weekend?
>> 
>> 
>> 
>> -----Original Message-----
>> From: timmmurphh at gmail.com <mailto:timmmurphh at gmail.com> [mailto:timmmurphh at gmail.com <mailto:timmmurphh at gmail.com>] 
>> Sent: Monday, August 14, 2017 12:20 PM
>> To: 'Tony Drews' <tony at tonydrews.com <mailto:tony at tonydrews.com>>
>> Cc: ryan.murphy at fdlco.wi.gov <mailto:ryan.murphy at fdlco.wi.gov>; Steve Yott <tr4 at wi.rr.com <mailto:tr4 at wi.rr.com>>
>> Subject: RE: [Fot] Grattan VSCDA next weekend?
>> 
>> Sadly, we won't be able to make it.
>> 
>> I just got the block back from the machine shop last Tuesday. Not enough
>> time to get everything together and in the car and ready to go by Thursday.
>> The good news is the liner stick out is the 0.004 the machine shop said they
>> machined the liner seats and deck to. It's still a bitch to measure, as the
>> liners "wobble" on the seats and when you're looking for 0.004 it's a
>> problem. I finally have a decent dial indicator gage setup to at least give
>> repeatable readings. The figure 8's used was a set of copper ones I had
>> that measured 0.016. The machining was done on a single setup on a Rottler
>> floor type vertical 4-inch spindle milling/boring machine. These are
>> special purpose machine tools built specifically to machine engine parts,
>> such as blocks and heads. After all the machining to get the stick out
>> right I now have the pistons sticking up about 0.010 above the tops of the
>> liners. No problem with the composite Payen head gasket.
>> 
>> Thanks for all the responses to the Figure 8 gasket question. I did
>> eventually hear back from Moss UK. They said their steel fig. 8's are 0.016
>> and their copper fig. 8's were 0.018. No explanation regarding the
>> different thickness other that they have not had reported problems with the
>> gaskets. I'm not sure where the 0.020 thick copper ones that I measured
>> came from or for that matter, the 0.016 thick copper ones I'm using.
>> 
>> Tim & Ryan Murphy
>> 1961 ('61, it is Comm. No. 511 and was built in 1961 per British Heritage)
>> TR4 #317 BRG
>> 
>> -----Original Message-----
>> From: Fot [mailto:fot-bounces at autox.team.net <mailto:fot-bounces at autox.team.net>] On Behalf Of Tony Drews via
>> Fot
>> Sent: Saturday, August 12, 2017 6:21 PM
>> To: fot at autox.team.net <mailto:fot at autox.team.net>
>> Subject: [Fot] Grattan VSCDA next weekend?
>> 
>> Anyone else going to Grattan next weekend?
>> 
>> Vroom, vroom, Tony Drews
>> 
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