Bolt sizes for engine mounting

[Tim Donald]

I'm assembling a Ripmax Bolero and have the Ripmax IC installation pack but the supplied bolts for engine mounting are M4.  I am using a Saito 82 and the maximum size of bolt that would fit through the lugs is 3.5mm (same on the recommended OS 55Ax).  I have some M3 nuts / bolts but they seem quite thin compared to the supplied M4.  Will M3 stainless steel hex cap bolts be strong enough for the Saito?  Otherwise I could drill out the lugs on the engine for M4 but don't want to do that in case it weakens it.   I could source some M3.5 bolts / nuts but I can't understand why Ripmax would supply bolts that don't fit the engine that they recommend?  Am I missing something?

Thanks for any help !

 

[john stones 1 - Moderator]

Don't drill out your lugs Tim, get some 3.5mm they'll be fine.

[Tim Donald]

OK Thanks John I won't drill the lugs.  Do you think the M3 that I have would be ok?  It would save me faffing around ordering stuff.  A bit annoying that Ripmax supply the wrong bolts in the mounting pack - you wonder who designs these things.

 

[john stones 1 - Moderator]

Yep they would probably be o.k, I would buy some 3.5mm myself though, some for future use and to ensure good fit in the lugs and no elongating the holes.

[Denis Watkins]

Would always use the M4s on the engine mount to the bulkhead, and as John states for the motor.

Edited February 27, 2023 by Denis Watkins

[Tim Donald]

Thanks John / Denis.  Yes the M4 for the mount to bulkhead is fine.  OK I will try to get some M3.5 - they don't seem very common though - most places seem to only have M3 and M4.  Thanks for your help.

 

[jrman]

Have you tried Model  Fixings ?

https://www.modelfixings.co.uk/cap_screws.htm#Cap Screws

[Keith Berriman]

Beaten to it 

Model Fixings 

[kc]

Yes Model Fixings and see their note that No6 UNC is almost same as M3.5 - cheaper too.

[Paul De Tourtoulon]

3 mm is more than enough,

 

 Have a look here,,,

 

https://www.engineeringtoolbox.com/metric-bolts-minimum-ultimate-tensile-proof-loads-d_2026.html

[Simon Chaddock]

My Dad had a saying about bolts.

"You can support the weight of a man on one 10BA bolt" and that is just 1.7 mm diameter.

Using 3mm will be plenty strong enough but you would need a thick washer to ensure the bolt load was evenly distributed into the lug.  

[Don Fry]

1 hour ago, Tim Donald said:

Thanks John / Denis.  Yes the M4 for the mount to bulkhead is fine.  OK I will try to get some M3.5 - they don't seem very common though - most places seem to only have M3 and M4.  Thanks for your help.

 

I think that’s an Imperial/ Metric clash, as KC says, that’s a UNC ( measured in inches hole, converted to an unpopular metric bolt.)
Use the smaller 3 mm metric, washer needed, or buy some UNF. But either will work. The motor won’t fall off.
Stress is reserved, I note you have no tomatoes, to important stuff. I ate the first of the spring asparagus up from Spain this weekend. If you don’t get any, don’t worry, I will oblige. Honest. 

[john stones 1 - Moderator]

Tomato joke eh, least we got a Lettuce that outlasted a P.M. 😁

[kc]

According to Model Fixings the 6 UNC bolt is just 1/5th of a thou bigger than M3.5.

 

I note that many electrical fittings have M3.5 bolts so there might be some spare in electrical stuff.   Might have Phillips heads though.

 

Many electric fliers would willingly swop four M3.5 for the M3 that are often needed for electric motors.  I would!  Worth asking clubmates? 

[Tim Donald]

Great advise and help as always on here.  Thanks everyone.  I've now fitted it with the 3mm bolts that I had and I think it will be fine.  I will keep an eye on it anyway just in case.  Good to know about the 6UNC size being so similar to M3.5 for future reference.  

[Eric Robson]

I've always used m3 cap heads for mounting my Saito 82's and recently put a Hurricane in vertical after the flight battery failed, the engine had to be dug out of the ground which was soft fortunately and the 3mm bolts had sheared off, thicker bolts may have broken the engine lugs. After a good clean up and check the engine was started up and run fine. It is now mounted in an old but good Acrowot to flight test it.

[Robert Welford]

Yes - using underside engine bolts was recommended years ago (early radio + free flight) to protect engine lugs from  breaking off in the inevitable crash 😞 These days our equipment is  significantly more reliable and hopefully we all crash less often 🙂

[Engine Doctor]

3m Nuts and bolts are more than adequate for a Saito 82 assuming you use washers  . 0.5 mil clearance on the lugs is fine provided the mount is the correct size for the engine and the Crank case fits snugly between the mounting arms . If you want to use  3.5 mil bolts and are having trouble then give Rapid RC a ring , they have plenty in stock but 3mil m will be ok . Typical of  Ripmax to supply 4mm nuts and bolts for a recommended AS 55 engine. If it was a 180 FS then ok . No wonder we see so many engines with drilled out lugs . Like a lot of hardware provided with ARTF's its either rubbish or just not up to the job .

[Tim Donald]

Thanks Engine Doctor,  3mm now installed.  I must say the Bolero so far has been disappointing.  Shipped with snapped servo tray / former just glued back together,  very poorly fitting canopy / hatch (big gap at the back  where they meet but are made at different angles).  It's taken me longer to assemble and sort out the problems then building a kit would have done.  Also lots of mods needed to use a 4 stroke as no provision for throttle / servo on the opposite side etc.  Hope it flys well to make up for the effort.  

[Martin Harris - Moderator]

I stand to be corrected by a qualified mechanical engineer but as far as I'm aware, bolts are designed to pass through holes with clearance and retain components by their clamping force.  Other than tensile strength, the main benefit of using the "correct" size is less potential for misalignment during assembly - often, critical alignments will employ accurately ground locating pins.